Kaushik Bharati, PhD
Zika virus (ZIKV) is a Flavivirus spread by infected Aedes mosquitoes. Although the virus has been known since 1947, its notoriety spread in 2015 with major outbreaks in Brazil and other Latin American countries as well as the Caribbean. Cases of microcephaly, in which babies are born with abnormally small heads, were reported in large numbers and sent the public health systems into overdrive in these countries. The situation worsened so much that the World Health Organization (WHO) in February 2016 declared ZIKV to be a Public Health Emergency of international proportions.
Zika: A Problematic Virus
Although ZIKV is primarily spread by the bite of infected mosquitoes, it has been reported to be spread through blood transfusion as well as sexual contact. Most people infected with ZIKV develop mild or no symptoms. Those who do develop symptoms, usually exhibit fever, joint pain, muscle pain, rash, conjunctivitis, and general fatigue. The disease usually resolves itself within a week. However, the emergence of complications like microcephaly through infected pregnant mothers has stimulated intense research in a bid to increase our understanding of this unassuming virus. Indeed, studies have shown that the transplacental transmission of the virus occurs in the first trimester of pregnancy. It is now believed that microcephaly could just be the “tip of the iceberg”, as with further research, it is slowly emerging that other congenital brain abnormalities also occur. Importantly, Guillain-Barré syndrome is another complication where autoantibodies are targeted against the nervous system, which can cause paralysis and even death.
Zika Virus: Potential for Global Spread
The danger of ZIKV lies in the fact that it can be spread to hitherto unaffected areas. The mosquito vector is distributed across the globe in all tropical climes, and this same vector also spreads diseases like dengue and chikungunya, thereby further complicating matters. This, coupled with the phenomenal increase in air travel, can send the virus anywhere across the globe within a matter of 24 hours, where the vector mosquito would be waiting to spread it further. Indeed, this is already happening, as the virus has made inroads to USA as well as other continents. Four cases of ZIKV infection were reported from Florida in July 2016, and as of December 30, 2016, the state of Texas has reported 294 cases of illness due to ZIKV. Several cases of imported ZIKV infections have been reported from North America, Europe, Asia, and Australia. These cases included infected pregnant women. There have also been reports of sexual transmission of ZIKV from Europe and USA.
Zika Research: Current Status
Studies have indicated that following the bite of an infected Aedes mosquito, the virus is spread from the skin via the draining lymph nodes and blood to the peripheral tissues and visceral organs. However, till date, there are very few cell types for which ZIKV tropism has been established. Experimental studies have mainly focused on the possible implications of placental and neural cells in viral pathogenesis. The neurotropism of ZIKV has been recently established using a mouse model. Research has also concentrated on identifying the host factors involved in the restriction of infection and the development of antiviral immune responses. Several of these host factors have already been identified. Moreover, genome-wide screenings have identified a number of human genes that are essential for ZIKV replication and could provide an answer to developing new antiviral therapies.
Vaccine development is complicated by the fact that ZIKV shares cross-reacting antigenic epitopes with other flaviviruses so that prior exposure to a flavivirus infection could aggravate the immune response against a heterologous antigen. The same problem applies to the development of immunodiagnostics against ZIKV. Nevertheless, several ZIKV vaccine candidates have been developed and are undergoing evaluation in clinical trials. Therefore, 2017 appears to be an exciting year for ZIKV research, and we might see a safe and effective vaccine on the horizon!