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Full Text:   in newborns and Guillain Barre´Syndrome (GBS). Recently, this virus has posed a pandemic threat to global human population. The virus is transmitted primarily by a mosquito (Aedes aegypti), whereas, other routes of viral transmission includes monkey bite, coitus and body fluids such as blood, semen, and saliva which needs further confirmaton. Furthermore, implementation of appropriate mechanical, chemical and biological control methodologies can help to avoid mosquito bites and ultimately limit the viral spread. Another potential approach could be the development of ZIKV resistant mosquito strains such as Wolbachia-harboring mosquitoes which can reduce the incidences of ZIKV by lowering the availability of vectors for virus transmission. In-use ZIKV diagnosis relies on serological, molecular and few other advanced detection methods. Nowadays, worldwide researchers and biomedical and pharmaceutical authorities are paying attention on designing and developing effective vaccines, drugs, medicines, and therapeutics to counter this virus. The reasons behind the shift of virus virulence need to be explored by understanding the genetic and molecular characteristics, host-pathogen interactions and immunobiology of the virus, which would help to design effective antiviral drugs, prophylactics, and vaccines. The present review highlights various aspects of ZIKV and its clinical manifestations, progress, and advances in developing effective diagnostics, vaccines and drugs/therapeutics along with adopting suitable prevention and control strategies to tackle this deadly emerging disease.     
 
1 Introduction Zika virus (ZIKV) is a mosquito-borne virus of the Spondweni serocomplex, genus Flavivirus, family Flaviviridae. Owing to the climate changes like global warming, increasing population dynamics, fast globalization and travel, the human population is facing a rising emergence and outbreaks of mosquito-borne viruses such as Dengue virus, Japanese encephalitis virus, Chikungunya virus, West Nile virus and Zika virus (Chen & Wilson, 2010; Dhiman et al., 2010; Dhama et al., 2013a; Hubalek et al., 2014; Medlock & Leach, 2015; Parham et al., 2015; Carneiro & Travassos, 2016; Gautret & Simon, 2016; Musso et al., 2017). Soon after the deadly outbreaks of Ebola virus in Western Africa, the most recent emerging virus threatening the global human population is the Zika virus (ZIKV), declared as an emergency situation (Public Health Emergency of International Concern) on February 1, 2016 by the World Health Organization (WHO) for its quick spread, affecting large human population in different countries with pandemic threats (Dhama et al., 2015; Chang et al., 2016; Chen & Hamer, 2016; ECDC, 2016; Gulland, 2016; Higgs, 2016; Singh et al., 2016; Krauer et al., 2017; Singh et al., 2017). Being remained harmless for six decades (first reported in 1947), the sudden emergence of ZIKV with higher virulence, speedy spread and inducing severe clinical manifestations  (especially associated with the cases of microcephaly, unexpected fetal anomalies during gestation period as placental insufficiency, hampered fetal growth, impaired Central Nervous System (CNS) of fetus, fetal death and other neurologic disorders as well as a cluster of Guillain-Barré syndrome) along with little knowledge on suitable prevention and therapeutic measures created  massive threats for the human health (Zanluca & dos Santos, 2016; Cao-Lormeau et al., 2016; Duhaime-Ross, 2016; Gatherer & Kohl, 2016; Petersen et al., 2016a; Samarasekera & Triunfol, 2016; Singh et al., 2016; Molko et al., 2017). Earlier reports on ZIKV were limited to Africa and Asia, while now it has worldwide presence  (Hayes, 2009; Heang et al., 2012; Grard et al., 2014; Brown, 2015; Chang et al., 2016; Fauci & Morens, 2016; Lucey & Gostin, 2016; Vest, 2017; Zhang et al., 2017). It is noteworthy that since the first report of the disease from the African continent, only 14 cases in humans had been reported before its first large epidemic in the year 2007 on the Island of Yap (Duffy et al., 2009; Marano et al., 2016; Reveiz et al., 2017), followed by the largest outbreak in French Polynesia (October 2013 to April 2014) (Cao-Lormeau et al., 2014; Reveiz et al., 2017). After that in 2015, ZIKV expanded its horizons and also emerged in Vanuatu, Fiji, Solomon and Samoa (ECDC, 2016; Musso et al., 2017). The sequences of ZIKV from Brazil and Suriname upon phylogenetic analysis have revealed their Asian origin. Pacific Islands have been found to be the region from where the virus has entered into the Latin America during some sports events during the year 2014 (Waggoner & Pinsky, 2016). Since 2015, 76 countries and territories around the world have reported ZIKV transmission, predominantly the South Americas, where Brazil was the most affected country with more than 1.4 million affected cases of Zika alone (Jamil et al., 2016; WHO, 2016; Krauer et al., 2017). With the noticed severity of the recent ZIKV outbreak, several countries geared up for epidemiological investigations to know out further disease spread and the serious health concerns (Van Kerkhove et al., 2016; Reveiz et al., 2017). Notably, in Asia the incidence of ZIKV infection is relatively low when viral detection was done by employing reverse transcription-polymerase chain reaction (RT-PCR). Viremia induced by ZIKV infection is relatively low; thereby decreasing the probability of detecting the virus in blood samples in acute cases. This has led the researchers to make the interpretation of the results by taking extra caution (Shan et al., 2016; Duong et al., 2017). In Asia and America, the strain difference (as far as the infectivity is concerned) could have been responsible for the fundamental difference in the epidemiology as well as the burden of ZIKV infection. The Asian lineage of ZIKV is responsible for most of the recent outbreaks in Asia as well as America (Haddow et al., 2012). There is a requirement of using viruses generated de novo from diverse geographical as well as clinical sources to describe the pattern of ZIKV infection in Asia and America (Setoh et al., 2017). ZIKV infection impacts any nation’s economy adversely besides being a toll on human health (Jamil et al., 2016). International agencies including WHO, Pan American Health Organization (PAHO), the Consortium for the Standardization of Influenza Seroepidemiology (CONSISE), Institute Pasteur, the International Severe Acute Respiratory and Emerging Infection Consortium (ISARIC), Fiocruz, and others have taken up collective steps and are coordinating to synchronize the ongoing research on this virus and complications associated with it (Reveiz et al., 2017). The emergence of this virus with increased virulence and potential of rapid spread posed pandemic threats to global human population (Chitti et al., 2016; Petersen et al., 2016a). Of note, till 2015 nearly 100 research articles were available in Pubmed and Pubmed Central, while now in January 2018, there are more than 3500 research articles on different aspects of the virus and the disease it causes, which altogether reflects the high concerns and wide attention this pathogen has gained in just past 20 months. Researchers in many countries are trying hard to counter ZIKV and the Zika fever by carrying out detailed virological, pathological and molecular studies, developing rapid diagnostics, finding out potential drugs, prophylactics, vaccines as well as adopting appropriate prevention and control measures (Keasey et al., 2017; Munjal et al., 2017a; Rather et al., 2017; Shankar et al., 2017; Sharma & Lal, 2017; Singh et al., 2018a). Rapid diagnostics are now existing for detecting ZIKV infection, many drug and vaccine candidates have also been identified, but still any effective / approved treatment or vaccine is practically lacking against this virus (Dyer, 2016a; Fernandez & Diamond, 2017; Munjal et al., 2017a; Munjal et al., 2017b; Passi et al., 2017; Singh et al., 2018b). Appropriate prevention and control strategies include limiting the spread/bite of the vector mosquitoes by checking their population expansion, safe precautions during sexual intercourse and blood transfusions, avoiding travel to Zika endemic countries and surveillance and monitoring are the only feasible options to keep ZIKV infection under limits (Rather et al., 2017; von Seidlein et al., 2017). Here, we present a compilation on the Zika virus/ Zika fever, covering different aspects of the virus and the disease it causes, and describes the ongoing progress and advances being made in the field of designing and developing diagnostics, vaccines, drugs along with prevention and control measures to be adapted to combat this viral pathogen of high public health concerns. 