(CTN News) – More than 2,400 people became sick with malaria in the Ethiopian city of Dire Dawa in early 2022. An invasive mosquito species that is spreading across Africa is responsible for the spike in infections, according to scientists.
Presented at the American Society of Tropical Medicine and Hygiene annual meeting on November 1, the study provides evidence that the invasive vector can contribute to malaria outbreaks.
It is concerning to note that the species can thrive in urban environments, thus exposing millions of people to the threat of malaria.
Mosquito Anopheles stephensi, native to India and the Persian Gulf, is a major vector for Plasmodium parasites, which cause malaria in humans (SN: 10/26/20).
Africa is home to the primary malaria vector, Anopheles gambiae. In 2012, Asa stephensi was first reported on the African continent in Djibouti. Since then, the species has turned up in other African countries such as Ethiopia, Somalia and Nigeria.
It wasn’t clear what kind of burden the invasive mosquito could bring to Africa, says Fitsum Girma Tadesse, a molecular biologist at the Armauer Hansen Research Institute in Addis Ababa, Ethiopia.
In the eight years after the mosquito’s arrival in Djibouti, the country reported a 40-fold increase in yearly malaria cases, Tadesse says. But no one had directly linked A. stephensi to the increase.
The researchers tracked 80 patients in the city who had sought care for at a local or university clinic, as well as 210 patients who had sought treatment for other reasons, and they screened the patients’ household members for malaria.
The team also scanned the patients’ neighborhoods for the presence of mosquito adults and larvae within a 100-meter radius of households, or in the cases of students that visited a clinic, dormitories.
The team found that the malaria patients primarily lived near water sources used by the invasive mosquito, A. stephensi. Households and dorms close to aquatic habitats harboring A.
stephensi larvae were 3.4 times as likely as those not close to such water sources to have a family or dorm member test positive for malaria.
And most of the adult mosquitoes the team caught — 97 percent — were of the invasive species, the only mosquito species that the researchers found carrying Plasmodium parasites.
A. stephensi breeds in water storage containers that are common in rapidly expanding urban areas, says Tadesse. In rural areas, the native mosquito species, Aedes gambiae, often uses natural water sources such as small pools, which are more prevalent in rural areas.
Therefore, the concern is that with the expansion of A. stephensi along with urbanization in Africa, the mosquito may be able to exploit many new water sources.
It expands the problem from a predominantly rural problem to an urban one, according to Teun Bousema, an epidemiologist at Radboud University in Nijmegen.
Another study from 2020 estimated that an additional 126 million people in cities would be at risk of contracting malaria if the invasive mosquito spread widely across the continent.
When was the biggest malaria outbreak?
Over this period we see a clear rise-peak-fall trend, increasing from around 850,000 deaths in 1990; peaking at around 965,000 in 2004; and then declining (although at varying rates) to around 650,000 in 2019.
