Scientists wrestle with possibility of second Zika-spreading mosquito

It’s not yet clear whether Asian tiger mosquitoes will be important virus carriers and worsen outbreak

Aedes mosquitoes

DOUBLE TROUBLE OR NOT  Researchers don’t yet know whether the Asian tiger mosquito (Aedes albopictus, right) will join the yellow fever mosquito (Ae. aegypti, left) as a notable spreader of Zika virus in the current outbreak. A two-mosquito scenario would expose more people.

Both: James Gathany/CDC

Sure, mosquitoes spread Zika virus. Scientists have already identified the yellow fever mosquito (Aedes aegypti) as a major spreader in the Americas of Zika and its risk of birth defects and possible paralysis. But Ae. aegypti may not be the only culprit. Recent evidence raises concerns that a relative, the Asian tiger mosquito (Ae. albopictus), might also play a role.

A Mexican lab, for instance, recently detected Zika virus for the first time in an Ae. albopictus collected in the Americas, the World Health Organization and Pan American Health Organization announced in April. Just finding the virus in a mosquito doesn’t prove the species will spread the disease in a major way in the Americas, says Phil Lounibos of the University of Florida’s medical entomology lab in Vero Beach. But if Asian tiger mosquitoes do turn out to be important in driving the Zika outbreak northward in the Americas, researchers predict more people and more places could face disease risk.

For Ae. aegypti, the evidence is clear that it readily transfers the Zika virus, the WHO declares. Ae. aegypti is a household cockroach of a species, thriving around people; it frequents human houses, bites (and bites repeatedly) during the day, and breeds in plant saucers and other minipools in yards.

DOUBLE JEOPARDY Circles show the top cities at risk of Zika jumping from current outbreak sites (map released in March) based on local mosquito habitat plus air travel from infected zones. If Ae. aegypti proves the only important spreader, not as many cities are at risk (solid circles). Adding Ae. albopictus (open circles) expands the risk farther north.L.M. Gardner et al/Lancet Infectious Diseases 2016

Asian tiger mosquitoes also thrive around humans, and in many places, they have a taste for human blood (SN: 6/29/13, p. 26). (Contrary to popular paranoia, many mosquitoes don’t.) They can spread Zika relatives such as dengue — in fact, Ae. albopictus has been responsible for at least modest outbreaks of viral disease on its own, as evidenced by surges of dengue cases in Hawaii and in southern China.

Researchers have their reasons for worrying that Ae. albopictus can transmit Zika as well. Lab work has shown that the virus can travel from blood in the mosquito gut to the salivary glands and then reproduce there vigorously enough to render future bites infectious (although recent lab tests find a low rate of virus transmission). And the mosquito is a major suspect in a 2007 Zika outbreak in Gabon.

These worrying factors, however, still leave a lot of uncertainty about whether Ae. albopictus will prove important in Zika’s sweep through the Americas. In general, Ae. albopictus isn’t as dedicated to biting humans as Ae. aegypti, says Michael Reiskind of North Carolina State University in Raleigh. And while Ae. albopictus’ range reaches much farther north than that of Ae. aegypti, the virus replicates more slowly in mosquitoes as temperatures drop. Therefore, milder temperatures in northern mosquito seasons can render mosquitoes a bit less dangerous by dragging out the time between drinking virus-tainted blood and building up enough virus to pass it on.

Figuring out the role of Ae. albopictus in the current sweep of Zika through the Americas is “critically important,” says Lauren Gardner of the University of New South Wales in Sydney.  Gardner specializes in analyzing how widespread human systems, such as transportation, affect the spread of disease. In the May issue of Lancet Infectious Diseases, she and colleagues argue that adding Ae. albopictus as a second spreader of Zika virus makes a big difference in how the epidemic might play out worldwide. In North America for instance, with just Ae. aegypti as the major vector, the researchers predict a high risk of Zika virus spreading mostly in Florida, Louisiana and Texas. But having Ae. albopictus as a second important vector raises the specter of local mosquitoes spreading Zika in chillier places such as Canada.

Although Ae. albopictus is a competent vector in lab tests, the real world has so many more variables that actual outcomes are difficult to predict, Reiskind says. “Could albopictus spread Zika? Absolutely,” he says. “Will it?” That’s the question.

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