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Brazilian health workers have been spraying for Aedes aegypti mosquitoes, but some health experts wonder if another mosquito specie should also be examined as a possible vector for the Zika virus.Felipe Dana/The Associated Press

When Brazilian President Dilma Rousseff announced new funding to fight the Zika epidemic in her country on Wednesday, she emphasized how mysterious the virus is: "We know that the transmission of the Zika virus happens through the bite of the Aedes aegypti mosquito, but the rest of our knowledge … is still below what's necessary for us effectively to protect the health of our people."

But Ms. Rousseff was wrong. Her government does not know even this one fact about Zika. Although there is good cause to suspect that Aedes aegypti, the mosquito species with distinctive black-and-white striped legs, is the agent of infection, there are no confirmed findings of the virus in aegypti in Brazil.

Nearly a year after Zika began to be reported here, and five months after the government declared a public-health emergency, no one has yet found proof that Aedes aegypti is spreading Zika or whether it may be one among several vectors. One critical question is whether the virus can be transmitted by another mosquito – possibly one in the genus Culex, which is vastly more common than Aedes aegypti.

Yet Brazil, on the advice of the World Health Organization, has based its entire response to the Zika crisis on combatting this one mosquito species. And the political and economic crisis gripping the country has sidelined an already haphazard and underfunded response.

In Brazil, some 907 babies have been confirmed to have microcephaly (abnormally small heads – the hallmark of this crisis) or other brain defects related to congenital Zika virus infection, with another 4,293 cases under investigation; 146 deaths and stillbirths are being related to the virus. It is also being blamed for a surge in cases of the neuroparalytic disorder Guillain-Barré syndrome. At least 1.5-million Brazilians have been infected with the Zika virus to date.

Two researchers who are analyzing mosquitoes trapped in the homes of people with Zika infection say they "may" have found the virus, but this is not finalized or published data. A handful of other entomologists – none of them organized into a national surveillance network – are trapping and studying aegypti, but have not found Zika in any specimens they have caught.

Claudio Maierovitch, director for communicable diseases in Brazil's Ministry of Health, defended the country's focus on Aedes aegypti, saying there is considerable research from Africa showing that the virus is carried by this mosquito species; that Aedes aegypti is confirmed as the vector of other closely related flaviviruses, such as dengue and yellow fever in Brazil; and that the pattern of the Zika epidemic overlaps with the maps of the presence and density of the mosquito.

"In Brazil, we have relied, since the beginning of the epidemic, on the official information from the World Health Organization that the mosquitoes from the Aedes genus are primarily responsible for the transmission of the Zika virus," he said.

The WHO, in an e-mailed response to questions from The Globe and Mail, said: "The current epidemiological evidence strongly suggests that Aedes is the main vector."

But some Brazilian entomologists – and international experts who recently travelled here to examine the role of the mosquito in the Zika outbreak – are alarmed. They agree that Aedes aegypti could well be a vector but they say too little is known to be focusing the response solely on this species, particularly since the strain of the virus circulating in Brazil is behaving in ways, and having an impact, markedly different from that known previously.

Zika never before had been known to be transmissible sexually, which this virus strain is, or to cause the fetal brain damage. Researchers have now established that an outbreak in French Polynesia in 2013 was the same strain. But the virus was not found in Aedes aegypti there, said Constancia Ayres, an entomologist studying Zika in mosquitoes in the Recife branch of Brazil's foremost public-health research institute, the Oswaldo Cruz Foundation (Fiocruz). And it wasn't found in an outbreak on the Pacific Island of Yap in 2007 either.

"I believe Aedes can transmit the virus," she said. "But I believe it is possible that Culex is the major vector in Brazil."

Another key question here is whether sexual transmission is acting as a greater driver of the epidemic than was previously suspected. It is difficult to prove this in an area with an outbreak – everyone who is likely to have had sex with someone infected is also likely to have been bitten by a mosquito.

All of this matters because a public-health response must be tailored to the vector. Aedes aegypti feeds during the day, for example, so Brazil's Health Ministry is encouraging people, especially pregnant women, to use repellent and wear long sleeves. But Culex species feed at night – and so a key prevention strategy is bed nets, which are not widely used here. Aedes thrives in small patches of water around human housing; Culex breed in sewage.

Brazil is investing in transgenic mosquitoes that would wipe out the species – but it's Aedes aegypti that has been genetically modified. And if the vector is something else entirely than mosquitoes?

Grayson Brown, a professor of entomology at the University of Kentucky, who co-chaired a summit on Aedes and Zika held in the northeastern Brazilian city of Maceio 10 days ago, said participants left uncertain what the vector is.

"We're going based on evidence with Zika from places where there is a longer period of study, but the problem is the strain of Zika circulating has changed a lot – and we saw with Chikungunya [another mosquito-born virus] that it's possible for these things to jump species," Dr. Brown said.

But the Zika epidemic has been entirely sidelined here by the intense political drama playing out in Brasilia. Ms. Rousseff faces imminent impeachment and virtually the entire political establishment is embroiled in a mammoth corruption investigation. The country is in its worst recession in 100 years and public budgets have been slashed.

"The problem here now is the financial crisis in Brazil: Imagine that you ask for a budget for a project $1-million [U.S.] – the answer is going to be no," said Margareth Capurro, an entomologist with the University of Sao Paulo whose analysis of 289 Aedes aegypti mosquitoes collected in the homes of patients with suspected Zika is the only one so far potentially to show the virus here. Testing a single bug for the virus costs the equivalent of $11; ideally researchers would be doing thousands and thousands, she said.

