Scientists are warning that the invasion of an insecticide-resistant mosquito could change Africa’s “landscape of malaria” after research showed it caused an unprecedented urban outbreak in Ethiopia.
An investigation into a steep rise in cases in the Ethiopian city of Dire Dawa during a dry season this year identified the mosquito as the cause of the outbreak. Scientists said there is strong evidence that it could prompt surges of malaria in areas typically less affected by the disease.
The eastern city of about 500,000 people is not usually a malaria hotspot, recording just 205 cases in 2019, the scientists said.
However, between January and May this year, when rains were patchy and infections were expected to be low, there were about 2,400 cases.
Research presented this week at the annual meeting of the American Society of Tropical Medicine and Hygiene (ASTMH), showed most (97 percent) of the adult mosquitoes collected in the area were Anopheles stephensi, a mosquito usually found in India and Iran, which is thought to have arrived in Africa in 2012.
Most of Africa’s 228 million annual cases of malaria are still caused by the Anopheles gambiae mosquito, whose population ebbs and flows seasonally, thriving in the rainy months, particularly in rural areas, but receding as a threat in the dry season.
However, since it arrived in Africa, Anopheles stephensi has alarmed health officials with its ability to resist some standard insecticides and survive all year in natural water sources and pools that have collected in human-made storage containers.
“Our findings indicate it can survive anywhere, so we need to be looking for this mosquito in places where, previously, people were not expecting to find it,” said Fitsum Tadesse, a lead scientist at the Armauer Hansen Research Institute in Addis Ababa.
“Malaria in Africa is typically associated with rainy seasons in rural areas, but this mosquito produced a 10-fold spike in infections in just three weeks in an urban area during a dry season,” he added.
Once seen only in parts of south Asia and the Arabian peninsula, the Anopheles stephensi mosquito was detected in Africa in Djibouti in 2012.
That year, the tiny coastal country was edging towards elimination of malaria and reported just 27 presumed or confirmed malaria cases. By 2020, the caseload had exceeded 73,000 .
Anopheles stephensi has since been found in four other African countries, including Nigeria, on the other side of the continent. In September, the WHO said it was launching an initiative aimed at stopping the spread of the insect.
“This mosquito’s ability to persist in the dry season and in urban environments has the potential to alter the landscape of malaria in Africa,” US Centers for Disease Control and Prevention staff member Sarah Zohdy said.
“It could cause malaria to expand from a predominantly rural disease to a rural and urban challenge that also affects Africa’s rapidly growing and densely populated cities, where infection rates have been comparatively low,” she added.
More than 40 percent of the continent’s population live in urban environments, and in 2020 a study projected that 126 million additional people in African cities could be at risk of malaria as a result of the Anopheles stephensi mosquito.
However, Tadesse warned that if the mosquitoes of Dire Dawa were elsewhere, that could be an underestimate.
In 2020, approximately 627,000 people, mostly children under five years old, died of malaria, according to the WHO, 96 percent of whom were in sub-Saharan Africa.
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