2 Etiology, Epidemiology and Transmission / Spread Zika virus belongs to the genus Flavivirus, family Flaviviridae (Gold & Josephson, 2016; Paz-Bailey et al., 2017; Uncini et al., 2017). It is a ss-RNA virus of 11 kb genome size with single open reading frame (ORF) encoding a polyprotein with 03 structural proteins (capsid, pre-membrane, envelope) and 07 non-structural (NS) proteins (NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5) (Musso & Gubler, 2016; Kumar et al., 2017). Among these NS5 has multiple roles and protein E plays a pivotal role in binding and fusion of virus as an important basic step for the establishment of ZIKV infection (Dasti, 2016). Various antigenic epitopes are present in the third domain of the E protein and thus essentially targeted as far as the serological tests are concerned. It is also an important target for vaccine development. It is assumed that the global spread of the ZIKV is due to loss of the E protein glycosylation site: N154. This helps the virus to adapt to a broader range of vector mosquitoes (Faye et al., 2014; Chan et al., 2016). From Zika forests of Uganda, the first ZIKV isolation was done in 1947 from rhesus monkey. ZIKV strain 766 particle size ranged between 30 and 45 nm (Dick, 1952). Recently, the viral structure has been elucidated using cryo-electron microscopy which could assist in designing of antiviral drugs and vaccines (Sirohi et al., 2016). Consequent to early 2015 Zika disease outbreak in Brazil, the Zika fever spread rapidly across South and Central America, North America, Latin America, Mexico, Colombia, French Polynesia, Cook Islands, Haiti, Panama, Trinidad and Tobago, Samoa, and American Samoa (Campos et al., 2015; Zanluca & dos Santos,  2016; Rodriguez-Morales, 2015; Nereida, 2015; Summers et al., 2015; Chang et al., 2016; Chen & Hamer, 2016; ECDC, 2016; Hennessey et al., 2016; Higgs, 2016; Pastula et al., 2016; Petersen et al., 2016b). As per the updates of the Pan American Health Organization (PAHO) vector-borne ZIKV infections have been confirmed in 48 countries and sexual transmission in 5 countries regionally. Imported cases have been recorded in people visiting areas where the epidemic infection is prevalent and subsequently returning to Europe, Asia-Pacific and the Americas (Pan American Health Organization/ World Health Organization, 2016; Goorhuis et al., 2016). ZIKV autochthonous (locally transmitted) cases have been reported from many countries or territories (Zanluca & dos Santos, 2016; Calvet et al. 2016a). The year 2016 also marked the report of first ZIKV infection in Taiwan and PCR followed by further analysis showed that it was Asian lineage virus having close proximity with Cambodian virus (Huang et al., 2016). The ZIKV transmission mainly takes place via mosquito (Aedes sp.-Aedes aegypti and A. albopictus) in an anthroponotic manner (human-to-vector-to-human), perinatal transmission, sexual intercourse, and blood transfusion (Besnard et al., 2014; Musso et al., 2014, Musso et al., 2015; Franchini & Velati, 2015; Carneiro & Travassos, 2016; Marcondes & Ximenes, 2016; Shoaib et al., 2016). Aedes aegypti was originated from A. aegyptiformosus, a zoophilic tree hole mosquito from African countries while Aedes albopictus is a zoophilic forest mosquito, belongs mainly to Asian countries. A. aegypti mosquito is a most important vector in spreading of ZIKV (Kaddumukasa et al., 2014; Carneiro & Travassos, 2016). Apart from Aedes mosquito vector other vectors like Culex spp. have also been reported. Studies suggested that around ten species of genus Aedes, Culex perfuscus, Anopheles coustani and Mansonia uniformis have also been reported as a vector for ZIKV (Ayres, 2016). Culex quinquefasciatus was also suspected to play a role in ZIKV transmission as ZIKV RNA was detected from these mosquitoes from North Eastern Brazil and later from China. Thus there may an array of vectors involved in ZIKV transmission which has to be elucidated swiftly to prevent further transmission (van den Hurk et al., 2017). ZIKV transmission from infected pregnant woman occurs through transplacental route and thus affects the brain of the fetus (Roa, 2016; Torjesen, 2016). The mosquito A. aegypti and A. albopictus mainly transmit ZIKV in urban areas while in niche ecotypes A. hensili and A. polynesiensis are found to be the vectors. Even though the dominance of the vector A. albopictus is mainly in Asia but the vector has got potential to spread ZIKV infection globally because of its invasive nature and broader distribution geographically (Grard et al., 2014; Ledermann et al., 2014; Weaver et al., 2016). In the tropical and subtropical climate, the rainy season is conducive for the Aedes mosquitoes allowing them to breed and most of the people suffer from mosquito bite during the day (from dawn to dusk) (Ibrahim, 2016). Brain lesions in the non-human primate fetus have developed due to subcutaneous inoculation of ZIKV. Susceptibility in neonatal pigs is also on the higher side (Waldorf et al., 2016; Darbellay et al., 2017). ZIKV transmission from mother to fetus is high in rhesus macaques when the Asian lineage of the virus is inoculated subcutaneously. Such study is also indicative of the possibilities of mother-fetus transmission in case of human. Studies suggested the transfer of ZIKV from a pregnant mother to fetus and from lactating mother to newborn as well (Jamil et al., 2016). Following inoculation through rectum or vagina in macaques (adult) the rate of infection was found high (Haddow et al., 2017; Nguyen et al., 2017). Some of the cases of asymptomatic ZIKV infections have been found to spread through transfusion of blood during acute viremia. The role of saliva to spread the viral infection is still under study (Oster et al., 2016; Petersen et al., 2016a). Reports are suggestive of ZIKV transmission through infected monkey bite also (Leung et al., 2015). The isolation of ZIKV from semen is an indication of transmission through coitus (Musso et al., 2015). The first confirmatory case of sexually transmitted ZIKVinfection is from France in 2016 in a woman where her husband acquired the infection during his recent visit to Brazil (Elgot et al., 2016). Recently, a study was conducted to know the site of ZIKV replication in the genital organs which aid in the sexual transmission of the virus. Stromal mesenchymal stem cells and epithelial cells of the human prostate were studied with 3 different ZIKV isolates, and the result showed that ZIKV infection is higher in stromal cells than epithelial cells of the prostate. Thus, findings confirmed that replication of ZIKV can occur in prostate and can spread further through semen (Spencer et al., 2017). RT-in situ PCR of the sperm showed that ZIKV was confined to the midpiece of the spermatozoa and hence it was speculated that receptors for ZIKV might be present in the midpiece of the sperm (Bagasra et al., 2017). Present knowledge on ZIKV shows that transmission can occur from male to a female partner, and male can shed ZIKV through semen, but there is no evidence regarding the shedding of virus from the female genital tract. Further research may pave way to find the exact mechanism of transmission and the receptors responsible for ZIKV infection (Epelboin et al., 2017). 3 Zika Fever - Clinical picture, Pathology and Pathogenesis   Infection with ZIKV were earlier reported to be asymptomatic in 50 to 80% cases, and the symptoms of  illness being mild and self-limiting, a febrile disease of 3-7 days with no mortality and lesser hospitalizations (Duffy et al., 2009; CDC, 2016; Reveiz et al., 2017). The symptoms are more or less similar to dengue infection expressing fever, anxiety, joint pain, body ache, indicating no special reason to consider Zika virus as the primary cause of illness unless until any neurological or auto-immune disorder develop (Jamil et al., 2016). However, the most recent outbreaks of ZIKV revealed microcephaly in newborn babies (abnormal small heads and brains), and congenital neurological malformations and disabilities in babies, while in adults it mainly presented Guillain-Barré syndrome (GBS); apart from these visual impairment cases have also been recorded (de Araújo et al., 2016; Brasil et al., 2016; Cauchemez et al., 2016; Cao-Lormeau et al., 2016; Dos Santos et al., 2016; Cuevas et al., 2016;  Martines et al., 2016a; Mlakar et al., 2016; Schuler-Faccini et al., 2016; Shuaib et al., 2016; Rodriguez- Morales, 2016; Reveiz et al., 2017; Ventura et al., 2017). It is reported that ZIKV may not cause congenital brain abnormalities or GBS by itself and some unknown cofactors might be associated along with this virus to cause such serious complications (Reveiz et al., 2017). GBS, an immune-mediated ascending flaccid paralysis, is implicated within a month of ZIKV infection (Willison et al., 2016; Krauer et al., 2017). In patients suffering from GBS, respiratory muscles become gradually weak which led to difficulty in breathing and lead to death or lifetime impaired functions of affected muscles (Jamil et al., 2016). Recently, transient myocarditis has also been found associated with ZIKV infection, and researchers advise electrocardiogram and troponin tests if cardiac signs are suspected of ZIKV infection (Aletti et al., 2017). It is also speculated that hearing loss may also be associated with ZIKV. A study among 104 infants supposed to be infected with ZIKV in Brazil showed 9% cases affected by hearing loss. Further detailed studies are warranted to unearth the relationship between ZIKV and hearing loss (Mittal et al., 2017). The possibility of vertical transmission of ZIKV is documented in the literature. Calvet et al. (2016b) reported the presence of the ZIKV genome in the amniotic fluid along with the presence of anti-ZIKV IgM antibodies. This implies the ability of the virus to reach the fetal environment thereby suggestive of the possibility of vertical transmission. The presence of ZIKV in the brain tissues of the fetus is also suggestive of vertical transmission (Mlakar et al., 2016). But as IgM cannot cross the placenta, its presence in the fetus is an indication of vertical transmission of ZIKV to cause fetal infection. The GBS is reported to have an incidence of 0.24 per 1000 cases of ZIKV infections (Cao-Lormeau et al., 2016; Uncini et al., 2017). A