Mosquito-borne viruses, like dengue and chikungunya, tend to come in seasonal waves, flaring up in the summer or wet seasons, and fading in the winter or dry seasons. Zika is expected to behave similarly, though hopefully future flare-ups will be less intense than the current epidemic as people in affected populations develop immunity.
One way these diseases can survive the winter, even as mosquito populations dip, is in eggs. There is evidence that dengue, West Nile, and chikungunya can be vertically transmitted—that is, mosquitoes can sometimes pass the viruses along to their offspring. A new study published in the American Journal of Tropical Medicine and Hygiene has found evidence that Zika can also be vertically transmitted.
Researchers from the University of Texas Medical Branch infected populations of Aedes aegypti, the main vector for Zika, and Aedes albopictus, which can also spread the virus, but is less likely to bite humans. They collected the infected females’ eggs, raised them, and then tested the adult offspring for Zika. In the Aedes aegypti, some of the offspring tested positive, at a rate of 1 Zika-infected mosquito per 290 offspring. (None of the Aedes albopictus offspring had Zika, but the researchers tested fewer of them, and it’s possible they have a small enough vertical transmission rate that the study just didn’t pick it up.)
That’s not a very high rate, but when you consider that the population of Aedes aegypti in Zika-infected areas likely numbers in the millions, it’s not insignificant.
Vertical transmission is “not going to change the epidemiology very much,” says Robert Tesh, a professor of pathology at UTMB. “I think what it means is it’s a way for the virus to overwinter. Here in Galveston, it’s still quite warm now. We have aegypti. In October it’ll start to get cool. But their eggs survive the winter. In April or March it’ll start to warm up again and we’ll get rain. The containers where the eggs are will fill with water, and you’ll have another generation of mosquitoes. If even just a few of those eggs are infected, when the larvae hatch, they’re also infected.”
As my colleague Adrienne LaFrance has reported, Aedes aegypti are notoriously hardy mosquitoes. They can breed in the tiniest amounts of water, and their eggs can survive for more than a year. This research underscores that spraying for adult mosquitoes may not be enough to get Zika under control. Larvicides can help get the bugs before they can bite, but Tesh says those “don’t necessarily kill the eggs, because they’re often in protected places, and the eggshell of the Aedes is quite resistant.”
Genetically modified mosquitoes could help, since they reduce the population by breeding with wild mosquitoes to create offspring that can’t survive to adulthood. The Food and Drug Administration recently approved a trial of genetically modified mosquitoes made by the company Oxitec, which would be released in Key Haven, Florida, but due to opposition from residents, the local mosquito control authority hasn’t approved it yet.
“Oxitec’s solution is targeted specifically for Aedes aegypti mosquitoes,” Derric Nimmo, Oxitec’s senior scientist on the Florida Keys trial, told me in an email. But both he and Tesh say that genetically modified mosquitoes wouldn’t be a silver bullet for the vertical transmission problem. “Since Aedes aegypti eggs can lie dormant for over six months and hatch at any time, the Oxitec solution, as with any vector control solution, may require repeat releases to maintain the suppression of populations of Aedes aegypti,” Nimmo writes.