New Zika Transmission Pathway May Enable Virus to Survive Winter Months

To paraphrase a well-known commercial slogan, Zika may not be going anywhere for a while.

Researchers in the Department of Pathology at the University of Texas Medical Branch Institute for Human Infection and Immunity in Galveston have confirmed so-called “vertical transmission” of Zika from female Aedes aegypti carrying the virus to their larvae. Their findings suggest that this pathway for transmission may enable the virus to “survive during adverse conditions,” such as the colder and drier climate conditions of the winter months.

“This isn’t going to impact much on the transmission dynamics of the virus; it doesn’t mean there are going to be a lot more new cases,” study co-author Robert B. Tesh, MD, told Contagion. “What it does show, however, is that there is a mechanism for the virus to survive a season in which there are no adult mosquitoes.”

Dr. Tesh explained that in Galveston, where he also lives, adult Aedes aegypti mosquitos found in the region typically die in the late fall, when it starts to get colder. However, their eggs survive the winter, and many will hatch in the spring, as temperature warm and it begins to rain. “A new generation of mosquitos will emerge,” he continued. “And, if you have a few Zika-infected eggs, they’ll be infected when they hatch, and they’ll bite new people, who will infect new people, and start the cycle all over again.”

For the study, published online on August 29th by The American Journal of Tropical Medicine and Hygiene, Dr. Tesh and his colleagues, who have explored the vertical transmission of numerous viruses among insect vectors in the past, inoculated 100 females in 2 established laboratory colonies of Aedes aegypti and Aedes albopictus mosquitos intrathoracically with a Zika virus stock containing 10 plaque forming units (PFU)/mL. Ten days after infection, mosquitoes were fed defibrinated sheep blood, using a Hemotek membrane feeding system, and then isolated. Eggs from the first oviposition of the infected Aedes aegypti and Aedes albopictus females were hatched and reared to adults, using standard procedures.

In all, 10 of the parent female mosquitoes of each species were frozen for testing (by RT-PCR) 16 days post-infection. The mean virus titer in the 10 Aedes aegypti females was 6.13 log₁₀ PFU/mosquito compared to 6.35 log₁₀ PFU/mosquito in the 10 Aedes albopictus females. A total of 69 pools consisting of 1,738 female adult Aedes aegypti were tested, and 6 of the 69 pools were indirect fluorescent antibody technique-positive, indicating that “one or more of the progeny in the pool were infected” with Zika virus. Based on these results, the authors estimated a minimum filial infection rate of 1:290 for Aedes aegypti. None of the 32 Aedes albopictus pools, comprising 803 adults, were positive in the indirect fluorescent antibody technique test.

From their earlier work, Dr. Tesh and his colleagues know that this pattern of vertical transmission for Zika mirrors that of another mosquito-borne infection, Dengue. In fact, as with Dengue, Dr. Tesh believes that human and mosquito populations will eventually develop “herd immunity” to Zika and the virus will “die out” for a few years, before re-emerging. Other experts have already predicted that it’s likely the world will see new outbreaks of Zika about 10 years after the current outbreaks are under control, which may take 2 to 3 years.

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.