recent analysis of Zika-associated GBS from seven countries (Brazil, the Dominican Republic, Colombia, Honduras, El Salvador, Venezuela and Suriname) documented the rapid surge of GBS (2.0–9.8 times higher) as compared to the pre-Zika era (Parra et al., 2016; Dos Santos et al., 2016; Uncini et al., 2017). This remarkable upsurge in GBS cases could inundate hospital and intensive care resources with well-equipped healthcare services (Uncini et al., 2017). It is noteworthy that there is involvement of the Asian/ Pacific lineage of the virus in case of microcephaly cases in Latin America. The rate of restoring the walking capacity in patients with GBS at the post-infection stage is found to be 56% (Cao-Lormeau et al., 2016; Teruya & Versalovic, 2017). The other sequel of the disease is the visual impairment in congenital cases which is mainly due to chorioretinal atrophy, mottling of retinal pigment, retinal vasculature and optic nerve abnormalities (Benzekri et al., 2017; Ventura et al., 2017). The congenital Zika syndrome is also associated with arthrogryposis, brain parenchymal atrophy, intracranial calcification, ventriculomegaly, hypoplasia of corpus callosum, brain stem and cerebellum (Mehrjardi et al., 2017; Sousa et al., 2017). The recent ZIKV epidemic in Latin America has also been implicated with observations of severe fetal abnormalities such as spontaneous abortion, stillbirth, microcephaly, hydranencephaly, and placental insufficiency, which might lead to limiting the intrauterine growth of fetuses (Ferguson et al., 2016; Musso & Gubler, 2016; Kumar et al., 2017). The brain development of the fetus is inhibited by ZIKV which is evident from the neural stem cell death in case of this viral infection. Studies have been conducted in mice time-to-time wherein intraperitoneal injection of the infected materials has resulted in the development of neuropathological lesions involving the brain. There is very less fatality as far as the acute infection is concerned, but certainly, it has been reported in Colombian children suffering from sickle cell disease. There has been no any report of haemorrhagic symptoms in patients infected with ZIKV (Koenig et al., 2016; Tian et al., 2016). There are still ongoing researches for understanding the mechanism of development of microcephaly along with other neurological disorders in case of ZIKV infection (Araujo et al., 2016; Faizan et al., 2016). There may be involvement of the neural progenitor cells, and most importantly direct suppressive effects of the NS4A and NS4B proteins of the virus on the process of neurogenesis is evident. Cellular death is the outcome of stem cell infection of brain thereby reducing the genesis of neurons further ultimately resulting in retarded brain growth (Li et al., 2016; Nayak et al., 2016). It has been found that the subcutaneous inoculation of ZIKV in guinea pig can result in viral invasion and replication in the brain. It has also been shown previously that in mice viz., A129 and AG129 (that have a defect in interferon responses) the concentration of the virus in the brain, as well as spinal cord, is high. In primates (non-human) the viral RNA has been demonstrated after subcutaneous injection of the infected material. However, there is the scope of further researches to determine and to increase our understanding regarding the exact mechanisms involved in microcephaly mediated by ZIKV (Lazear et al., 2016; Kumar et al., 2017). Ancestral analysis of the ZIKV showed that a single point mutation at 139 position of viral polyprotein from serine to asparagine (S139N) led to the increased infection of both mouse and human neural progenitor cells. This alteration also led to higher cases of microcephaly and mortality in infant mice. This study also revealed that this mutational change occurred before French Polynesia ZIKV outbreak in 2013 and it remained constant throughout the outbreak (Yuan et al., 2017). Anomalies of the eye may develop due to tropism/ affinity of the virus towards the developing ocular cells (Li et al., 2016). It is important to note that due to interference with the mitotic function ZIKV can cause microcephaly (Bullerdiek et al., 2016). Immune-mediated damage of the fetal brain cells may occur if the virus crosses the placental barrier (Wang et al., 2017a). The neural progenitor cells can be infected by ZIKV thereby producing viremia in the infected mother. This further leads to transfer of the virus maternally (from blood to the fetus) resulting in teratogenic effect (viz., neurological lesions). There is a decrease in posterior white matter along with white matter gliosis (bilateral) in the fetal brain infected with ZIKV in non-human primates. Apoptotic and mitotic figures may appear (Tang et al., 2016; Waldorf et al., 2016). Atrophy of the brain of the fetus along with coarse calcification in the white matter of frontal lobes and reduction of circumference of the brain s revealed by ultrasonography analysis (Melo et al., 2016). Recently, it was found that ZIKV also leads to testicular atrophy upon infection in mouse thereby suggesting serious complications of non-vector transmission of this virus along with reproductive deficiency in males (Uraki et al., 2017). Prolonged viremia is a feature in pregnant women due to replication of the virus in fetus and placenta (Suy et al., 2016). Thrombocytopenia and hemorrhagic signs have been most recently reported (Boyer Chammard et al., 2017). The ZIKV enters the host cell by binding to virus-specific (AXL, DC-SIGN, Tyro3, and members of the TIM and TAM families of phosphatidylserine receptors) or general (Sulfated polysaccharides) receptors, and after endocytosis via clathrin-coated pits ZIKV replicates in the cell cytoplasm (Hamel et al., 2015; Nowakowski et al., 2016). After entry, Flavivirus (Dengue virus) has been reported to activate RIG-I, MDA-5, and TLR3 genes which recognize various pathogen-associated molecular patterns (PAMPs), that plays a role in innate antiviral immunity (Streblowb et al., 2015; Surasombatpattana et al., 2011). The virus derived PAMPs have been reported to stimulate the expression of transcription factor IRF7, which on binding to interferon-stimulated response element and lead to the expression of IFN-α and IFN-β and several other antiviral genes like OAS2, ISG15, and MX1 (Honda et al., 2005). ZIKV induces autophagy of the infected cell which serves a dual purpose of both activating viral replication as well as an anti-viral effect (Olagnier et al., 2016). Various pro-inflammatory cytokines (IFN-γ, IL-18, IL-6, TNF-α) and chemokines (CCL2, CCL5, CCL7, CXCL1, CXCL10) induced after viral infection are responsible for systemic inflammation caused by ZIKV. High levels of cytokines such as IP-10, IL-6, IL-8, VEGF, MCP-1 and G-CSF demonstrated in the amniotic fluid of ZIKV infected pregnant women can be responsible for brain malformation of the fetus (Ornelas et al., 2017). Activation of the cytokines and chemokines can lead to damage of the tissues. ZIKV can also activate apoptosis and glial cells (Wang et al., 2017b). A non-apoptotic form of cell death can also be induced by ZIKV which is caspase-independent and associated with the appearance of large cytoplasmic vacuoles derived from the endoplasmic reticulum (Monel et al., 2017). Dengue virus antibodies in humans are highly cross-reactive to ZIKV (Priyamvada et al., 2016). Due to the cross-reactivity of anti-flaviviral antibodies, ZIKV infection is facilitated by antibody-dependent enhancement (ADE) phenomenon (Dejnirattisai et al., 2016). Memory T cells elicited against Dengue virus can identify later ZIKV infection and prior Dengue infection influence the speed, quality, and magnitude of T cell response against ZIKV (Grifoni et al., 2017). An overview on Zika virus entry, interferon induction, and autophagy during viral infection is depicted in Figure 1. NS1 protein of ZIKV plays an important role in escaping from immune reponse through complement antagonism. The secreted hexamer molecule (sNS1) present in extracellular environment interacts with the complement system and leads to progeny virus survival (Conde et al., 2016). ZIKV NS4A and NS4B protein inhibits the activation of Akt-mTOR signaling pathway (Asif et al., 2017), which is an important step in regulation of development, proliferation, and inhibition of autophagy in neuronal proginator cells (Franke, 2008). Also, NS5 protein of ZIKV further inhibit interferon pathway through degrading STAT2 (Laurent-Rolle et al., 2014) in a UBR4 independent method (Morrison et al., 2013). 4 Advances in Diagnosis, Monitoring and Surveillance Since there is no pathognomonic clinical sign observed in ZIKV infection, the diagnosis of ZIKV can be made by isolation and identification of the virus, serological diagnosis by ELISA, genomic detection by RT-PCR and employing other advanced diagnostics (Singh et al., 2016; Singh et al., 2018a). Clinical samples for diagnosis include urine, serum, saliva, amniotic fluid, placenta and cerebrospinal fluid (Paz-Bailey et al., 2017). Histopathology and immunohistochemistry of placenta and umbilical cord can also be carried out to detect the presence of ZIKV (Landry & George, 2017). Isolation of ZIKV can be attempted from mosquitoes in newborn Swiss albino mice following various routes of inoculation namely intracerebral, subcutaneous and intraperitoneal (Marchette et al., 1969; Way et al., 1976). Cell culture can also be employed for isolation of virus using various cell line like rhesus monkey kidney cells (LLC- 192 MK2), Vero cells, and mosquito origin cells like C6/36 (A. albopictus origin), MOS61 or AP-61 cells (A. psuedoscutellaris origin) (Barzon et al., 2016; Waggoner & Pinsky, 2016). Tests like serum neutralization tests, complement fixation test, and Figure 1 Zika virus (ZIKV) entry, induction of interferons and autophagy: The paradigm during ZIKV infection
