“Growing” malaria mosquitoes have gotten more and more tough to manage

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Anopheles malaria mosquitoes in Africa and Latin America evolve in response to human activity, studies on both continents have shown.

Anopheles gambiae, the major malaria vector in Africa, is developing increasing resistance to pyrethroid insecticides, according to a study published in Nature Scientific Reports this month. Synthetic pyrethroids are the most common insecticide used to control malaria mosquitoes in the world.

The research team from Kenya, Ghana and the US says the indiscriminate use of pyrethroid insecticides in agriculture and health programs could be responsible for the moderate and high-intensity resistance.

The team collected mosquitoes in western Kenya and analyzed the results of a number of tests, including exposure of the mosquitoes to insecticides over several generations.

Anopheles gambiae, the main malaria vector in Africa. Photo credit: Jim Gathany, USCDCP / Pixnio (public domain image).

While the younger generations showed greater insecticide resistance, the researchers also found that reducing the use of the chemicals for at least 15 generations reduced resistance. This restored the mosquito’s susceptibility to the pyrethroid insecticides in a process that took an average of two years.

“Health officials should change insecticides every year to combat mosquito resistance,” lead author Maxwell Machani, entomology expert at the Kenya Medical Research Institute, told SciDev.Net.

“The best way to control this mosquito is to improve forest management and avoid illegal settlements in green spaces.”

Laura Cristina Multini, University of São Paulo

However, Maria Anice Mureb Sallum, a professor in the Faculty of Public Health at the University of São Paulo, says deeper research is needed to generalize the results.

“Other An. Gambiae populations could be tested in other regions of Africa with different insecticide exposure to see if the same mechanism works, ”says Sallum.

City adaptation

A study in Brazil published last month (October 22) shows that Anopheles cruzii, the primary malaria vector in Brazil’s Atlantic forest region, is increasingly adapting to urban environments. This region includes São Paulo and Rio de Janeiro, the most populous cities in the country.

The researchers suspect that deforestation and the invasion of human settlements into the forest promote genetic diversity among the populations of An. cruzii. Because the mosquito has fewer food sources such as monkeys in its natural habitat, it is more common in cities that forage among humans.

“This leads to their genetic structure becoming more diverse, which is what the An. cruzii more strongly represented in urban environments, ”says lead author Laura Cristina Multini, epidemiologist at the University of São Paulo.

The team is not yet aware of the possible effects of this genetic variation, but the study suggests an increased likelihood of malaria outbreaks in cities in the Atlantic forest.

The researchers analyzed mosquitoes collected in urban, peri-urban and rural areas of Parelheiros south of the city of São Paulo. They found greater genetic variation between mosquitoes in the ground than in tree tops – suggesting an ongoing evolutionary process.

Since it can fly long distances, the An. cruzii is hard to control, Multini told SciDev.Net.

“The best way to control this mosquito is to improve forest management and avoid illegal settlements in green spaces,” she says. If more animals need to be fed, the insect is likely to stay in the canopy, its preferred habitat.

Studies across Brazil are needed to understand if similar patterns occur elsewhere, Sallum said. Sallum, who did not take part in either research, says both studies suggest that human activities promote mosquitos’ adaptability.

“Since human actions force a response from mosquitoes, we ultimately influence their development,” says Sallum.

The research carried out in Brazil was supported by FAPESP, a SciDev.Net donor.

This piece was produced by SciDev.Net’s desk for Latin America and the Caribbean.

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