Asian mosquito causes malaria outbreaks in African cities

A species of mosquito that arrived from Asia a decade ago appears to be responsible for major malaria outbreaks in Africa. Especially in cities, where malaria was previously rare. This is shown by researchers from Radboud university medical center and others, published in Nature Medicine. The mosquito is spreading rapidly and threatening public health.

Malaria control measures resulted in a gradual reduction in the number of malaria cases in recent decades. However, malaria burden has been increasing again in recent years. In 2022, there was a major malaria outbreak at Dire Dawa University in Ethiopia, with 1,300 students falling ill. This was unexpected, because it was the dry season, while malaria normally typically occurs during the wet season. Even more strikingly, the outbreak occurred in a large urban setting while it is traditionally a disease of rural areas. An international research team unraveled this mystery.

The researchers not only mapped the spread of malaria, but also investigated the mosquitoes in Dire Dawa. They conclude that the number of malaria cases has increased 10-fold in three years. They showed that this increase was caused by a mosquito species, Anopheles stephensi, that has only been living in Africa for a decade. This is the first time this invasive mosquito has been shown to be responsible for an increase in malaria.

Bottle cap

This imported mosquito originally came from India, and probably arrived in Ethiopia in Africa in 2012. The mosquito spread rapidly to countries such as Sudan, Somalia, Eritrea, Yemen, Kenya, Ghana and Nigeria. ‘We didn't find this mosquito in more countries yet, but that doesn't mean it's not there. We may just have not looked hard enough’, says lead researcher Fitsum Tadesse. He is a researcher affiliated to Radboudumc and the Armauer Hansen Research Institute in Ethiopia.

The mosquito A. stephensi is successful in Africa, especially in cities, because of certain characteristics. For example, this mosquito is resistant to high temperatures and drought, and therefore does not depend on the wet season, like traditional African malaria mosquitoes do. In addition, the mosquito is very flexible in picking sites for egg laying and reproduction. Even a small amount of water, such as in a bottle cap on the street or water storage containers provide suitable places to lay eggs.

Problem causer

The fact that this invasive mosquito thrives in cities poses a major threat to public health in Africa. Malaria researcher at the Radboudumc Teun Bousema: ‘Areas where many people live together are highly suitable for malaria outbreaks. People in cities also have little immunity to malaria and can fall seriously ill’. Tadesse adds: ‘Doctors in cities have little experience with diagnosis. And importantly, the number of people living in cities in Africa is growing, we expect half the population by 2030.’

A. stephensi also presents a number of other challenges for African populations. For example, this mosquito can be highly resistant to insecticides. Also, the reported outbreak involves parasites that have acquired mutations that make them both difficult to detect by rapid diagnostic tests and less sensitive to malaria treatment. This makes the mosquito a real problem causer.

Breeding sites

Can A. Stephensi still be eliminated from Africa? ‘It is too late now and this should have been done years ago’, says Bousema. ‘But there is a lot to gain with good policy. That means clearing all possible breeding sites from house to house, measuring properly where mosquitoes and larvae are, and making sure mosquitoes cannot enter water storage. That way we can still achieve much better control of this mosquito.’