The usual suspect has been caught, not red-handed but red-bellied. Since the beginning of the Zika virus outbreak in Brazil, health authorities and researchers have strongly suspected that the mosquito Aedes aegypti, known for spreading several deadly viruses, was also guilty of spreading Zika from one person to another. But direct evidence had been hard to find. Now, researchers in Rio de Janeiro, Brazil, report that they have found the Zika virus in wild-caught A. aegypti. The researchers did not find the virus in other mosquito species they captured in neighborhoods where Zika was spreading, which strengthens the case that A. aegypti is the main vector driving the outbreak.
That mosquito species, which is ubiquitous in urban areas across Brazil and much of Latin America, is known to spread several closely related viruses, including dengue, chikungunya, and yellow fever. Researchers had confirmed that the species could be infected with Zika and that the virus could multiply and infect the mosquito’s saliva—a requirement for it being able to spread the virus. But no one in Latin America had found a wild-living specimen that was carrying the virus.
Despite the hundreds of thousands of human cases, it’s not easy to find infected mosquitoes, explains Oliver Brady, an entomologist at the University of Oxford. “Finding the virus in a mosquito is extremely difficult,” he says. “They infect people and die before anyone shows up at the hospital” with disease symptoms.
To try to nail down solid evidence against A. aegypti, Ricardo Lourenço-de-Oliveira, an entomologist at the Oswaldo Cruz Foundation (Fiocruz) in Rio de Janeiro and his colleagues collected mosquitoes from homes and streets in neighborhoods where people complaining of Zika symptoms lived. Over 10 months they collected more than 1500 mosquitoes, identified them, and tested them for the presence of Zika and other viruses. Nearly half were A. aegypti, and most of the rest were Culex quinquefasciatus, another common mosquito in urban Brazil that some have suspected of transmitting Zika. Roughly 5% of the collected mosquitoes were other species. A species called A. albopictus, which can also transmit Zika in the lab and has been found infected with the virus in Mexico, made up only about 2% of the catch, Lourenço-de-Oliveira says. That means it’s unlikely to be a major vector in urban areas, he says. The researchers pooled mosquitoes of the same species when they tested for viruses. They found Zika virus in three sets of A. aegypti mosquitoes, but none of the other species.
The researchers plan to submit their results for publication once they have finished analyzing insects caught earlier this month, Lourenço-de-Oliveira says, but they announced their finding today because they wanted to let health authorities know as soon as possible. He and his colleagues are also examining whether any of the insects they caught were infected with dengue, chikungunya, or other viruses.
The new findings do not rule out other mosquito species as possible vectors, but they do provide some reassurance that Zika is likely following the familiar patters seen in dengue and chikungunya outbreaks, says Philip McCall, an entomologist at the Liverpool School of Tropical Medicine un the United Kingdom. That suggests that large outbreaks are less likely outside the range of A. aegypti. If the virus were easily spread by Culex species or A. albopictus, the regions threatened by serious outbreaks would be much larger.
How much A. albopictus is helping spread Zika is still an open question. It has been shown to be a Zika vector in Africa, but in Latin America it bites humans less frequently than A. aegypti does. Both A. albopictus and A. aegypti can transmit dengue in the lab, but in areas where A. albopictus lives but A. aegypti is lacking “you don’t have huge dengue outbreaks,” Brady notes.
*Correction, 24 May, 11:37 a.m.: The photo originally posted with this story showed Aedes albopictus, not Aedes aegypti.