The Zika Virus (ZIKV) is a flavivirus which was originally discovered in Uganda in the Zika Forest in 1947. It was a pathogen which had been highly neglected until an unexpected outbreak in 2007 caused a widespread epidemic mainly in Micronesia and eventually spread to the South Pacific and the Americas. Most notably, in 2015, a large outbreak occurred in Brazil where around 30,000 cases of infection were reported (Weaver et al., 2016). It is thought that the ZIKV was introduced to French Polynesia via Asia which, in turn, created a rapid spread the South Pacific. The complete structure is known to be quite similar to that of flaviviruses, especially that of the dengue virus, as well as West Nile and Japanese encephalitis viruses (Gong et al., 2016). The virus was examined under certain temperatures and showed to be thermally stable under 40°C, contrasting to the dengue virus. Due to this, there is still much to be discovered about the structure and functions of the ZIKV (Kostyuchenko et al., 2016). It has been suggested that there is a correlation between the ZIKV and disorders such as genital microcephaly which occurs in foetuses as well as Guillain-Barré syndrome. In recent years, there has been more of an emphasis on research in order to discover more about the virus with the hopes of controlling it and creating a suitable vaccine with assistance from other flaviviruses (Barzon et al., 2016).
The initial modern outbreak of the ZIKV in 2007 occurred in Gabon in Libreville in 2007 where both humans and mosquitos from the same are tested positive for ZIKV after being screened with RT-PCR. As well as that, the island of Yap in Micronesia experienced an outbreak where the infected were affected by intense fevers an in some cases, conjunctivitis. Around 73% of the population of the island were infected and it is thought that the probable causing vector of this was Aedes (Stegomyia) hensilli. After this in 2013, French Polynesia was affected with ZIKV and it is thought that the virus was transported from somewhere in Southeast Asia. As this outbreak was occurring, the transmission of the virus through blood banks and human transferal via urine, saliva and semen were all determined as potential risks of being infected by the virus. Most notably, a large outbreak of ZIKV occurred in Brazil in 2015 and it is thought that the virus entered the country during the World Sprint Championships in 2014 as several Pacific countries were involved in the games where ZIKV outbreaks initially occurred. It is estimated that around 1.5 million positive cases have been reported in Brazil since the beginning of the outbreak (Weaver et al., 2016).
There is much yet to be discovered about ZIKV, however its similarities to the dengue virus (DENV) gives the ability to comprehend its structures and properties. The growth of ZIKV is mostly associated with the transmission of the A. aegypti vector. It is the current understanding that the origin of ZIKV in Africa separated into two lineages. The first of these being the African lineage where the virus is fuelled by the prototype MR766 strain, isolated in 1947, and the second being the Asian cluster which is driven by prototype P6-740 strain that was first discovered in Malaysia in 1966. In the past 2-3 years, a branch of the second lineage has arisen in the Americas with its distinct characteristic being the ability for the virus to replicate rapidly especially among inferior populations where their immune systems would be regarded as naïve. Each lineage shows differences in 90% of the nucleotide (Figure 1) sequences as well as around 59 amino acids (Gong et al., 2016)).
ZIKV contains an icosahedral envelope, similar to flaviviruses, which has an RNA genome that is single stranded and is around 10.7 kb in length. Several replications of the capsid protein is entailed in this. There are three structural proteins that are created by the processing of around 3500 amino acids, which are encoded by the genome, by cellular and viral proteases. The three proteins are the membrane protein (prM), the capsid (C) and the envelope (E). These proteins create the particle of the virus and initiate the attachment, entry and encapsidation of the virus. As well as that, replication of the genome is assisted by seven proteins that are non-structural and they also aid in the processing of polyproteins. Replication of the virus is located in the cytoplasm of cells that are infected. The viral particle is uncoated and the viral genome is released after endocytosis occurs and initiates viral fusion with endosomal membranes. This endocytosis is regulated by a cellular receptor that is still yet to be discovered. Translation of the viral genome into a single polyprotein then occurs at the endoplasmic reticulum (ER). Matured viral proteins are created after cellular proteases and NS2B-NS3 protease process the polyprotein. NS proteins as well as NS5 RNA-dependent RNA polymerase are components of a replication complex that assists in catalysing the synthesis of the genome of the virus, along with the ER. The C protein then allows for the packaging of the newly created genomes which then attain the envelope, during which time they bud from the ER. The immature virions are then carried via secretory pathways. Exocytosis then occurs which releases the mature virions that are created after E protein undergoes glycosylation and furin protease cleaves prM. It was shown that there are similarities in the structure of ZIKV with the structures of DENV and WNV due to observations using cryoelectron microscopy. There are 180 replications of prM and E proteins that are contained in the spikey surface of immature ZIKV virions. These are heterodimers that held up in the lipid membrane. The E proteins in the smooth surface of ZIKV mature virions are antiparallel homodimers that assist in the cleavage of M proteins that are located flat on the lipid envelope (Barzon et al., 2016).