(1) a. AXL,  b. DC-SIGN,  c. Tyro3,  d.  Phosphatidylserine receptors (TIM / TAM families) or e. Sulfated polysaccharide receptors mediate ZIKV entry, (2) through clathrin-mediated endocytosis,              (3) ZIKV is released and replicated inside the cytoplasm, (4)Viral RNA is recognized by TLR3,                                      (5) Receptor complex activates phosphorylation of IRF3 and IRF7, (6) Phosphorylated IRF3 and IRF7 moves into the nucleus and binds to antiviral gene elements to express OAS2, ISG15 and MX1, (7) TLR3-ZIKV complex is degraded by                                 (8) Formation of autophagosome and (9) Fusion of autophagosome to lysosome, and      (10) Autophagy enhances ZIKV replication.       Figure 1 Zika virus (ZIKV) entry, induction of interferons and autophagy: The paradigm during ZIKV infection
(1) a. AXL,  b. DC-SIGN,  c. Tyro3,  d.  Phosphatidylserine receptors (TIM / TAM families) or e. Sulfated polysaccharide receptors mediate ZIKV entry, (2) through clathrin-mediated endocytosis,              (3) ZIKV is released and replicated inside the cytoplasm, (4)Viral RNA is recognized by TLR3,                                      (5) Receptor complex activates phosphorylation of IRF3 and IRF7, (6) Phosphorylated IRF3 and IRF7 moves into the nucleus and binds to antiviral gene elements to express OAS2, ISG15 and MX1, (7) TLR3-ZIKV complex is degraded by                                 (8) Formation of autophagosome and (9) Fusion of autophagosome to lysosome, and      (10) Autophagy enhances ZIKV replication.  
Haemagglutination inhibition tests are also used for ZIKV diagnosis (Fagbami, 1979; Monath et al., 1980). It is helpful to collect paired serum samples during the infection to know the status of infection which can be achieved by the use of IgG and IgM level through ELISA (Pyke et al., 2014). Detection of ZIKV IgG in the mother during pregnancy can help to find the relationship between ZIKV and congenital abnormalities associated with it (Sumita et al., 2016). Since, IgM does not cross placenta its detection in neonates serum is an important marker for neonatal infection while CSF act as a good indicator of neurologic infection (Cordeiro et al., 2016). Cross-reaction with other flavivirus family members is the major limitation with ELISA, hence plaque reduction neutralization test (PRNT) could be employed for detecting ZIKV to overcome this disadvantage (Granger et al., 2017). Other developments using recombinant non-structural protein 1 (NS1) of ZIKV based ELISA showed a lesser cross-reaction with dengue virus (Steinhagen et al., 2016). Lateral flow assay based on IgG/IgM antibodies of ZIKV has been marketed by Chembio Diagnostic Systems (Acharya et al., 2016). Multiplex microsphere immunoassay (MIA) is highly useful in diagnosis ZIKV infection by using asmall volume of thespecimen (Wong et al., 2017).  For detecting and quantifying neutralizing antibodies (virus-specific) plaque reduction neutralization test (PRNT) which is having higher specificity than ELISA is performed. The false positivity in theserological test can be eliminated by performing this assay along with ELISA (Charrel et al., 2016; Rabe et al., 2016). Several laboratory parameters viz., blood cell count; lactate dehydrogenase in the serum; rise in the concentration of markers (protein) are indicative of ZIKV infection. Assays based on PCR have been approved by the United States Food and Administration (USDA) (Plourde & Bloch, 2016).    RT-PCR has been used to detect ZIKV genome from saliva, blood, amniotic fluid and amniotic fluid (Faye et al., 2008; Hills et al., 2016; Rather et al., 2017). A recent report states that urine is a better choice for detection of ZIKV by RT-PCR (Bingham et al., 2016). Urine sample offers the ease of sample collection hence identification of ZIKV can be carried out easily with RT-PCR (St George et al., 2017). Altona Diagnostics markets RealStar Zika RT-PCR kit has been reported sensitive and specific in ZIKV detection (L'Huillier et al., 2017). SYBR based real-time RT-PCR has been developed that can detect up to 1 PFU/mL (Xu et al., 2016a). Real-time RT-PCR assay targeting 5'-untranslated conserved region (5'-UTR) was developed that can detect 5-10 ZIKV RNA copies/reaction, and this assay was very specific for ZIKV detection (Chan et al., 2017a). DNA sequencing NS5, NS3, and envelope gene can help to identify the strains of ZIKV (Fonseca et al., 2014; Grard et al., 2014; Tognarelli et al., 2015). Aptima assay (automated Panther system) for ZIKV detection is based on transcription-mediated amplification (TMA) of highly conserved sequences in NS2 and NS4/5 regions of ZIKV RNA in urine and serum samples. This test is rapid, and yields result in 3.5 hours performed in a single tube (Ren et al., 2017). Recently, multiplex microsphere immunoassay has been developed for ZIKV diagnosis. This test combinedly detects structural envelope proteins of flaviviruses and differentially non-structural proteins (NS1, NS5) thus making it more specific as well (Wong et al., 2017). The diagnosis of ZIKV is also essential to establish its treatment. Recently, a novel real-time multiplex PCR was developed to detect ZIKV and Chikungunya virus. The sensitivity of the developed assay was 0.5 and 1 PFU for Chikungunya virus and ZIKV, respectively (Liu et al., 2017). One-week post infection detection of IgM antibodies (virus-specific) along with neutralizing antibodies is possible. The virus can be detected by employing another rapid test like RNA-biosensors and RT-loop-mediated isothermal amplification (RT-LAMP) too (Tappe et al., 2014; Gourinat et al., 2015). The RT-LAMP assay has been reported to be highly specific and sensitive as compared to RT-PCR and real-time PCR (Wang et al., 2016a). Recently, another RT-LAMP was developed to detect ZIKV RNA from urine, and serum samples and the sensitivity was 10 times higher than Real-time RT- PCR, and it could detect 1.2 RNA copies/μl of the sample (Calvert et al., 2017). At later stages of viral infection, the real-time RT-PCR (rRT-PCR) technique has been found useful diagnostic tool for detecting viral load in urine as well as serum (Gourinat et al., 2015). RT-LAMP clubbed with lateral flow assay showed detection limit of even single copy number of ZIKV (Lee et al., 2016). Other recent diagnostic techniques like RT-isothermal recombinase polymerase amplification assay (RT-RPA) based on NS2A region of ZIKV was found to be specific and sensitive (Abd El Wahed et al., 2017). Advances in the diagnostics have led to the development of a RT-LAMP assay employing smartphone which is provided with chromaticity algorithm to scan the fluorescent light (Priye et al., 2017). Other techniques like nucleic acid sequence based amplification (NASBA) and RT- strand invasion based amplification (RT-SIBA) were developed for the early diagnosis of ZIKV (Pardee et al., 2016; Eboigbodin et al., 2016). Since, the adverse effects of ZIKV infection are devastating thus it is essential to develop a specific antibody-based nano-enabled electrochemical immunosensing system for rapid diagnosis and immediate care of patients (Kaushik et al., 2017). Both molecular, as well as serological tests, must be performed in the case of congenital infection. To detect viral antigen immunohistochemistry is recommended. Analysis of the cerebrospinal fluid; placenta or umbilical cord is mandatory to detect a congenital form of ZIKV infection (Martines et al., 2016b; Staples et al., 2016). The recent advent of the nerve electrophysiology has been attributed to play a critical role in ZIKV associated GBS diagnosis by verifying the     in newborns and Guillain Barre´Syndrome (GBS). Recently, this virus has posed a pandemic threat to global human population. The virus is transmitted primarily by a mosquito (Aedes aegypti), whereas, other routes of viral transmission includes monkey bite, coitus and body fluids such as blood, semen, and saliva which needs further confirmaton. Furthermore, implementation of appropriate mechanical, chemical and biological control methodologies can help to avoid mosquito bites and ultimately limit the viral spread. Another potential approach could be the development of ZIKV resistant mosquito strains such as Wolbachia-harboring mosquitoes which can reduce the incidences of ZIKV by lowering the availability of vectors for virus transmission. In-use ZIKV diagnosis relies on serological, molecular and few other advanced detection methods. Nowadays, worldwide researchers and biomedical and pharmaceutical authorities are paying attention on designing and developing effective vaccines, drugs, medicines, and therapeutics to counter this virus. The reasons behind the shift of virus virulence need to be explored by understanding the genetic and molecular characteristics, host-pathogen interactions and immunobiology of the virus, which would help to design effective antiviral drugs, prophylactics, and vaccines. The present review highlights various aspects of ZIKV and its clinical manifestations, progress, and advances in developing effective diagnostics, vaccines and drugs/therapeutics along with adopting suitable prevention and control strategies to tackle this deadly emerging disease.     
