Insecticide-resistant newcomer caused unprecedented urban outbreak in Ethiopia and can survive the dry season, scientists say
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 say it is the strongest evidence to date 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. But between January and May this year, when rains were patchy and infections would be 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%) 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 228m 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.
But since it arrived in Africa Anopheles stephensi has alarmed health officials with its ability to resist some standard insecticides and survive all year, round, particularly in pools that have collected in human-made water storage containers often seen in urban neighbourhoods. It is also found in natural water sources.
“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 G Tadesse, from 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 World Health Organization said it was launching a new 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,” said Sarah Zohdy, from the US Centers for Disease Control and Prevention.
“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.”
More than 40% 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. But 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% of whom were in sub-Saharan Africa.