The political crisis now gripping the nation isn't helping either, she added. "Let's say that tomorrow I get a commitment from the minister of health for $3-million – but I don't know if next week he's going to be the minister," she said.

Ms. Rousseff's announcement of new funding was for $320-million in grants and loans to cover everything from vaccine and diagnostics research to commercialization of technologies to control mosquitoes – with the focus solely on Aedes aegypti.

Dr. Ayres of Fiocruz said she is also desperate for resources – human and financial – to expand surveillance. She is finalizing a study that she says shows Culex quinquefasciatus, a species ubiquitous in Brazil and also found as far north as Canada, can transmit Zika in laboratory conditions – although this is a long way from showing it can happen in the wild. She is trapping and studying both Culex and Aedes species, but has not yet found the virus in either. If it turns out that Brazil, urged by the WHO, bet wrong on which is the vector, the implications are frightening, she said.

Scientists outside Brazil increasingly share her fears. Fiona Hunter, an entomologist with Brock University in St. Catharines, Ont., who attended the Aedes summit, said she is deeply concerned that the country might be making a mistake in its response. "The fact that there are no data as far as I can tell that Aedes aegypti is driving this Zika epidemic just flabbergasts me."

Dr. Hunter was involved in the first surveillance to find the vector for West Nile in Canada. To do that, she said, scientists set traps to gather every mosquito existent in an area, then tested and retested dozens of species to establish beyond doubt which one was driving the epidemic.

"That's how research into a new emerging disease ought to be done," she said, and the vicious behaviour of Zika in Brazil merits treating it as a new disease. "But they didn't figure it out – the Brazilians automatically assumed it's aegypti. I hear physicians here on the radio in Canada saying you don't have to worry ever about Zika – well, yes you do. There are implications for the spread of this virus around the world."

Dr. Capurro in Sao Paulo said that she would like to be analyzing 2,000 mosquitoes a month but is constrained by the lack of a national network. "I need connections with hospitals [to identify prospective capture sites] and municipal administrations – who do I even call?" she said.

Dr. Ayres said that just last week, months into the emergency, WHO's regional branch contacted her to discuss a network to do viral surveillance in mosquitoes.

Davis Ferreira, a virologist who is vice-director of the Microbiology Institute at the Federal University of Rio de Janeiro, has analyzed about 500 mosquitoes, both aegypti and Culex, looking for Zika, and not found it. He recently changed his search protocol to use a new sequence of viral RNA, which he hopes will yield new results.

"We don't have the surveillance we should," he said, and he was unsure what other researchers might have found or be looking for. "Maybe it's time for us to join forces and see what others are doing."

One other entomologist, Ricardo Lourenco de Oliveira who works with the Rio office of Fiocruz, said he "has found a few" mosquitoes with Zika in the approximately 600 mosquitoes he has analyzed but has no data to make public yet.

The WHO's Vector Control Group in Geneva said by e-mail that more research is required but Brazil is not making a mistake by targeting Aedes aegypti.

"The current vector control recommendations which are targeting Aedes species, have also been proven very efficient against domestic Culex," wrote infectious-disease expert Raman Velayudhan and entomologist Florence Fouque. "Methods include insecticide residual spraying, indoor space spraying, larval control, elimination of breeding sites and personal protection including the use of repellents."

A range of experts consulted by The Globe emphasized that it is not necessarily surprising that Zika would not have yet been found in Aedes aegypti in Brazil – although all expressed surprise there is no nationally co-ordinated effort looking for it.

Even in an epidemic, finding the virus in an insect is no simple thing, they said. Only one in every 1,000 insects is estimated to be infected, even at the height of an outbreak. Only a few mosquitoes live long enough to become infectious (that is, they bite someone with the virus, it establishes itself in the insect body and travels to the salivary glands where the mosquito can then secrete it in someone else it bites.) And, Dr. Ayres said, her research suggests the virus doesn't live as long in a mosquito as dengue, for example, which means the window in which a researcher might find it is brief.

Dr. Brown in Kentucky said there are a host of other logistical challenges, ranging from the way the mosquito gets battered in a fan trap to the need to get samples quickly on to dry ice or "chill tables," both of which are scarce in Brazil. "At this point, I'm willing to believe [the fact it hasn't been found] is due to sampling technique and logistical limitations as much anything," he said.

The Brazilian strain of Zika is racing through Latin America now. In Puerto Rico, where up to a quarter of the population is expected to be infected with the virus in the coming months, researchers have not yet found the virus in Aedes either. Roberto Barrera, chief of entomology and ecology activity in the dengue branch of the U.S. Centers for Disease Control in Puerto Rico, said that his lab has a huge backlog of mosquitoes to analyze.

Samples from November and December did not have Zika, and mosquitoes trapped since then have not yet been analyzed. He called it "unexpected" that no researcher in Brazil has yet found Zika in Aedes in areas with transmission.

"Zika is a newly circulating virus, so even though personally I don't believe you're going to find Culex being a vector it wouldn't hurt to investigate it ," he said. "The conservative approach is to target aegypti – but who knows? It will be a tragedy if Zika is being transmitted by Culex, and there's also no reason why Aedes aegypti wouldn't be able to transmit it as well – so it's double trouble."

With files from Manuela Andreoni

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