'Concerning' Malaria Surge After Asian Mosquito Invades African City

SEATTLE -- Early this year an alarming surge of malaria was reported out of Dire Dawa, a small urban center in Ethiopia, sparking enough concern that the Ethiopian government called in a research team to determine what was causing the uptick.

From week 2 to week 5 of 2022, malaria cases in the city increased from 27 to 260, and the city ultimately reported 2,400 cases by May, when the area is usually still very dry and malaria infections are rare, according to Fitsum G. Tadesse, PhD, of Armauer Hansen Research Institute in Addis Ababa, Ethiopia.

"Malaria is normally a rural disease in Africa," Tadesse told MedPage Today in an interview at the American Society of Tropical Medicine & Hygiene annual meeting, where research was presented on how the unusual outbreak spread. "In Dire Dawa, that was 10 times the number of malaria cases in a very short time in an urban area. It was very concerning."

After performing tests on 617 mosquitoes in Dire Dawa, the research team determined that 97% were Anopheles stephensi, an insecticide-evading, dry season-thriving mosquito, once seen only in parts of South Asia and the Arabian Peninsula. Of the mosquitoes they tested, 4.5% were infected with Plasmodium falciparum, the protozoan parasite that causes malaria in humans.

In a case-control study conducted to see how the disease was spreading, the research team found that family members of infected individuals were 5.6 times more likely to test positive on a rapid diagnostic test compared with family members of uninfected people (95% CI 3.1-10.0, P<0.001), Tadesse reported.

Furthermore, households with positive larvae in aquatic habitats were 3.4 times more likely to have at least one positive family member versus those without positive larvae habitats (95% CI 1.4-8.1, P=0.006).

"This mosquito is different than the ones that usually transmit malaria parasites in Africa. It thrives in man-made water containers that you have in the expanding cities, where infrastructure development is slow. The containers can be clean or dirty, or they can live in natural habitats too ... this mosquito has high plasticity," Tadesse noted. "This is the mosquito you will find in any lab in the U.S. where they study mosquitoes, because it is so highly adaptable."

As of September 29, the World Health Organization launched an initiative with the Global Malaria Programme to stop the spread of the mosquito, declaring that "the rapid growth of many African cities, coupled with the invasion and spread of this highly efficient and adaptable malaria vector, could undermine the gains made in reducing the burden of the disease."

The CDC and the U.S. President's Malaria Initiative are also currently working on efforts to control the mosquito before it further expands its territory.

Study Details

To determine the extent to which the mosquito had caused the outbreak and how it was spreading disease -- specifically if the mosquitoes were infecting people within households in containers and reservoirs used to store water -- Tadesse and colleagues followed the household family members and contacts in student dormitories of infected patients from Goro Health Center from April to June. They also looked for the presence of the mosquitoes in containers or other aquatic reservoirs.

To establish a control group for the study, family members and friends of 210 people without malaria, also recruited from the health center, were tested for malaria and their households were scanned for evidence of the invasive mosquito.

The team also looked for the mosquito in neighboring villages, and found it in smaller numbers, indicating an extending reach.

Tadesse and team hypothesized that the mosquito migrated on trucks in containers or other trade vehicles from Djibouti, and it could do the same in other parts of Africa.

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