RIO DE JANEIRO, Brazil, Wednesday July 27, 2016 – Brazilian scientists have found the Zika virus in the Culex quinquefasciatus mosquito, which is far more common and widespread than the species previously identified as the vector for the virus.
The findings of research by the South American country’s top public health institute, the Oswaldo Cruz Foundation (Fiocruz), suggest that Brazil may need to change its Zika response strategy, and comes as a blow to a nation in the grip of an outbreak less than two weeks before tens of thousands of visitors arrive for the Olympics.
The virus was discovered in Culex quinquefasciatus in the north-eastern city of Recife, regarded as “ground zero” for the rash of microcephaly cases caused by the virus.
Culex is 20 times more common there than Aedes aegypti, which until now has been identified as the main vector, moreover.
Constancia Ayres, the entomologist who conducted the research and who insisted that it was risky to focus exclusively on Aedes aegypti as the country struggled to respond to the Zika outbreak, described the development as “very bad news for Brazil.”
“We have a national programme for controlling Aedes, but we have nothing for Culex. So if Culex is an important vector then we have to start from zero.”
Responding to the announcement, Brazil’s Ministry of Health said that it remains convinced that Aedes aegypti is the most important vector.
A ministry spokesperson added that ultimately the findings change nothing because the public-health response is the same regardless of the mosquito species.
Dr Ayres and other entomologists strongly disagree, however. Culex quinquefasciatus has some significantly different behaviours than Aedes aegypti, and those habits necessitate very different response strategies, she said.
For a start, Aegypti bites during the day, while Culex quinquefasciatus is nocturnal, making bed nets, which few Brazilians currently use, the best form of protection.
Aegypti, furthermore, breeds in clean water, while Culex favours polluted water, like the sewage canals that snake through Recife.
“The only explanation for how fast Zika is going through Latin America is that it’s transmitted by more than Aegypti – and that means you do a different response, and you sample for mosquitoes differently,” Fiona Hunter, a professor of entomology at Brock University who is working on Zika transmission in the Caribbean, told the Globe and Mail.
Doubts were also expressed back in March, when several Brazilian and international entomologists expressed public concern over Brazil’s Zika response plan and its focus on Aedes aegypti, saying it was dangerous to be focusing all resources on that vector without more evidence, especially given that this strain of Zika was behaving in markedly different ways from what had been seen before elsewhere.
At that time, the Ministry of Health said the Brazilian response was based on recommendations from the World Health Organization (WHO).
In turn, the WHO said that it was relying on “current epidemiological evidence” even though Zika was not found in an Aedes aegypti mosquito there until May.
Now, in what could turn out to be a futile move, the Ministry of Health is investing in testing genetically modified mosquitoes as a Zika-control strategy – but the species modified is Aedes aegypti.
More Research Needed
Dr Ayres said that more research is necessary to establish which of the two species is the primary vector in Brazil in order to answer questions such as whether one focuses more on humans than on animals, which species bites more often, which transmits more virus in a bite and which lives longer.
Dr Ayres’s research has implications far beyond Brazil: Culex quinquefasciatus, which is known to carry West Nile virus, has a far wider range than Aedes aegypti, which is found only in tropical and subtropical regions.
Meanwhile, Brazil’s Ministry of Health reports 1,709 cases of confirmed congenital Zika, with 102 deaths and stillbirths caused by the virus. More than 3,000 other cases are still under investigation.
In a welcome lull, rates of new Zika infection have fallen with the onset of the Brazilian winter and less of the warm, damp weather that boosts mosquito breeding.