Scientists in Recife, Brazil, have found Zika in a species of mosquito that was not thought to carry the virus, raising concerns about a much wider outbreak than has been anticipated in the U.S.
Since early in the current outbreak, the Aedes aegypti mosquito has been regarded as the lone mosquito species capable of passing Zika to humans. But when researchers at the government-run Oswaldo Cruz Foundation collected about 5,000 mosquitoes from homes in Recife, the birthplace and epicenter of the current crisis, they found that the vast majority were not Aedes, but rather Culex.
Culex mosquitoes are known to transmit West Nile virus in the states and elsewhere. They are less aggressive than Aedes but far more widespread.
Constancia Ayres, a lead scientist on the project, said the findings proved that Culex mosquitoes might be at least partly responsible for the outbreak. "It can transmit Zika," she said in a press conference last week. "What we need to know now is which species is the most important—if Culex is the primary vector or the secondary vector."
Other scientists have resisted that conclusion. "We need more research to see if it's a competent vector," says Matthew Aliota, an infectious disease researcher at the University of Wisconsin-Madison who has studied the Zika virus in Culex mosquitoes. "Just because you find virus in a Culex mosquito doesn't mean the mosquito can transmit the virus."
To understand why, consider how the virus is transmitted from mosquito to human. When a virus is first taken up by the mosquito in a bloodmeal, it lands in the mosquito's gut, where it replicates itself. To infect another human, though, the virus needs to do more than that: it needs to spread from the gut to the salivary glands, where it can be spit out in a subsequent blood meal.
That doesn't happen as efficiently in some mosquitoes as in others; in part because of anatomical barriers that can block viral replication. "A mosquito might pick up a virus when feeding and that virus might replicate in the midgut but still might not disseminate to secondary tissues," says Aliota.
In fact, several studies, both in the field and in the lab (and including one by Aliota), have failed to find Zika virus in the salivary glands of many mosquito species, including Culex.
Aedes vs. Culex
The Centers for Disease Control and Prevention says Aedes aegypti mosquitoes remain the most significant driver of the current crisis. Even with sexual transmission emerging as very real problem, scientists at the CDC and elsewhere maintain that Zika is first and foremost a mosquito-borne illness, and that the virus is transmitted by this one mosquito above all others. "The body of scientific evidence to date clearly points to Aedes," says CDC press officer Candice Hoffmann.
The Aedes aegypti is an aggressive daytime biter that thrives in tropical and subtropical climates. Its preference for human habitats has complicated mosquito-control efforts, because aerial spraying is less effective at killing mosquitoes that avoid the open air above lakes and large bodies of water.
The control measures that work best for Aedes species (namely integrated pest management, which involves ridding yards of standing water and fortifying the screens on doors and windows) are time-consuming and labor-intensive.
Culex mosquitoes are nighttime biters that like to breed in the organic-rich water of lakes, streams, and the like. If additional research were to point to Culex as a transmitter of Zika, strategies for combating the current global outbreak would need to be adapted accordingly. In short, it could mean more aerial spraying, and more vigilance encouraged at night as opposed to the day.
In the meantime, your best bet is to avoid all mosquito bites, by using an effective insect repellent and taking other steps that can keep biting bugs away.
