A species of mosquito originally from Asia threatens to put tens of millions of city-dwellers in Africa at higher risk of catching malaria as the invading insect spreads throughout the continent, according to a study published in the Proceedings of the National Academies of Science of the US.
Malaria – which killed 400,000 people in 2018, mainly children in Africa – is caused by parasites that roughly 40 mosquito species spread among humans when they feed.
The Anopheles gambiae group of mosquito species are the main drivers of malaria's spread in Africa, but these insects dislike the polluted puddles seen in cities and haven't learned to lay their larvae in urban fresh water tanks.
For these reasons, most malaria transmission in Africa occurs in rural areas.
In a study, medical entomologist Marianne Sinka, of the University of Oxford, charted the spread of another species, Anopheles stephensi, which originated in Asia. This species has learned to slip through cracks to access water tanks, favouring those made from brick and cement.
"It's the only one that's really good at getting into central urban areas," Sinka told AFP. Anopheles stephensi caused a major outbreak in Djibouti City in the Horn of Africa in 2012, a city where malaria hardly existed, and has since been observed in Ethiopia, Sudan and elsewhere.
Sinka and colleagues combined location data for the species with spatial models that identified the environmental conditions characterizing its preferred habitat: high-density urban areas where it is hot and rainfall is plentiful.
Their study found that 44 cities are "highly suitable" locations for the insect, putting 126m more Africans – mainly around the equatorial regions – at risk of malaria, compared to today. "That means that Africa, which has already got the highest burden of malaria, could have an even bigger impact," said Sinka, with 40% of the continent's population in urban areas.
Unlike African mosquitoes, which like to bite humans when at night when it is cool, Anopheles stephensi can feed in the evening when it is warmer, making bed nets less effective.
So, installing mosquito nets on windows, soaking the walls in insecticides, and covering the body are better ways to protect against this species.
Longer-term, the most effective measure is to target the larvae: eliminate stagnant water and tightly seal water tanks from intrusion. These methods proved effective in India, said Sinka.
In 2012, an unusual outbreak of urban malaria was reported from Djibouti City in the Horn of Africa and increasingly severe outbreaks have been reported annually ever since. Subsequent investigations discovered the presence of an Asian mosquito species; Anopheles stephensi, a species known to thrive in urban environments. Since that first report, An. stephensi has been identified in Ethiopia and Sudan, and this worrying development has prompted the World Health Organisation (WHO) to publish a vector alert calling for active mosquito surveillance in the region. Using an up-to-date database of published locational records for An. stephensi across its full range (Asia, Arabian Peninsula, Horn of Africa) and a set of spatial models that identify the environmental conditions that characterize a species’ preferred habitat, we provide evidence-based maps predicting the possible locations across Africa where An. stephensi could establish if allowed to spread unchecked. Unsurprisingly, due to this species’ close association with man-made habitats, our maps predict a high probability of presence within many urban cities across Africa where our estimates suggest that over 126 million people reside. Our results strongly support the WHO’s call for surveillance and targeted vector control and provide a basis for the prioritization of surveillance.
ME Sinka, S Pironon, NC Massey, J Longbottom, J Hemingway, CL Moyes, KJ Willis
Medical Xpress report