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A banner on Zika virus outbreak is pictured on the opening of the World Health Assembly, with some 3,000 delegates from its 194 member states on May 23, 2016 in Geneva.FABRICE COFFRINI/AFP / Getty Images

A Brazilian researcher says he has found the Zika virus in a wild-caught Aedes aegypti mosquito, providing the first evidence to support the government's theory of how the disease is being transmitted in the country.

Brazil is in the grip of a Zika epidemic, with a new strain of the once-benign virus causing devastating fetal brain defects. There are 1,332 confirmed cases of fetal Zika syndrome, and another 3,332 under investigation. The virus has also been linked to a spike in cases of Guillain-Barré syndrome, which causes paralysis.

The Brazilian government and the World Health Organization (WHO) have built a response to the virus on the assumption that it is being transmitted by Aedes aegypti, the mosquito species that is known to be the principal vector in the country for dengue fever, which is a related virus.

But until this announcement, which is as-yet unpublished research by the Oswaldo Cruz Foundation (Fiocruz) in Rio, no one had found Zika in a wild mosquito in the country, which concerned entomologists.

Ricardo Lourenço de Oliveira, who heads the institute's laboratory for mosquito-borne disease, announced Monday that he had found Zika in adult mosquitoes trapped in the homes of people with confirmed cases of the disease. He said that in a total of 1,500 mosquitoes trapped in Rio over the past 10 months, half were of the Aedes aegypti species. The presence of Zika in these was confirmed by molecular analysis that found the virus's RNA and by identifying individual virus particles in the insects.

"The discovery of the presence of the virus in the mosquito reinforces that the A. aegypti must be the most frequent transmission path of Zika in Brazil," he said in a statement.

As The Globe and Mail has reported, entomologists in Brazil and abroad have expressed concern that, given the intensity of Brazil's epidemic, no mosquitoes carrying Zika had yet been caught here – meaning that the health strategy was built on responding to one species of mosquito without proof that it was, in fact, the vector.

"It's very good news that it has been found here in Brazil – I did think it was very strange that no one had found it," said Constancia Ayres, an entomologist studying Zika transmission with Fiocruz in Pernambuco in northeast Brazil. But she said she does not consider this sufficient to conclude that Aedes aegypti is the principal vector. Her own work is examining the Culex genus of mosquito, which is much more common in Brazil, and which she fears may yet prove to be the principal vector.

Dr. de Oliveira said his samples also contained Culex quinquefaciatus and another species of Aedes mosquito, albopictus, but none had the virus. An albopictus carrying the Zika virus was recently reported by Mexico's Ministry of Health, the only other confirmed vector finding in the Americas.

Dr. de Oliveira said this finding follows on March work at Fiocruz that showed that aegypti can transmit Zika, and he called identifying the virus in wild mosquitoes the second key step in responding to the epidemic: This is "fundamental to understanding the transmission of the virus, estimating the risk of spreading the disease and guiding the control actions," he said.

"In reality, the percentage of A. aegypti that are infected with the virus and transmit the disease is very low. On the other hand, the Aedes is a domestic mosquito that lives close to people and benefits from breeding spots inside homes, where they can put their eggs." When they live close to dense human populations, they can feed often and have many breeding spots. They also live longer, "which favours the infection process and the spreading of diseases such as dengue, Zika and chikungunya," he added.

Last week, the WHO announced that the first cases of the "Asian strain" of Zika, the one causing the neurological defects in Brazil, had been confirmed in Africa, after sequencing the virus found in patients in the West African island nation of Cape Verde.

"The findings are of concern because it is further proof that the outbreak is spreading beyond South America and is on the doorstep of Africa," said Matshidiso Moeti, the WHO's regional director for Africa. As of May 8, there were 7,557 suspected cases of Zika in Cabo Verde, and four babies born with related neurological defects.

While Zika was discovered in Uganda in 1947 and was thought to be a mild cousin of dengue, the virus has undergone a mutation that causes it to attack both fetal neurological systems and those of some adults, who in rare cases develop Guillain-Barré syndrome after infection. The strain now racing through the Americas is also sexually transmissible, which was previously unknown behaviour for the virus.

One question of major concern to public health officials is whether the new Zika strain will prove as virulent elsewhere, or if there is something specific to the Brazilian epidemic causing the surge of affected babies. Officials with the U.S. Centers for Disease Control estimate that up to 80 per cent of the population of Puerto Rico may eventually be infected by Zika, which is racing through the island; there are 249 confirmed cases there so far, and one confirmed case of fetal Zika syndrome.

Meanwhile Colombia, which has the highest number of confirmed Zika cases after Brazil, there have been reports of five babies born so far with the syndrome, from 15,038 pregnant women with confirmed or suspected Zika.

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