Zika virus, the pathogen once obscure now widely feared to cause birth defects and other problems, has spread far very quickly since an outbreak was first noticed in northern Brazil in early 2015 it reached more than 40 countries throughout the Americas, even making the Cape Verde islands off the west coast of Africa. More than a million people have been infected.
As public health officials are trying to contain the epidemic, researchers are racing to answer a key question with important implications for which risk areas, and what methods might work to slow its spread: What mosquitoes transmit the virus? Responding to the question is a challenge. Scientists need evidence of both mosquitoes bred and wild caught laboratory to make the case that a given species is guilty.
Last week, a Rio de Janeiro team announced it had caught several Aedes aegypti infected Zika first-infected mosquitoes found in Brazil. Species, the yellow fever mosquito, has long been the main suspect, but some scientists believe that the virus must have Zika other carriers have spread so quickly and have field studies and laboratory courses for solve the problem. Until the evidence is, "we must not jump to conclusions," said Duane Gubler, a virologist at the medical school Duke-NUS in Singapore.
A. aegypti won suspicion because it spreads dengue and chikungunya and yellow fever is common in urban areas of Brazil where major outbreaks occurred and throughout Latin America. But evidence of wild mosquitoes infected Zika was lacking. It is more difficult than one might expect to find them. In dengue outbreaks, says Sander Koenraadt, entomologist at Wageningen University in the Netherlands, researchers generally find that less than 1% of the sampled mosquitoes are infected with the dengue virus, even when people get sick. "You have to watch a lot of mosquitoes to find [infected ones]," Gubler said. Mosquitoes "infect people and die before anyone shows up at the hospital" with the symptoms of the disease, said Oliver Brady, an entomologist at the University of Oxford in the UK.
For insects to transmit a virus they must infected blood from a human or animal and become infected themselves. The virus then travel from the gut to their saliva. Only a few species are susceptible to particular viruses.
To test whether a given species is able to transmit a virus, researchers feed on insects on the infected blood in the laboratory and a week later to collect saliva from them. If saliva contains infectious virus, the species is considered a "competent" vector. Not all competent laboratory vectors spread the disease, however. This depends on several factors such as the frequency with which species stings if it feeds mainly on humans or other animals, and how long he lives. To confirm that a species transmit the disease, researchers also need to find mosquitoes infected with the virus in nature.
You have to watch a lot of mosquitoes to find [infected ones].
the team that reported the first mosquitoes Zika-infected in Brazil, led by Ricardo Lourenço de Oliveira, entomologist at the Oswaldo Cruz Foundation (Fiocruz) in Rio de Janeiro, vacuumed mosquitoes from homes and streets in parts of Rio de Janeiro where people lived complain Zika symptoms. Over 10 months, they collected more than 1,500 mosquitoes, identified and tested pooled samples of the same sex and species for the presence of Zika and other viruses. Nearly half were A. aegypti , and most of the others were Culex quinquefasciatus , another common mosquito in urban Brazil. About 5% were other species. A species called A. albopictus , widely known as the Asian tiger mosquito, which can also transmit Zika in the laboratory and was found infected by the virus in Mexico and Gabon, made only about 2% of the capture, Lourenço de Oliveira said. They found Zika virus in three sets of women A. aegypti mosquitoes, but none of the other species.
The absence of viruses in C. quinquefasciatus is somewhat reassuring, Lourenço de Oliveira said, but the case is not closed. Constância Ayres, entomologist at Fiocruz in Recife, Brazil, said his laboratory has evidence that the species is a possible vector; they found the Zika virus in the saliva of C. quinquefasciatus who had fed infected blood. (His team presented its work for publication.)
Lab tests can be misleading, however. "There is a classic discrepancy between what you see in the lab and what happens in nature," says Brady. " albopictus and aegypti are both very competent in the laboratory "as vectors of dengue." But in Europe, where we have generalized albopictus and almost no aegypti , you do not have huge dengue outbreaks. "
Ayres and others are still looking for Zika in nature. She and her colleagues have collected and identified over 5,000 mosquitoes in the Recife region since March, from homes where patients lived Zika confirmed and emergency care centers. it expects the money to the promised subsidy before it can perform the reaction tests polymerase chain to find the virus mosquitoes are carriers, she said.
Culex mosquitoes transmit several viruses related to Zika, and it would not be particularly surprising if both Culex and Aedes species could spread Zika, Ayres said. Gubler should Culex is a plausible carrier. He noted that several relatives Zika propagated by Culex mosquitoes, including West Nile virus, targeting the nervous system, which also seems to Zika.
If Culex Mosquitoes can transmit Zika virus, which will slow its spread even more difficult. C. quinquefasciatus is located as far north as Iowa and Indiana in the US, although people there are protected by mosquito nets and other factors. In Latin America, most vector control methods are targeted A. aegypti . These efforts have made barely a dent in curbing the spread of the virus so far Zika said Paul Reiter, an entomologist at the Pasteur Institute in Paris. Targeting multiple vectors at once will only make the task more difficult. "If [ C. ] quinquefaciatis is a vector," he said, "we can not say enough about mosquito control."
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