At the conclusion of a three-day Zika research and development meeting on Wednesday, WHO's Marie-Paule Kieny said that "evidence is missing" that the classical ways of fighting dengue have made any substantial dent in cases. She says the same challenge might apply to Zika.
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"Everything that was done in the country to control [mosquitoes] apparently didn't work," said Jorge Kalil, director of the Butantan Institute in Sao Paolo, Brazil, who attended the meeting.
"The problem right now is it's very difficult to fight the [mosquito], there are billions and billions of insects."
Kalil said Brazilian officials may try a more targeted approach calling for more involvement from villages and individuals. Kalil was also optimistic that the coming winter season might help reduce mosquito populations.
"Certainly it is worth continuing to try to use this method for the lack of other interventions, but what the scientists said is that there is an urgent need to also put in place studies to evaluate whether it has a benefit or not," Kieny said.
Kieny also noted another possible complication: that other mosquito species beyond Aedes aegypti might spread Zika. She said that while scientists have observed that other mosquito species can carry the virus, it's unclear if they can actually infect people.
Last month, WHO declared the explosive spread of Zika in the Americas to be a global emergency, due to its link to the spike in the number of babies born with abnormally small heads and the rise in a rare neurological syndrome that can cause paralysis and death.
So far, Zika has triggered outbreaks in 41 countries, although confirmed cases linking Zika to babies with birth defects have only been seen in Brazil and French Polynesia. Nine countries have reported a spike in cases of Guillain-Barre syndrome, a neurological condition that typically affects people after infections.
She said work is being done by more than 30 companies to develop a better diagnostic Zika test, since current tests often mix up Zika and dengue infections.