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Invasive mosquito species linked to unusual malaria outbreak

Scientists have linked an invasive mosquito to an unusual outbreak of malaria in Ethiopia. Anopheles stephensi, native to southern Asia, was first identified in Africa a decade ago in the Republic of Djibouti, which borders Ethiopia. It has since spread to at least four other countries in sub-Saharan Africa.

Now, amid lingering questions about whether the insect’s presence is significantly driving cases of malaria on the continent, researchers have confirmed that people infected in an uncommon dry season outbreak of the disease were more likely to have the mosquito living close to their homes.

The find, reported on 01 November at the American Society of Tropical Medicine and Hygiene (ASTMH) meeting in Seattle, is the most direct evidence yet that ties the invasive insect to increasing malaria cases, says Martin Donnelly, an evolutionary geneticist at the Liverpool School of Tropical Medicine (LSTM) who was not involved in the study. “It is a big step forward” in understanding how An. stephensi is likely to make malaria much more difficult to control in Africa, where it kills more than half a million people each year, most of them children under age 5.

Unlike most of the mosquitoes in Africa that transmit the parasites that cause malaria, An. stephensi is a city dweller. Most African mosquitoes lay their eggs in rainy-season puddles, but it thrives in artificial water sources such as cisterns and barrels of clean drinking water. That enables the insect to stay active during dry seasons, which traditionally provide a respite from the disease.

In Djbouti and elsewhere, malaria cases went up at the same time that An. stephensi was identified, but scientists were not sure whether the new mosquito or other factors were to blame. To better understand the role of the invasive insect, Fitsum Tadesse, a molecular biologist at the Armauer Hansen Research Institute in Addis Ababa, Ethiopia, and colleagues tracked cases of malaria in Dire Dawa, a city in eastern Ethiopia that experienced a large outbreak earlier this year. In 2019, the city only had 205 cases all year. This year, Dire Dawa recorded more than 2,400 cases in the first half of the year.

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