How invasive South Asian mosquitos could threaten malaria elimination efforts in Africa
Anopheles stephensi mosquitoes, which first arrived on the African continent in 2012, have been linked to an unprecedented urban outbreak of malaria in Ethiopia.
- 4 November 2022
- 4 min read
- by Linda Geddes
An invasive insecticide-resistant mosquito native to South Asia is fast expanding its range and may have been responsible for a tenfold spike in malaria infections in Eastern Ethiopia earlier this year.
The discovery, which was reported at the Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) earlier this week, provides some of the most compelling evidence to date that the mosquito could cause infections to surge in areas of Africa with previously low rates of disease.
“This is not like any other malaria-carrying mosquito we have seen in Africa before. [...] 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.”
Most malaria in Africa is transmitted by Anopheles gambiae mosquitoes, which are most prevalentduring the rainy season. However, in 2012, Anopheles stephensi mosquitoes were detected in the small East African country of Djibouti – the first time this species had been recorded in Africa. The mosquitoes, which are the primary transmitter of malaria in urban areas of India and Iran, are thought to have arrived accidentally in shipping containers.
At that time, Djibouti, which has a population of about one million people, had been close to eliminating malaria, but cases have been steadily rising ever since, with a 40-fold increase since 2013. An. stephensi is a prime suspect.
“This is not like any other malaria-carrying mosquito we have seen in Africa before,” said Dr Sarah Zohdy, a disease ecologist with the US Centers for Disease Control and Prevention (CDC). “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.
“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.”
Now, new evidence suggests An. stephensi may also have been responsible for a recent outbreak of malaria in Dire Dawa, a city of about 500,000 people in eastern Ethiopia that typically records only about 200 cases a year.
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Between January and May 2022, when rain was scarce and malaria infections would usually be rare, about 2,400 cases were reported in the city. An investigation by Dr Fitsum Tadesse, a molecular biologist with the Armauer Hansen Research Institute in Addis Ababa, Ethiopia, found that An. stephensi was responsible for the surge.
More worryingly, they also discovered that the mosquito was resistant to common insecticides used to control malaria via treated bed nets and indoor 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 Tadesse.
“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 neighbourhoods.”
The report came as Zohdy announced that the species may now be widespread in and around households across nine states in Sudan. They have also recently been detected in Nigeria, on the other side of the continent.
The concern in that these countries may follow the pattern of Djibouti, where there was a lag of several years between the detection of An. stephensi and an increase in malaria infections, Zohdy warned.
In 2020, there were an estimated 241 million cases of malaria worldwide and 627,000 deaths, with African countries accounting for 95-96% of them.
According to recent estimates, some 126 million additional people could be at risk of malaria if An. stephensi is able to continue its march across the African continent. However, Tadesse warned that this could be an underestimate, as the assumption had been that the species predominantly breeds in containers, such as household water tanks and garden pots. He said they also found plenty of An. stephensi mosquitoes at the edge of streams and nearby rivers.
“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,” said Tadesse.
Stopping their spread will require continued surveillance, as well as efforts to limit breeding sites in urban areas.