 
1 Introduction Zika virus (ZIKV) is a mosquito-borne virus of the Spondweni serocomplex, genus Flavivirus, family Flaviviridae. Owing to the climate changes like global warming, increasing population dynamics, fast globalization and travel, the human population is facing a rising emergence and outbreaks of mosquito-borne viruses such as Dengue virus, Japanese encephalitis virus, Chikungunya virus, West Nile virus and Zika virus (Chen & Wilson, 2010; Dhiman et al., 2010; Dhama et al., 2013a; Hubalek et al., 2014; Medlock & Leach, 2015; Parham et al., 2015; Carneiro & Travassos, 2016; Gautret & Simon, 2016; Musso et al., 2017). Soon after the deadly outbreaks of Ebola virus in Western Africa, the most recent emerging virus threatening the global human population is the Zika virus (ZIKV), declared as an emergency situation (Public Health Emergency of International Concern) on February 1, 2016 by the World Health Organization (WHO) for its quick spread, affecting large human population in different countries with pandemic threats (Dhama et al., 2015; Chang et al., 2016; Chen & Hamer, 2016; ECDC, 2016; Gulland, 2016; Higgs, 2016; Singh et al., 2016; Krauer et al., 2017; Singh et al., 2017). Being remained harmless for six decades (first reported in 1947), the sudden emergence of ZIKV with higher virulence, speedy spread and inducing severe clinical manifestations  (especially associated with the cases of microcephaly, unexpected fetal anomalies during gestation period as placental insufficiency, hampered fetal growth, impaired Central Nervous System (CNS) of fetus, fetal death and other neurologic disorders as well as a cluster of Guillain-Barré syndrome) along with little knowledge on suitable prevention and therapeutic measures created  massive threats for the human health (Zanluca & dos Santos, 2016; Cao-Lormeau et al., 2016; Duhaime-Ross, 2016; Gatherer & Kohl, 2016; Petersen et al., 2016a; Samarasekera & Triunfol, 2016; Singh et al., 2016; Molko et al., 2017). Earlier reports on ZIKV were limited to Africa and Asia, while now it has worldwide presence  (Hayes, 2009; Heang et al., 2012; Grard et al., 2014; Brown, 2015; Chang et al., 2016; Fauci & Morens, 2016; Lucey & Gostin, 2016; Vest, 2017; Zhang et al., 2017). It is noteworthy that since the first report of the disease from the African continent, only 14 cases in humans had been reported before its first large epidemic in the year 2007 on the Island of Yap (Duffy et al., 2009; Marano et al., 2016; Reveiz et al., 2017), followed by the largest outbreak in French Polynesia (October 2013 to April 2014) (Cao-Lormeau et al., 2014; Reveiz et al., 2017). After that in 2015, ZIKV expanded its horizons and also emerged in Vanuatu, Fiji, Solomon and Samoa (ECDC, 2016; Musso et al., 2017). The sequences of ZIKV from Brazil and Suriname upon phylogenetic analysis have revealed their Asian origin. Pacific Islands have been found to be the region from where the virus has entered into the Latin America during some sports events during the year 2014 (Waggoner & Pinsky, 2016). Since 2015, 76 countries and territories around the world have reported ZIKV transmission, predominantly the South Americas, where Brazil was the most affected country with more than 1.4 million affected cases of Zika alone (Jamil et al., 2016; WHO, 2016; Krauer et al., 2017). With the noticed severity of the recent ZIKV outbreak, several countries geared up for epidemiological investigations to know out further disease spread and the serious health concerns (Van Kerkhove et al., 2016; Reveiz et al., 2017). Notably, in Asia the incidence of ZIKV infection is relatively low when viral detection was done by employing reverse transcription-polymerase chain reaction (RT-PCR). Viremia induced by ZIKV infection is relatively low; thereby decreasing the probability of detecting the virus in blood samples in acute cases. This has led the researchers to make the interpretation of the results by taking extra caution (Shan et al., 2016; Duong et al., 2017). In Asia and America, the strain difference (as far as the infectivity is concerned) could have been responsible for the fundamental difference in the epidemiology as well as the burden of ZIKV infection. The Asian lineage of ZIKV is responsible for most of the recent outbreaks in Asia as well as America (Haddow et al., 2012). There is a requirement of using viruses generated de novo from diverse geographical as well as clinical sources to describe the pattern of ZIKV infection in Asia and America (Setoh et al., 2017). ZIKV infection impacts any nation’s economy adversely besides being a toll on human health (Jamil et al., 2016). International agencies including WHO, Pan American Health Organization (PAHO), the Consortium for the Standardization of Influenza Seroepidemiology (CONSISE), Institute Pasteur, the International Severe Acute Respiratory and Emerging Infection Consortium (ISARIC), Fiocruz, and others have taken up collective steps and are coordinating to synchronize the ongoing research on this virus and complications associated with it (Reveiz et al., 2017). The emergence of this virus with increased virulence and potential of rapid spread posed pandemic threats to global human population (Chitti et al., 2016; Petersen et al., 2016a). Of note, till 2015 nearly 100 research articles were available in Pubmed and Pubmed Central, while now in January 2018, there are more than 3500 research articles on different aspects of the virus and the disease it causes, which altogether reflects the high concerns and wide attention this pathogen has gained in just past 20 months. Researchers in many countries are trying hard to counter ZIKV and the Zika fever by carrying out detailed virological, pathological and molecular studies, developing rapid diagnostics, finding out potential drugs, prophylactics, vaccines as well as adopting appropriate prevention and control measures (Keasey et al., 2017; Munjal et al., 2017a; Rather et al., 2017; Shankar et al., 2017; Sharma & Lal, 2017; Singh et al., 2018a). Rapid diagnostics are now existing for detecting ZIKV infection, many drug and vaccine candidates have also been identified, but still any effective / approved treatment or vaccine is practically lacking against this virus (Dyer, 2016a; Fernandez & Diamond, 2017; Munjal et al., 2017a; Munjal et al., 2017b; Passi et al., 2017; Singh et al., 2018b). Appropriate prevention and control strategies include limiting the spread/bite of the vector mosquitoes by checking their population expansion, safe precautions during sexual intercourse and blood transfusions, avoiding travel to Zika endemic countries and surveillance and monitoring are the only feasible options to keep ZIKV infection under limits (Rather et al., 2017; von Seidlein et al., 2017). Here, we present a compilation on the Zika virus/ Zika fever, covering different aspects of the virus and the disease it causes, and describes the ongoing progress and advances being made in the field of designing and developing diagnostics, vaccines, drugs along with prevention and control measures to be adapted to combat this viral pathogen of high public health concerns. 2 Etiology, Epidemiology and Transmission / Spread Zika virus belongs to the genus Flavivirus, family Flaviviridae (Gold & Josephson, 2016; Paz-Bailey et al., 2017; Uncini et al., 2017). It is a ss-RNA virus of 11 kb genome size with single open reading frame (ORF) encoding a polyprotein with 03 structural proteins (capsid, pre-membrane, envelope) and 07 non-structural (NS) proteins (NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5) (Musso & Gubler, 2016; Kumar et al., 2017). Among these NS5 has multiple roles and protein E plays a pivotal role in binding and fusion of virus as an important basic step for the establishment of ZIKV infection (Dasti, 2016). Various antigenic epitopes are present in the third domain of the E protein and thus essentially targeted as far as the serological tests are concerned. It is also an important target for vaccine development. It is assumed that the global spread of the ZIKV is due to loss of the E protein glycosylation site: N154. This helps the virus to adapt to a broader range of vector mosquitoes (Faye et al., 2014; Chan et al., 2016). From Zika forests of Uganda, the first ZIKV isolation was done in 1947 from rhesus monkey. ZIKV strain 766 particle size ranged between 30 and 45 nm (Dick, 1952). Recently, the viral structure has been elucidated using cryo-electron microscopy which could assist in designing of antiviral drugs and vaccines (Sirohi et al., 2016). Consequent to early 2015 Zika disease outbreak in Brazil, the Zika fever spread rapidly across South and Central America, North America, Latin America, Mexico, Colombia, French Polynesia, Cook Islands, Haiti, Panama, Trinidad and Tobago, Samoa, and American Samoa (Campos et al., 2015; Zanluca & dos Santos,  2016; Rodriguez-Morales, 2015; Nereida, 2015; Summers et al., 2015; Chang et al., 2016; Chen & Hamer, 2016; ECDC, 2016; Hennessey et al., 2016; Higgs, 2016; Pastula et al., 2016; Petersen et al., 2016b). As per the updates of the Pan American Health Organization (PAHO) vector-borne ZIKV infections have been confirmed in 48 countries and sexual transmission in 5 countries regionally. Imported cases have been recorded in people visiting areas where the epidemic infection is prevalent and subsequently returning to Europe, Asia-Pacific and the Americas (Pan American Health Organization/ World Health Organization, 2016; Goorhuis et al., 2016). ZIKV autochthonous (locally transmitted) cases have been reported from many countries or territories (Zanluca & dos Santos, 2016; Calvet et al. 2016a). The year 2016 also marked the report of first ZIKV infection in Taiwan and PCR followed by further analysis showed that it was Asian lineage virus having close proximity with Cambodian virus (Huang et al., 2016). The ZIKV transmission mainly takes place via mosquito (Aedes sp.