Zika virus is an emerging threat unless contained especially in areas where the vector Aedes aegypti mosquito is in large numbers, including India. At Pune’s National Institute of Virology, scientists have demonstrated in their study how Indian mosquitoes, when infected with African strain of the zika virus (ZIKV), can easily transmit the infection to suckling mice. In their study published on March 29 this year in Intervirology, a team of scientists from National Institute of Virology, National Jalma Institute of Leprosy and other Mycobacterial diseases and others research institutes have shown how the Indian Aedes aegypti mosquito that transmits dengue and chikungunya viruses is easily susceptible to the zika virus.
The director of NIV, Dr Devendra Mourya, told The Indian Express, this study has shown that Indian mosquitoes pick up the ZIKV in a natural way. “We have shown their susceptibility when they were injected with ZIKV,” he said. Experiments were performed to understand the natural disease progression in infant mice after the bite of mosquitoes infected by the ZIKV. “We wanted to check how much time it took for the virus to replicate — cross the stomach gut barrier to the saliva, and through experiments we found that the Indian mosquito… is very susceptible to the zika virus,” Dr Mourya added.
The mosquitoes were experimentally infected with zika virus (African Strain MR-766) through the oral-feeding route, which is the natural mode by which mosquitoes get the virus. Infected mosquitoes were allowed to bite on infant and suckling mice. Sick mice were euthanised, and their organs were collected and subjected to three detection tests — real-time RT-PCR, histopathology, and Immunohistochemistry [IHC] — to check for the presence of the virus.
The results showed clinical symptoms such as trembling, solitary behaviour, nervous signs, no movement, and lethargy appeared in mice after 4-5 days of being bitten by the infected mosquitoes. In the terminal stages, mice became moribund following which they were euthanised. “These [infected] mosquitoes harbour the virus in their salivary glands, which explains its easy transmission when feeding on mice,” scientist Dr Pragya Yadav further explained.
In 2013, the first zika virus outbreak was reported in the Marquesas Islands in the southern Pacific Ocean. It subsequently spread to Brazil in May 2015. Health agencies in India closely monitored the situation in the country. Three cases identified in Gujarat, and one in Chennai did not reveal any travel history to zika virus endemic region, suggesting that the virus is not new in the country.
In India, more than 35,000 serum samples of febrile illness were tested, which yielded these four cases, suggesting a very low level of transmission of the virus. About 18,000 mosquitoes were tested — and this included about 500 mosquitoes from the Bapunagar in Ahmedabad, where two cases were reported. The virus, however, could not be detected. So far there has been no isolation of this virus. But scientists are not ruling out the possibility of mutations that may make these mosquitoes more susceptible to the virus, and result in an outbreak-like situation. In November last year, in the Indian Journal of Medical Research, scientists from influenza and entomology group of the Indian Council of Medical Research and National Institute of Virology said lessons need to be drawn from the episode when it was suggested that the chikungunya virus had disappeared from India in 1990-1999.
In 2000, a chikungunya virus strain that was isolated from mosquitoes in Yawat, Maharashtra, appeared in an epidemic form in the Indian Ocean in 2004. Molecular clock studies showed that the epidemic strain originated from the Yawat strain. Hence, these lessons can help explain the current low prevalence of zika virus in India. Like chikungunya virus, when ZIKV will show greater affinity to effectively attach to the virus-specific receptors in the guts of mosquitoes, it will have the potential to cause outbreaks. The virus still does not have priority as other flaviviruses such as dengue and chikungunya, Dr Mourya said.
“There is a need to explore possibilities for containing the virus. These, in turn, demand advanced knowledge regarding the natural replication cycle. Our data from this study suggests that mice are highly susceptible to natural infection with this zika virus strain and, hence should be explored as a suitable model for understanding the virus’s natural transmission and disease progression,” the NIV director added.