A mosquito species that arrived in Africa from Asia ten years ago appears to be responsible for major outbreaks of malaria there. Particularly in cities, where malaria was previously rare. This is evident from research by, among others, Radboud university medical center, published in Nature Medicine. This mosquito is spreading rapidly and threatens public health.
The number of malaria cases has been declining for decades, but has been increasing again in recent years. In 2022, there was a major outbreak of malaria at Dire Dawa University in Ethiopia, which sickened 1,300 students. Strange, because it was the dry season, while malaria normally mainly occurs during the wet season. In addition, malaria is a disease of the countryside, but Dire Dawa is a big city. An international research team unraveled this mystery.
The researchers not only mapped the spread of malaria, but also looked at the mosquitoes in Dire Dawa. They saw that the number of malaria cases increased tenfold in three years. They show that this increase is caused by a type of mosquito, Anopheles stephensi, who has only been living in Africa for a decade. This is the first time that a specific species of mosquito has been shown to be responsible for an increase in malaria.
This imported mosquito originally came from India, and probably arrived in Ethiopia in Africa in 2012. The mosquito quickly spread to countries such as Sudan, Somalia, Eritrea, Yemen, Kenya, Ghana and Nigeria. ‘We have not yet found this mosquito in more countries, but that does not mean that it is not there. We may not have measured well enough,” says lead researcher Fitsum Tadesse. He received his PhD from Radboud university medical center and now works at the Armauer Hansen Research Institute in Ethiopia.
The mosquito A. stephensi is so successful in Africa, especially in the cities, because of certain characteristics. This mosquito is resistant to high temperatures and drought, and is therefore not dependent on the wet season, like mosquito species that have lived in Africa for some time. In addition, the mosquito is very flexible in terms of breeding sites. A small amount of water, such as in a bottle cap on the street or in water storage tanks, provides a suitable place for laying eggs.
The fact that this invasive mosquito thrives so well in cities poses a major threat to public health in Africa. Malaria researcher at Radboud university medical center Teun Bousema: ‘Many people live together in cities and that increases the risk of malaria outbreaks. These city people have little natural resistance due to previous exposure and are violently ill.’ Tadesse adds: ‘Furthermore, doctors in cities have little experience with diagnosis. And finally, the number of people living in cities in Africa is growing, we expect half of the population by 2030.’
A. stephensi also presents a number of other challenges for the African population. For example, this mosquito is quite resistant to insecticides. The malaria parasites that this mosquito transmits also often have an abnormality in their DNA, which makes conventional anti-malarial medications less effective. And half of the parasites lack a protein, which means that the available rapid test for malaria no longer works. This makes the mosquito a real problem.
Can A. Stephensi yet to be exterminated in Africa? “Actually, it is already too late and this should have been done years ago,” says Bousema. ‘But we can still achieve a lot with good policy. This means cleaning up all possible breeding sites from house to house, carefully measuring where mosquitoes and larvae are, and ensuring that mosquitoes cannot access water storage. This way we can get this mosquito even better under control.’
Photo: Tiksa Negeri
About the publication
This article was published in Nature Medicine: Anopheles stephensi is implicated in an outbreak of Plasmodium falciparum parasites that carry markers of diagnostic resistance and candidate artemisinin resistance in Dire Dawa City, Ethiopia, January–July 2022. Tadele Emiru, Dejene Getachew, Maxwell Murphy, Luigi Sedda, …, Teun Bousema , Fitsum G Tadesse.