-Aedes aegypti and A. albopictus) in an anthroponotic manner (human-to-vector-to-human), perinatal transmission, sexual intercourse, and blood transfusion (Besnard et al., 2014; Musso et al., 2014, Musso et al., 2015; Franchini & Velati, 2015; Carneiro & Travassos, 2016; Marcondes & Ximenes, 2016; Shoaib et al., 2016). Aedes aegypti was originated from A. aegyptiformosus, a zoophilic tree hole mosquito from African countries while Aedes albopictus is a zoophilic forest mosquito, belongs mainly to Asian countries. A. aegypti mosquito is a most important vector in spreading of ZIKV (Kaddumukasa et al., 2014; Carneiro & Travassos, 2016). Apart from Aedes mosquito vector other vectors like Culex spp. have also been reported. Studies suggested that around ten species of genus Aedes, Culex perfuscus, Anopheles coustani and Mansonia uniformis have also been reported as a vector for ZIKV (Ayres, 2016). Culex quinquefasciatus was also suspected to play a role in ZIKV transmission as ZIKV RNA was detected from these mosquitoes from North Eastern Brazil and later from China. Thus there may an array of vectors involved in ZIKV transmission which has to be elucidated swiftly to prevent further transmission (van den Hurk et al., 2017). ZIKV transmission from infected pregnant woman occurs through transplacental route and thus affects the brain of the fetus (Roa, 2016; Torjesen, 2016). The mosquito A. aegypti and A. albopictus mainly transmit ZIKV in urban areas while in niche ecotypes A. hensili and A. polynesiensis are found to be the vectors. Even though the dominance of the vector A. albopictus is mainly in Asia but the vector has got potential to spread ZIKV infection globally because of its invasive nature and broader distribution geographically (Grard et al., 2014; Ledermann et al., 2014; Weaver et al., 2016). In the tropical and subtropical climate, the rainy season is conducive for the Aedes mosquitoes allowing them to breed and most of the people suffer from mosquito bite during the day (from dawn to dusk) (Ibrahim, 2016). Brain lesions in the non-human primate fetus have developed due to subcutaneous inoculation of ZIKV. Susceptibility in neonatal pigs is also on the higher side (Waldorf et al., 2016; Darbellay et al., 2017). ZIKV transmission from mother to fetus is high in rhesus macaques when the Asian lineage of the virus is inoculated subcutaneously. Such study is also indicative of the possibilities of mother-fetus transmission in case of human. Studies suggested the transfer of ZIKV from a pregnant mother to fetus and from lactating mother to newborn as well (Jamil et al., 2016). Following inoculation through rectum or vagina in macaques (adult) the rate of infection was found high (Haddow et al., 2017; Nguyen et al., 2017). Some of the cases of asymptomatic ZIKV infections have been found to spread through transfusion of blood during acute viremia. The role of saliva to spread the viral infection is still under study (Oster et al., 2016; Petersen et al., 2016a). Reports are suggestive of ZIKV transmission through infected monkey bite also (Leung et al., 2015). The isolation of ZIKV from semen is an indication of transmission through coitus (Musso et al., 2015). The first confirmatory case of sexually transmitted ZIKVinfection is from France in 2016 in a woman where her husband acquired the infection during his recent visit to Brazil (Elgot et al., 2016). Recently, a study was conducted to know the site of ZIKV replication in the genital organs which aid in the sexual transmission of the virus. Stromal mesenchymal stem cells and epithelial cells of the human prostate were studied with 3 different ZIKV isolates, and the result showed that ZIKV infection is higher in stromal cells than epithelial cells of the prostate. Thus, findings confirmed that replication of ZIKV can occur in prostate and can spread further through semen (Spencer et al., 2017). RT-in situ PCR of the sperm showed that ZIKV was confined to the midpiece of the spermatozoa and hence it was speculated that receptors for ZIKV might be present in the midpiece of the sperm (Bagasra et al., 2017). Present knowledge on ZIKV shows that transmission can occur from male to a female partner, and male can shed ZIKV through semen, but there is no evidence regarding the shedding of virus from the female genital tract. Further research may pave way to find the exact mechanism of transmission and the receptors responsible for ZIKV infection (Epelboin et al., 2017). 3 Zika Fever - Clinical picture, Pathology and Pathogenesis   Infection with ZIKV were earlier reported to be asymptomatic in 50 to 80% cases, and the symptoms of  illness being mild and self-limiting, a febrile disease of 3-7 days with no mortality and lesser hospitalizations (Duffy et al., 2009; CDC, 2016; Reveiz et al., 2017). The symptoms are more or less similar to dengue infection expressing fever, anxiety, joint pain, body ache, indicating no special reason to consider Zika virus as the primary cause of illness unless until any neurological or auto-immune disorder develop (Jamil et al., 2016). However, the most recent outbreaks of ZIKV revealed microcephaly in newborn babies (abnormal small heads and brains), and congenital neurological malformations and disabilities in babies, while in adults it mainly presented Guillain-Barré syndrome (GBS); apart from these visual impairment cases have also been recorded (de Araújo et al., 2016; Brasil et al., 2016; Cauchemez et al., 2016; Cao-Lormeau et al., 2016; Dos Santos et al., 2016; Cuevas et al., 2016;  Martines et al., 2016a; Mlakar et al., 2016; Schuler-Faccini et al., 2016; Shuaib et al., 2016; Rodriguez- Morales, 2016; Reveiz et al., 2017; Ventura et al., 2017). It is reported that ZIKV may not cause congenital brain abnormalities or GBS by itself and some unknown cofactors might be associated along with this virus to cause such serious complications (Reveiz et al., 2017). GBS, an immune-mediated ascending flaccid paralysis, is implicated within a month of ZIKV infection (Willison et al., 2016; Krauer et al., 2017). In patients suffering from GBS, respiratory muscles become gradually weak which led to difficulty in breathing and lead to death or lifetime impaired functions of affected muscles (Jamil et al., 2016). Recently, transient myocarditis has also been found associated with ZIKV infection, and researchers advise electrocardiogram and troponin tests if cardiac signs are suspected of ZIKV infection (Aletti et al., 2017). It is also speculated that hearing loss may also be associated with ZIKV. A study among 104 infants supposed to be infected with ZIKV in Brazil showed 9% cases affected by hearing loss. Further detailed studies are warranted to unearth the relationship between ZIKV and hearing loss (Mittal et al., 2017). The possibility of vertical transmission of ZIKV is documented in the literature. Calvet et al. (2016b) reported the presence of the ZIKV genome in the amniotic fluid along with the presence of anti-ZIKV IgM antibodies. This implies the ability of the virus to reach the fetal environment thereby suggestive of the possibility of vertical transmission. The presence of ZIKV in the brain tissues of the fetus is also suggestive of vertical transmission (Mlakar et al., 2016). But as IgM cannot cross the placenta, its presence in the fetus is an indication of vertical transmission of ZIKV to cause fetal infection. The GBS is reported to have an incidence of 0.24 per 1000 cases of ZIKV infections (Cao-Lormeau et al., 2016; Uncini et al., 2017). A recent analysis of Zika-associated GBS from seven