Mosquitoes can pass the Zika virus along to future generations in their eggs, researchers have found. That means winter may not stop Zika's spread.
Infected female mosquitoes can transmit the virus along to their offspring, according to a new study published in the American Journal of Tropical Medicine and Hygiene. That means that even once it gets too cold or dry for adult mosquitoes, their eggs—which can easily survive the dry season or winter—can hatch the next spring when it rains, producing Zika-infected larvae that grow into infected mosquitoes.
The Aedes aegypti, the mosquito known to carry Zika, is a creature of warm weather. But the study found that the virus, like mosquito eggs, has no such climate restrictions.
"[Transmission to offspring] is a mechanism to allow the virus to survive from one season to another," said researcher Robert Tesh, who chairs the pathology department at the University of Texas Medical Branch in Galveston. "This is one way for the virus to survive when there are no adult mosquitoes."
The only way mosquitoes were previously known to transmit the virus was by biting an infected human.
In their study, UTMB researchers began with two colonies of Zika-free mosquitoes—one of Aedes aegypti, and one of Aedes albopictus, another mosquito that has been linked to the virus. Then they infected some females of each. Once these females laid eggs, the adults were removed and frozen, while the eggs were allowed to hatch. This next generation of mosquitoes was allowed to grow into adulthood before being frozen, processed and tested.
None of the Aedes albopictus offspring were infected, but the Aedes aegypti offspring were infected at an estimated rate of one in 290.
That's a relatively low level of vertical transmission, Tesh says, referring to the passing of the virus from one generation to the next, and what happens in a laboratory doesn't always translate to the real world. But Tesh expects that vertical transmission of Zika from adult mosquito will. "This probably does occur in nature," he said.
Common methods to kill adult mosquitoes, like spraying, do not work on eggs. Eggs can be destroyed to stop them from hatching Zika-infected larvae—for instance, by adding certain bacteria to the standing water where they were laid—but it's difficult.
"They're so tiny," Tesh said, "if they're on a dark surface, you won't see them."