Wanted, dead or alive. In Kenya, Anopheles stephensi is now the number one enemy of health authorities. This malaria-carrying mosquito, which has spread to several East African countries in recent years, was first spotted in December 2022 in Turkana and Marsabit counties, two desert territories 600 km north of Nairobi, where hopper samples were collected.
In a country with 3.4 million cases of malaria per year, the presence of this invasive species, more dangerous for humans than other Anopheles mosquitoes, alarms specialists. “If we let it reach metropolises like Nairobi, it will be a sign that malaria is back in our cities. This is our greatest fear,” said Dr. Omar Ahmeddin, head of the Ministry of Health’s malaria program.
Native to Southeast Asia, Anopheles stephensi appeared in the Horn of Africa a decade ago in Djibouti, before gradually reaching Ethiopia, Somalia and now Kenya. “This is one of the most significant developments in malaria in the last fifty years,” said Seth Irish, an entomologist at the World Health Organization (WHO).
Ineffective larvicides
According to a study by the Liverpool School of Tropical Medicine, the species could threaten 126 million Africans if it spreads to major cities across the continent. But the counter-attack promises to be all the more difficult to carry out as the behavior of the mosquito is still the subject of studies, notes the WHO in a document from September 2022 which describes the strategy for the fight against malaria.
Not only does Anopheles stephensi carry the two deadliest strains of plasmodium parasites (falciparum and vivax) that cause malaria, but it is resistant to common chemicals. So far, larvicides and indoor spraying have proved largely ineffective.
Finally, Anopheles stephensi adapts very well to different climatic and environmental conditions, spreading rapidly in areas normally spared, at altitude or in cities. The mosquito is particularly fond of stagnant water (basins, puddles, cisterns, flowerpots, rice fields, springs or jerrycans) and is not repelled by pollution, unlike the classic Anopheles mosquitoes (Anopheles gambiae and Anopheles funestus), which proliferate especially near lakes and swamps, far from built-up areas.
The spread of the species inside the major cities of Kenya would prove catastrophic. “In Nairobi and the highlands, a large part of our population is not immune to malaria, especially children under 5 years old. There could be very severe cases, ”fears Dr. Patrick Amoth, director general at the Ministry of Health.
The Djiboutian precedent is particularly worrying in this respect. As the small state was poised to eradicate malaria in 2012, with only 27 cases recorded, the sudden arrival of Anopheles stephensi caused the number of cases to skyrocket.
Vaccine campaign
To try to stem the phenomenon, the Kenyan Ministry of Health has planned to distribute 18 million mosquito nets across the country in 2023. But this measure could quickly prove useless insofar as Anopheles stephensi does not bite the night but more in the afternoon and at dusk.
“We are continuing to spray insecticides and distribute mosquito nets, and we are focusing our efforts on educating the people of northern Kenya, telling them never to let water sit on the surface in their jerry cans or reservoirs, because that’s where the danger lies,” says Patrick Amoth, who is coordinating the response in the north of the country.
Kenya is one of the three pioneer countries (along with Ghana and Malawi) in which the first malaria vaccine is being tested. The vaccination campaign, which takes place on the shores of Lake Victoria, has led to a 30% reduction in severe cases. Encouraging results that prompted the WHO and the government to extend the vaccination campaign to 25 cantons in early March. Other African countries could benefit from the same program next year.
Could the appearance of Anopheles stephensi put a halt to efforts to fight malaria? “It is still early to draw conclusions, the most important thing is to quickly fight against the mosquito as soon as it appears,” says Dorothy Achu, head of tropical diseases at the WHO Africa office. Although significant resources are deployed in vector control, malaria still kills nearly 619,000 children a year in Africa. The continent records 96% of deaths in the world.