The composition of the E protein includes three ectodomains (DI, DII and DIII), a singular transmembrane domain as well as around five hundred amino acids. DII and DIII are connected via DI, which acts as a tunnel between the two and also assists in acknowledging entry cofactors and cellular receptors. Fusion is driven by insertion of a hydrophobic sequence of DII into the endosomal membrane of the host cell. Neutralising antibodies can associate with DIII which has a structure similar to that of an immunoglobulin that hold a site suitable for receptor-binding. Similarities in the E proteins of both DENV and ZIKV can be seen, however there are distinctions in the number of glycosylation sites present in both. DENV contains two sites while ZIKV only holds a single glycosylation site. The neurovirulence and transmission of the virus may be affected by this change in structure (Kostyuchenko et al., 2016). In accordance with this, the fusion loop of the ZIKV E protein contains an area beside it that is positively charged which can have a resulting influence on viral attachment to hosts. The prM protein allows for fusion-competent virions to be created. The Asian and African lineages show a structural difference due to a set of amino acids in the prM protein. Viral encapsidation requires the use of the C protein which has a hydrophobic and RNA binding domains that enable communication with the membrane (Barzon et al., 2016).
The NS1 protein present in ZIKV has a similar structure to that of the ones in flaviviruses and mainly operates during maturation in order to assist in the replication of the virus as well as immune response evasion. There is little information available about the NS2A protein, however it is known that it is a component in the replication complex and is hydrophobic and small in size. NS2B also has a comparable small size which associates and assists NS3 protein. This complex is activated through a hydrophobic domain to the ER membrane. NS3 contains an N-terminal and C-terminal domain. The N-terminal is the site of polyprotein cleavage with which a tight homodimer is created by ZIKV protease. While genome replication takes place, the C-terminal helicase unwinds the secondary structure of RNA. This helicase in DENV exists as dimer, while in ZIKV it is a monomer. The proteins NS4A and NS4B are both elements of the replication complex which are hydrophobic and assist in rearrangements of the ER membrane. Lastly, the largest of the non-structural proteins is the NS5 protein. The structure is that of a homodimer which has an N-terminal domain that contains methyltransferase as well as a C-terminal consisting of RNA-dependent RNA polymerase (RdRp). These domains help with RNA synthesis and capping the genome at the 5’ end (Barzon et al., 2016).
The bite of Aedes spp. mosquitoes is what primarily transmits ZIKV between humans. The mosquito receives the virus after a blood meal which then reaches the salivary gland after being replicated in the body. The new host then receives the virus after being injected. Viral replication is initiated at the inoculation site of the human skin. The virus is then expanded in the lymph node after it is transported there from the skin, which then leads to the virus particles replicating in the bloodstream, allowing peripheral tissues to be reached via haematogenous dissemination. The RNA of ZIKV can be detected in the body usually after the initial 10 days of infection. This would be in accordance with the first evidence of symptoms. High levels of the excreted virus can be found in saliva, urine and fluids from the body after the first weeks of being infected. This is consistent with infection from other flaviviruses, however, the ZIKV provides a dissimilar feature in which the virus remains to be detected in semen excretion, even after it has been cleared from the blood. This mechanism in the testis for the virus is still yet to be fully understood. Immune response begins quickly after infection where IgM antibodies can be visible as shortly as 10 days post infection which is then followed by the visibility of the IgG antibodies. IgM antibodies against the virus can exist in the body for up to a year. IgG antibodies can be detected for longer periods of time in the body and possible remain there for the rest of the lifetime of the human (Barzon et al., 2016).
Since the modern outbreaks of ZIKV has occurred, there has been a noticeable correlation with the virus and that of Guillain-Barré syndrome (GBS) and microcephaly (MC) in newborns. GBS is an autoimmune disorder that can be activated by bacterial or viral infections that can result in an eventual paralysis of the lower limbs after initially starting out as a weakness in the reflexes of tendons. The initial direct link between the virus and the disease was first recognised in French Polynesia in 2013. In this investigation, 42 cases of GBS were studied and in 41 of the cases (98%) presented anti-ZIKV IgG or IgM while 100% of the cases showed to have neutralising antibodies against ZIKV. This is a large contrast to the control group of people with febrile illness which overall showed 56% of people to have the antibodies as well. In the area at the time, there was a 66% infection rate of ZIKV, leading to the statistic of the risk of developing GBS to be 0.24 per 1000 cases of ZIKV infections (Weaver et al., 2016).
Several months following the outbreak of ZIKV in Northeast Brazil, there was an incline in the amount of cases of microcephaly in newborns. This is a disease in which the head circumference in a foetus is measured in order to determine an accurate age of the baby, but the size of the head is smaller than it should be. The average acceptable size for the head of a baby is usually within to 10%ile to the 90%ile, however, there is no overall accepted measurement to determine if a newborn has microcephaly. If a universal cut off was introduced, it is possible that there would be false positive and negatives diagnosed. The Society of Maternal Fetal Medicine (SMFM) provided a suggestion that microcephaly should be tested further if the circumference of the head is below three standard deviations for the gestational age mean, and should be positively diagnosed if it is lower than five standard deviations of the mean. There a several different cut off percentiles for countries of varied races and ethnic backgrounds. It has proved to be quite difficult to positively diagnose microcephaly as similar effects could be caused by genetic factors as well as environmental factors, but it is the stunt in brain development that must be looked out for. It is suggested that this halt in brain growth is in correlation with the infection caused by congenital ZIKV (Weaver et al., 2016).
In conclusion, scientific research of the Zika Virus has rapidly inclined in the past three years due to the excessive cases reported since 2013. There is still much to be understood about the virus, but the structural factors which are similar to that of DENV allows for some interpretation of the virus, and enables the development of possible vaccines. As well as that, the correlating incline with cases of the virus and Guillain-Barré syndrome and microcephaly have promoted further investigations into the mechanisms of the virus and their antibodies. This research, along with control management of the patients and virus have been implemented and continue to be updated new information of the virus continues to be obtained.