U.S. Study: Adult Mosquitoes Can Pass Zika to Their Offspring

Killing only adult mosquitoes may not end outbreaks of the virus

New research from the University of Texas Medical Branch (UTMB) in Galveston has shown that female Aedes aegypti mosquitoes can pass the Zika virus to their eggs and offspring.

The recent Zika virus outbreak in Florida has dramatically increased efforts to remove A. aegypti mosquitoes. The new findings highlight the importance of including larvicide in the efforts to curb the spread of the Zika virus. The findings were reported in the American Journal of Tropical Medicine and Hygiene.

“The implications for viral control are clear,” said lead author Dr. Robert Tesh, director of the World Reference Center for Emerging Viruses and Arboviruses at UTMB. “It makes control harder. Spraying affects adults, but it does not usually kill the immature forms—the eggs and larvae. Spraying will reduce transmission, but it may not eliminate the virus.”

“Since Zika virus has emerged as a global health emergency, most research has focused on the virus and its effects on humans. There is far less research on the virus in its mosquito host,” Tesh added. “But if you want to control Zika, you also have to know about the behavior of this virus in mosquitoes.”

Zika virus has been found to cause microcephaly in newborns whose mothers were infected during pregnancy. The World Health Organization (WHO) has declared Zika’s spread an international health emergency, and WHO and the United States government have urged pregnant women and their partners not to travel to 45 countries—most of them in the Caribbean and Latin America—where Zika virus is active.

A. aegypti is also known to be expanding its range northward. In the U.S., it is especially abundant in Florida, the Gulf Coast, Arizona, and California, with sporadic records in other Southern, Mid-Atlantic, and Midwestern states.

To determine whether female mosquitoes that carry Zika virus pass it on to their offspring, Tesh and his colleagues injected laboratory-reared A. aegypti with the virus. The mosquitoes were fed, and within the next week they were laying eggs. The researchers collected and incubated the eggs and reared the hatched larvae until adult mosquitoes emerged. Culture of these adults found Zika virus in one of every 290 mosquitoes tested.

“The ratio may sound low,” Tesh said, “but when you consider the number of A. aegypti in a tropical urban community, it is likely high enough to allow some virus to persist, even when infected adult mosquitoes are killed.”

Mosquitoes are known to pass other viruses on to their offspring, including dengue and yellow fever—both of which are also transmitted by A. aegypti. West Nile and St. Louis encephalitis viruses can also be passed on in eggs of Culex mosquitoes. The authors note that vertical transmission appears to provide a survival mechanism for the virus during adverse conditions, such as cold periods in temperate regions and hot dry seasons in tropical zones, or when many people become immune because of prior infection or vaccination.

Source: University of Texas Medical Branch; August 29, 2016.