By Sharon Atieno

Experts raise alarm that the malaria landscape in Africa could change as a new study links a mosquito native to South Asia, India and the Persian Gulf specifically, to an urban outbreak of the disease in Ethiopia.

The outbreak occurred in Dire Dawa, a city in eastern Ethiopia that typically records only about 200 cases a year. But between January and May 2022, when rains are scarce and infections are particularly rare, about 2,400 cases were reported.

The study presented during the Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH), found that the surge was caused by a mosquito known as Anopheles stephensi (An. stephensi), which is also resistant to common insecticides used to control malaria via treated bednets and indoor insecticide spraying.

“Malaria in Africa is typically associated with rainy seasons in rural areas, but this mosquito produced a 10-fold spike in malaria infections in just three weeks in an urban area during a dry season,” said Fitsum G. Tadesse, PhD, a molecular biologist with the Armauer Hansen Research Institute in Addis Ababa.

“Also, unlike the mosquitos that typically transmit malaria parasites in Africa, this one is best known for its ability to thrive in man-made water storage containers like what you see in rapidly expanding urban neighborhoods.”

Most malaria in Africa is caused by a mosquito species known as Anopheles gambiae, with populations rising and falling with the rainy seasons that swell rural waterways. An. stephensi mosquitoes have long been the primary transmitter (vector) of malaria in urban areas of India and Iran. But they were never seen in Africa until 2012, when they were reported in Djibouti, one of Ethiopia’s neighbouring countries.

An. stephensi mosquitoes likely arrived in shipping containers, Tadesse said, but they did not appear to be a significant threat until 2020 when they were suspected as the cause of the steadily rising number of malaria infections in Djibouti since 2013.

This new evidence, coupled with recent evidence of An. stephensi mosquitoes in Nigeria, West Africa are raising concerns about a changing malaria landscape in Africa which suffers 95 percent of the world’s 627,000 annual malaria deaths.

“We should be careful about considering it as just a container breeder,” Tadesse said. “It can survive in man-made or natural water sources. It also can feed on animals or humans. Basically, 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.”

Sarah Zohdy, PhD, a disease ecologist and An. stephensi expert with the U.S. Centers for Disease Control and Prevention (CDC) who works with the U.S. President’s Malaria Initiative (PMI), a key partner for the Ethiopia study, said A.stephensi is not like any other malaria-carrying mosquito seen in Africa before.

“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,” she said. “It could cause malaria to expand from a predominantly rural disease to both a rural and urban challenge that also impacts Africa’s rapidly growing and densely populated cities, where infection rates have been comparatively low.”

“We’re fortunate that this team moved quickly to confirm in just a few months that An. stephensi mosquitoes were the source of an unusual dry season urban outbreak,” said ASTMH President Daniel Bausch.

“Robust surveillance to detect and investigate unusual outbreaks in African countries is essential for developing strategies to stop this invasive mosquito from derailing Africa’s efforts to eliminate malaria.”

Meanwhile, a separate study presented at the ASTMH Annual Meeting reported new evidence that An. stephensi mosquitoes are abundant in and around households across nine states in Sudan.

Zohdy said the concern with the detection of An. stephensi mosquitoes in Sudan—and also Nigeria—is that they could follow the pattern of Djibouti, where there was a lag of several years between the mosquito’s detection and its link to a surge of malaria infections.

She said that CDC and PMI, which is co-implemented by United States Agency for International Development (USAID) and CDC, are actively working to mitigate the threat of An. stephensi by utilizing enhanced vector and disease surveillance. PMI and CDC are also collaborating with the World Health Organization (WHO), which just launched an initiative last month to stop the expanding range of An. stephensi in Africa. In addition, PMI is leading the deployment of interventions informed by An. stephensi’s unique biology, including the development of ways to limit breeding sites in urban areas.