countries (Brazil, the Dominican Republic, Colombia, Honduras, El Salvador, Venezuela and Suriname) documented the rapid surge of GBS (2.0–9.8 times higher) as compared to the pre-Zika era (Parra et al., 2016; Dos Santos et al., 2016; Uncini et al., 2017). This remarkable upsurge in GBS cases could inundate hospital and intensive care resources with well-equipped healthcare services (Uncini et al., 2017). It is noteworthy that there is involvement of the Asian/ Pacific lineage of the virus in case of microcephaly cases in Latin America. The rate of restoring the walking capacity in patients with GBS at the post-infection stage is found to be 56% (Cao-Lormeau et al., 2016; Teruya & Versalovic, 2017). The other sequel of the disease is the visual impairment in congenital cases which is mainly due to chorioretinal atrophy, mottling of retinal pigment, retinal vasculature and optic nerve abnormalities (Benzekri et al., 2017; Ventura et al., 2017). The congenital Zika syndrome is also associated with arthrogryposis, brain parenchymal atrophy, intracranial calcification, ventriculomegaly, hypoplasia of corpus callosum, brain stem and cerebellum (Mehrjardi et al., 2017; Sousa et al., 2017). The recent ZIKV epidemic in Latin America has also been implicated with observations of severe fetal abnormalities such as spontaneous abortion, stillbirth, microcephaly, hydranencephaly, and placental insufficiency, which might lead to limiting the intrauterine growth of fetuses (Ferguson et al., 2016; Musso & Gubler, 2016; Kumar et al., 2017). The brain development of the fetus is inhibited by ZIKV which is evident from the neural stem cell death in case of this viral infection. Studies have been conducted in mice time-to-time wherein intraperitoneal injection of the infected materials has resulted in the development of neuropathological lesions involving the brain. There is very less fatality as far as the acute infection is concerned, but certainly, it has been reported in Colombian children suffering from sickle cell disease. There has been no any report of haemorrhagic symptoms in patients infected with ZIKV (Koenig et al., 2016; Tian et al., 2016). There are still ongoing researches for understanding the mechanism of development of microcephaly along with other neurological disorders in case of ZIKV infection (Araujo et al., 2016; Faizan et al., 2016). There may be involvement of the neural progenitor cells, and most importantly direct suppressive effects of the NS4A and NS4B proteins of the virus on the process of neurogenesis is evident. Cellular death is the outcome of stem cell infection of brain thereby reducing the genesis of neurons further ultimately resulting in retarded brain growth (Li et al., 2016; Nayak et al., 2016). It has been found that the subcutaneous inoculation of ZIKV in guinea pig can result in viral invasion and replication in the brain. It has also been shown previously that in mice viz., A129 and AG129 (that have a defect in interferon responses) the concentration of the virus in the brain, as well as spinal cord, is high. In primates (non-human) the viral RNA has been demonstrated after subcutaneous injection of the infected material. However, there is the scope of further researches to determine and to increase our understanding regarding the exact mechanisms involved in microcephaly mediated by ZIKV (Lazear et al., 2016; Kumar et al., 2017). Ancestral analysis of the ZIKV showed that a single point mutation at 139 position of viral polyprotein from serine to asparagine (S139N) led to the increased infection of both mouse and human neural progenitor cells. This alteration also led to higher cases of microcephaly and mortality in infant mice. This study also revealed that this mutational change occurred before French Polynesia ZIKV outbreak in 2013 and it remained constant throughout the outbreak (Yuan et al., 2017). Anomalies of the eye may develop due to tropism/ affinity of the virus towards the developing ocular cells (Li et al., 2016). It is important to note that due to interference with the mitotic function ZIKV can cause microcephaly (Bullerdiek et al., 2016). Immune-mediated damage of the fetal brain cells may occur if the virus crosses the placental barrier (Wang et al., 2017a). The neural progenitor cells can be infected by ZIKV thereby producing viremia in the infected mother. This further leads to transfer of the virus maternally (from blood to the fetus) resulting in teratogenic effect (viz., neurological lesions). There is a decrease in posterior white matter along with white matter gliosis (bilateral) in the fetal brain infected with ZIKV in non-human primates. Apoptotic and mitotic figures may appear (Tang et al., 2016; Waldorf et al., 2016). Atrophy of the brain of the fetus along with coarse calcification in the white matter of frontal lobes and reduction of circumference of the brain s revealed by ultrasonography analysis (Melo et al., 2016). Recently, it was found that ZIKV also leads to testicular atrophy upon infection in mouse thereby suggesting serious complications of non-vector transmission of this virus along with reproductive deficiency in males (Uraki et al., 2017). Prolonged viremia is a feature in pregnant women due to replication of the virus in fetus and placenta (Suy et al., 2016). Thrombocytopenia and hemorrhagic signs have been most recently reported (Boyer Chammard et al., 2017). The ZIKV enters the host cell by binding to virus-specific (AXL, DC-SIGN, Tyro3, and members of the TIM and TAM families of phosphatidylserine receptors) or general (Sulfated polysaccharides) receptors, and after endocytosis via clathrin-coated pits ZIKV replicates in the cell cytoplasm (Hamel et al., 2015; Nowakowski et al., 2016). After entry, Flavivirus (Dengue virus) has been reported to activate RIG-I, MDA-5, and TLR3 genes which recognize various pathogen-associated molecular patterns (PAMPs), that plays a role in innate antiviral immunity (Streblowb et al., 2015; Surasombatpattana et al., 2011). The virus derived PAMPs have been reported to stimulate the expression of transcription factor IRF7, which on binding to interferon-stimulated response element and lead to the expression of IFN-α and IFN-β and several other antiviral genes like OAS2, ISG15, and MX1 (Honda et al., 2005). ZIKV induces autophagy of the infected cell which serves a dual purpose of both activating viral replication as well as an anti-viral effect (Olagnier et al., 2016). Various pro-inflammatory cytokines (IFN-γ, IL-18, IL-6, TNF-α) and chemokines (CCL2, CCL5, CCL7, CXCL1, CXCL10) induced after viral infection are responsible for systemic inflammation caused by ZIKV. High levels of cytokines such as IP-10, IL-6, IL-8, VEGF, MCP-1 and G-CSF demonstrated in the amniotic fluid of ZIKV infected pregnant women can be responsible for brain malformation of the fetus (Ornelas et al., 2017). Activation of the cytokines and chemokines can lead to damage of the tissues. ZIKV can also activate apoptosis and glial cells (Wang et al., 2017b). A non-apoptotic form of cell death can also be induced by ZIKV which is caspase-independent and associated with the appearance of large cytoplasmic vacuoles derived from the endoplasmic reticulum (Monel et al., 2017). Dengue virus antibodies in humans are highly cross-reactive to ZIKV (Priyamvada et al., 2016). Due to the cross-reactivity of anti-flaviviral antibodies, ZIKV infection is facilitated by antibody-dependent enhancement (ADE) phenomenon (Dejnirattisai et al., 2016). Memory T cells elicited against Dengue virus can identify later ZIKV infection and prior Dengue infection influence the speed, quality, and magnitude of T cell response against ZIKV (Grifoni et al., 2017). An overview on Zika virus entry, interferon induction, and autophagy during viral infection is depicted in Figure 1. NS1 protein of ZIKV plays an important role in escaping from immune reponse through complement antagonism. The secreted hexamer molecule (sNS1) present in extracellular environment interacts with the complement system and leads to progeny virus survival (Conde et al., 2016). ZIKV NS4A and NS4B protein inhibits the activation of Akt-mTOR signaling pathway (Asif et al., 2017), which is an important step in regulation of development, proliferation, and inhibition of autophagy in neuronal proginator cells (Franke, 2008). Also, NS5 protein of ZIKV further inhibit interferon pathway through degrading STAT2 (Laurent-Rolle et al., 2014) in a UBR4 independent method (Morrison et al., 2013). 4 Advances in Diagnosis, Monitoring and Surveillance Since there is no pathognomonic clinical sign observed in ZIKV infection, the diagnosis of ZIKV can be made by isolation and identification of the virus, serological diagnosis by ELISA, genomic detection by RT-PCR and employing other advanced diagnostics (Singh et al., 2016; Singh et al., 2018a). Clinical samples for diagnosis include urine, serum, saliva, amniotic fluid, placenta and cerebrospinal fluid (Paz-Bailey et al., 2017). Histopathology and immunohistochemistry of placenta and umbilical cord can also be carried out to detect the presence of ZIKV (Landry & George, 2017). Isolation of ZIKV can be attempted from mosquitoes in newborn Swiss albino mice following various routes of inoculation namely intracerebral, subcutaneous and intraperitoneal (Marchette et al., 1969; Way et al., 1976). Cell culture can also be employed for isolation of virus using various cell line like rhesus monkey kidney cells (LLC- 192 MK2), Vero cells, and mosquito origin cells like C6/36 (A. albopictus origin), MOS61 or AP-61 cells (A. psuedoscutellaris origin) (Barzon et al., 2016; Waggoner & Pinsky, 2016). Tests like serum neutralization tests, complement fixation test, and Figure 1 Zika virus (ZIKV) entry, induction of interferons and autophagy: The paradigm during ZIKV infection
