NAIROBI, Kenya — The invasive mosquito species Anopheles stephensi, a known vector for malaria transmission in Asia and the Middle East, is rapidly spreading across northern Kenya, according to a new scientific study. First confirmed in the country in December 2022, the mosquito has now been identified in seven of 18 surveyed counties, predominantly along major transportation routes.
Kenyan researchers, in collaboration with the National Malaria Control Program (NMCP), conducted extensive entomological and genetic surveillance between January 2023 and June 2024, confirming 114 larval and 33 adult specimens of An. stephensi across counties including Turkana, Marsabit, Mandera, Samburu, Isiolo, Wajir, and Elgeyo Marakwet.
Genetic analyses of these mosquito populations uncovered three distinct genetic compositions, indicating multiple introductions of the species into the country. While most populations mirrored those found in southern Ethiopia, Turkana mosquitoes exhibited a unique genetic signature. Researchers warn that this genetic diversity could complicate control efforts.
“This pattern of spread is concerning,” said Dr. Mercy Kimani, a vector biologist involved in the study. “The ability of An. stephensi to thrive in urban and peri-urban settings, especially by breeding in artificial water containers, puts more people at risk—especially in cities traditionally considered low-risk for malaria.”
A Threat to Urban Areas
Unlike native African malaria vectors, An. stephensi thrives in environments altered by human activity, such as construction zones and densely populated towns. Its presence in Kenya echoes trends seen in other countries. In Djibouti, the mosquito’s arrival in 2012 coincided with a sharp rise in malaria cases. A similar surge was recorded in Ethiopia following the mosquito’s detection there in 2016.
The World Health Organization (WHO) has classified the rapid expansion of An. stephensi as a major threat to malaria control in Africa, prompting the launch of a dedicated initiative in 2022. An updated vector alert was issued in 2023.
Larval Sampling Reveals Urban Predominance
Larval sampling across the 18 counties revealed that 59% of identified breeding sites were in urban and peri-urban environments. Common breeding habitats included discarded car tires, plastic containers, water storage tanks, and pits at mining sites. In some areas, An. stephensi coexisted with native species like An. arabiensis and culicine mosquitoes, complicating vector control strategies.
“The detection of An. stephensi in counties like Elgeyo Marakwet suggests the mosquito is adapting quickly and may already be spreading beyond what’s currently documented,” added Dr. Kimani.
Challenges in Adult Sampling and Trapping
While larval samples were more readily found, adult sampling proved challenging. Only 33 adult An. stephensi mosquitoes were confirmed via molecular analysis over two years. The evaluation of various trapping methods in Marsabit County—considered a hotspot—showed mixed results. UV light traps and CDC light traps had limited success, while other methods such as human landing catches and BG-Sentinel traps yielded few specimens.
Mapping the Threat: Predictive Modeling Shows Wider Range
To guide future surveillance, scientists developed a predictive model identifying other high-risk areas for An. stephensi expansion. Using climatic, environmental, and population data, the model revealed that large swathes of Kenya—particularly urban regions with low rainfall and stable temperatures—may soon become suitable habitats for the mosquito.
“There are likely more counties at risk than those where we’ve confirmed presence,” said Dr. Wanjiru Otieno, a spatial epidemiologist involved in the modeling. “This underscores the urgency for proactive surveillance and targeted vector control.”
Urgent Need for Surveillance and Collaboration
The researchers emphasized the importance of improving sampling strategies, particularly for adults, and enhancing genetic monitoring to trace new introductions. They also called for cross-border collaboration to combat the transnational spread of the vector, especially along transportation corridors linking Kenya to Ethiopia and Somalia.
As malaria remains a significant public health challenge—accounting for an estimated 669,000 deaths globally in 2022, 94% of them in Africa—the growing footprint of An. stephensi in Kenya could derail progress in malaria control if not addressed promptly.
“Kenya must prioritize this threat,” said Dr. Otieno. “With informed surveillance and regional coordination, we still have a chance to contain this invasion before it reshapes malaria epidemiology in East Africa.”
About The Author
