A new malaria vector in Africa: Anopheles stephensi
Anopheles stephensi is gaining ground in Africa. This insect species, native to South Asia and the Arabian Peninsula, carries the two most deadly strains of the plasmodium parasites – falciparum and vivax – that cause malaria. It appeared in the Horn of Africa in 2012, in Djibouti, a country that had almost eliminated the disease, with only 27 recorded cases per year (compared with 73,000 in 2020). Anopheles stephensi has since also been detected in Ethiopia, Sudan, Somalia and, more recently, in Nigeria and Kenya.
Anopheles stephensi shows an alarming resistance to most insecticides, such as DDT and pyrethroids. The invasive species could undermine efforts to control and eliminate malaria in Africa. For the World Health Organization (WHO), the mosquito’s spread is “a major potential threat to malaria control in Africa”, where the disease burden is already particularly high, with 400,000 people dying from malaria in Africa every year.
According to a study by the Liverpool School of Tropical Medicine, the species could endanger 126 million people in Africa if it spreads to the continent’s major cities – a scenario specialists are particularly concerned about. Unlike other mosquitoes that mainly thrive near lakes and swamps, such as Anopheles gambiae, Africa’s most prevalent species, Anopheles stephensi can tolerate pollution and is highly adaptable to different climatic and environmental conditions. It can therefore survive in previously unaffected places, like cities, where it thrives in stagnant water, such as in storage containers. This allows it to breed all year round and remain active in the dry season.
Its behavior is also distinctive. While other African anopheles bite humans at night and usually indoors, stephensi bites at dusk when the air is still warm and does so outside, limiting the effectiveness of insecticidal nets. Other tools, such as indoor residual spraying, may not work to contain this species.
Responding to this challenge will be tough, with many studies on the mosquito still under way. The aim is to gain a better understanding of the new species and to explore the best ways of monitoring its behavior and controlling its spread in already-infested areas. “It must be emphasized that we still don’t know how far this species of mosquito has spread, or what problem it poses or could pose,” says Dr Jan Kolaczinski, who heads the Vector Control and Insecticide Resistance Unit of the WHO’s Global Malaria Programme. While its presence correlates strongly with the sharp increase in malaria cases, the extent to which stephensi could cause malaria outbreaks in other cities remains unknown.
A 2020 model taking into account various parameters (temperature, rainfall, etc.) suggests it could become established in 44 African cities with more than one million inhabitants, such as Bamako, Dakar or Cairo. The WHO is coordinating several initiatives in response to this risk.
One such initiative, launched by the WHO in September 2022, aims to support an effective regional response across the African continent. It is based on five pillars: increased collaboration between countries’ healthcare systems; stronger surveillance to determine the extent of the spread of Anopheles stephensi; improved information-sharing on the mosquito’s presence; the development of guidance for national malaria control programs; and prioritization of research to assess the impact of existing operations and tools. Lessons drawn from dealing with Anopheles stephensi in India, where the mosquito has caused malaria outbreaks in urban areas, could also be useful. For example, introducing strict regulations on water storage has proven effective in curbing the spread of the disease.
The RTS,S vaccine, approved by the WHO in 2021, also offers hope as it is designed to build up antibodies against Plasmodium falciparum, the deadliest form of malaria parasite. The WHO is currently coordinating a pilot programme to distribute the vaccine in parts of Ghana, Kenya and Malawi. In Kenya, a vaccination campaign on the shores of Lake Victoria has already reduced severe cases by 30%. These encouraging outcomes have prompted the WHO to extend the vaccination campaign to other African countries by 2025.