(1) a. AXL,  b. DC-SIGN,  c. Tyro3,  d.  Phosphatidylserine receptors (TIM / TAM families) or e. Sulfated polysaccharide receptors mediate ZIKV entry, (2) through clathrin-mediated endocytosis,              (3) ZIKV is released and replicated inside the cytoplasm, (4)Viral RNA is recognized by TLR3,                                      (5) Receptor complex activates phosphorylation of IRF3 and IRF7, (6) Phosphorylated IRF3 and IRF7 moves into the nucleus and binds to antiviral gene elements to express OAS2, ISG15 and MX1, (7) TLR3-ZIKV complex is degraded by                                 (8) Formation of autophagosome and (9) Fusion of autophagosome to lysosome, and      (10) Autophagy enhances ZIKV replication.       Figure 1 Zika virus (ZIKV) entry, induction of interferons and autophagy: The paradigm during ZIKV infection
(1) a. AXL,  b. DC-SIGN,  c. Tyro3,  d.  Phosphatidylserine receptors (TIM / TAM families) or e. Sulfated polysaccharide receptors mediate ZIKV entry, (2) through clathrin-mediated endocytosis,              (3) ZIKV is released and replicated inside the cytoplasm, (4)Viral RNA is recognized by TLR3,                                      (5) Receptor complex activates phosphorylation of IRF3 and IRF7, (6) Phosphorylated IRF3 and IRF7 moves into the nucleus and binds to antiviral gene elements to express OAS2, ISG15 and MX1, (7) TLR3-ZIKV complex is degraded by                                 (8) Formation of autophagosome and (9) Fusion of autophagosome to lysosome, and      (10) Autophagy enhances ZIKV replication.  
Haemagglutination inhibition tests are also used for ZIKV diagnosis (Fagbami, 1979; Monath et al., 1980). It is helpful to collect paired serum samples during the infection to know the status of infection which can be achieved by the use of IgG and IgM level through ELISA (Pyke et al., 2014). Detection of ZIKV IgG in the mother during pregnancy can help to find the relationship between ZIKV and congenital abnormalities associated with it (Sumita et al., 2016). Since, IgM does not cross placenta its detection in neonates serum is an important marker for neonatal infection while CSF act as a good indicator of neurologic infection (Cordeiro et al., 2016). Cross-reaction with other flavivirus family members is the major limitation with ELISA, hence plaque reduction neutralization test (PRNT) could be employed for detecting ZIKV to overcome this disadvantage (Granger et al., 2017). Other developments using recombinant non-structural protein 1 (NS1) of ZIKV based ELISA showed a lesser cross-reaction with dengue virus (Steinhagen et al., 2016). Lateral flow assay based on IgG/IgM antibodies of ZIKV has been marketed by Chembio Diagnostic Systems (Acharya et al., 2016). Multiplex microsphere immunoassay (MIA) is highly useful in diagnosis ZIKV infection by using asmall volume of thespecimen (Wong et al., 2017).  For detecting and quantifying neutralizing antibodies (virus-specific) plaque reduction neutralization test (PRNT) which is having higher specificity than ELISA is performed. The false positivity in theserological test can be eliminated by performing this assay along with ELISA (Charrel et al., 2016; Rabe et al., 2016). Several laboratory parameters viz., blood cell count; lactate dehydrogenase in the serum; rise in the concentration of markers (protein) are indicative of ZIKV infection. Assays based on PCR have been approved by the United States Food and Administration (USDA) (Plourde & Bloch, 2016).    RT-PCR has been used to detect ZIKV genome from saliva, blood, amniotic fluid and amniotic fluid (Faye et al., 2008; Hills et al., 2016; Rather et al., 2017). A recent report states that urine is a better choice for detection of ZIKV by RT-PCR (Bingham et al., 2016). Urine sample offers the ease of sample collection hence identification of ZIKV can be carried out easily with RT-PCR (St George et al., 2017). Altona Diagnostics markets RealStar Zika RT-PCR kit has been reported sensitive and specific in ZIKV detection (L'Huillier et al., 2017). SYBR based real-time RT-PCR has been developed that can detect up to 1 PFU/mL (Xu et al., 2016a). Real-time RT-PCR assay targeting 5'-untranslated conserved region (5'-UTR) was developed that can detect 5-10 ZIKV RNA copies/reaction, and this assay was very specific for ZIKV detection (Chan et al., 2017a). DNA sequencing NS5, NS3, and envelope gene can help to identify the strains of ZIKV (Fonseca et al., 2014; Grard et al., 2014; Tognarelli et al., 2015). Aptima assay (automated Panther system) for ZIKV detection is based on transcription-mediated amplification (TMA) of highly conserved sequences in NS2 and NS4/5 regions of ZIKV RNA in urine and serum samples. This test is rapid, and yields result in 3.5 hours performed in a single tube (Ren et al., 2017). Recently, multiplex microsphere immunoassay has been developed for ZIKV diagnosis. This test combinedly detects structural envelope proteins of flaviviruses and differentially non-structural proteins (NS1, NS5) thus making it more specific as well (Wong et al., 2017). The diagnosis of ZIKV is also essential to establish its treatment. Recently, a novel real-time multiplex PCR was developed to detect ZIKV and Chikungunya virus. The sensitivity of the developed assay was 0.5 and 1 PFU for Chikungunya virus and ZIKV, respectively (Liu et al., 2017). One-week post infection detection of IgM antibodies (virus-specific) along with neutralizing antibodies is possible. The virus can be detected by employing another rapid test like RNA-biosensors and RT-loop-mediated isothermal amplification (RT-LAMP) too (Tappe et al., 2014; Gourinat et al., 2015). The RT-LAMP assay has been reported to be highly specific and sensitive as compared to RT-PCR and real-time PCR (Wang et al., 2016a). Recently, another RT-LAMP was developed to detect ZIKV RNA from urine, and serum samples and the sensitivity was 10 times higher than Real-time RT- PCR, and it could detect 1.2 RNA copies/μl of the sample (Calvert et al., 2017). At later stages of viral infection, the real-time RT-PCR (rRT-PCR) technique has been found useful diagnostic tool for detecting viral load in urine as well as serum (Gourinat et al., 2015). RT-LAMP clubbed with lateral flow assay showed detection limit of even single copy number of ZIKV (Lee et al., 2016). Other recent diagnostic techniques like RT-isothermal recombinase polymerase amplification assay (RT-RPA) based on NS2A region of ZIKV was found to be specific and sensitive (Abd El Wahed et al., 2017). Advances in the diagnostics have led to the development of a RT-LAMP assay employing smartphone which is provided with chromaticity algorithm to scan the fluorescent light (Priye et al., 2017). Other techniques like nucleic acid sequence based amplification (NASBA) and RT- strand invasion based amplification (RT-SIBA) were developed for the early diagnosis of ZIKV (Pardee et al., 2016; Eboigbodin et al., 2016). Since, the adverse effects of ZIKV infection are devastating thus it is essential to develop a specific antibody-based nano-enabled electrochemical immunosensing system for rapid diagnosis and immediate care of patients (Kaushik et al., 2017). Both molecular, as well as serological tests, must be performed in the case of congenital infection. To detect viral antigen immunohistochemistry is recommended. Analysis of the cerebrospinal fluid; placenta or umbilical cord is mandatory to detect a congenital form of ZIKV infection (Martines et al., 2016b; Staples et al., 2016). The recent advent of the nerve electrophysiology has been attributed to play a critical role in ZIKV associated GBS diagnosis by verifying the
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