Larval stage co-occurrence and ecological niche partitioning of invasive Anopheles stephensi relative to native malaria vectors in Kenya

Malaria remains one of the most persistent challenges to global health, particularly in sub-Saharan Africa, where the vast majority of cases and deaths occur. For decades, efforts to control the disease have relied on tools like insecticide-treated nets and indoor spraying to stop the mosquitoes that carry the parasite. However, the mosquitoes themselves are adaptable, evolving resistance to chemicals and shifting their behaviors to survive. A newer and more immediate threat has emerged with the arrival of Anopheles stephensi, an invasive mosquito species originally from Asia. Unlike the native mosquitoes that typically breed in natural, temporary pools of water like rain puddles or hoof prints, this newcomer has shown a strong preference for man-made water containers. Its spread across Africa raises concerns that it could establish itself in new areas, potentially bringing malaria to regions previously safe from the disease, including urban centers and highland areas where the native vectors struggle to survive.

In Kenya, the first detection of this invasive mosquito occurred in 2022, followed by sightings in 2023 and 2024. To understand how this species is spreading and where it might go next, a team of researchers from the University of Nairobi, the Kenya Medical Research Institute, and international partners launched a large-scale surveillance effort. They worked with Community Health Promoters, local residents trained to spot and collect mosquito larvae, to monitor breeding sites across the country. The goal was to map the habitats where the invasive mosquito thrives and to see how it interacts with the native mosquitoes that have long been the primary carriers of malaria in the region. By comparing the environments where these different species live, the researchers hoped to identify specific features that could be targeted to stop the spread before it becomes unmanageable.

The study covered 61 locations across 21 counties, stretching from the arid north to the coastal regions. Over a period of nine months, the health promoters conducted thousands of surveys, dipping nets into water sources to collect mosquito larvae. These samples were then sent to a central laboratory where scientists used genetic testing to identify exactly which species they were dealing with. The results revealed a clear pattern in the landscape of the invasion. While the native malaria mosquito, Anopheles arabiensis, remained the most common species found overall, the invasive Anopheles stephensi was present in nearly a quarter of the collections. The data showed that this newcomer was not just appearing randomly; it was consistently found in specific types of environments that were quite different from those favored by the native species.

The researchers found that the invasive mosquito has a distinct preference for deep water storage tanks, particularly those located near animal shelters. In both rural villages and urban towns, these tanks were the most likely place to find the larvae. The study showed that water storage tanks were nearly three times more likely to harbor the invasive mosquito compared to other water sources. Furthermore, the presence of these tanks within 100 meters of where livestock are kept was a strong indicator of the mosquito's presence. This suggests that the species is taking advantage of the infrastructure people and animals use to store water, a resource that is essential in many parts of Kenya but also serves as a perfect breeding ground for this specific mosquito.

In contrast, the native mosquitoes were found in different settings. They were less likely to be in deep, man-made tanks and more likely to be in natural water bodies, shallow puddles, or areas with vegetation. The statistical analysis confirmed that the invasive and native mosquitoes rarely shared the same breeding sites. When the researchers looked at the data, they found a strong negative association between the two, meaning that if one species was present, the other was usually absent. This separation suggests that the invasive mosquito is carving out a unique ecological niche, one that the native mosquitoes are not currently using. This separation is significant because it means the invasive species is not simply competing with the locals for the same resources; it is exploiting a different part of the environment entirely.

The study also examined how the mosquito's presence changed with elevation and other environmental factors. The invasive mosquito was found to be less common in high-altitude areas, suggesting that temperature might limit how far up the mountains it can travel. It also preferred water that was clear and free of algae, and it avoided habitats that were heavily shaded or covered in vegetation. These preferences align with what has been observed in other parts of Africa where the species has invaded, reinforcing the idea that it is a container-breeding mosquito that thrives in human-modified landscapes. Interestingly, the study found that the mosquito was just as likely to be found in rural areas as in cities, challenging the assumption that it is purely an urban problem. In many rural parts of Kenya, households rely on similar water storage tanks as those in towns, providing the same ideal conditions for the mosquito to breed.

The researchers also looked at how the invasive mosquito interacted with other native species, such as Anopheles coluzzii and Anopheles gambiae. The data showed that the invasive mosquito rarely shared breeding sites with Anopheles coluzzii, indicating another layer of separation. However, the relationship with Anopheles gambiae was less clear, partly because this species was very rare in the samples collected. The lack of a strong pattern here does not necessarily mean they coexist comfortably; it may simply reflect that the study did not collect enough of this specific native species to draw a firm conclusion. What is clear, however, is that the invasive mosquito is establishing itself in a way that is distinct from the dominant native vectors.

These findings have important implications for how malaria control efforts are conducted in Kenya. Because the invasive mosquito relies so heavily on water storage tanks, particularly those near animal shelters, these specific sites offer a clear target for intervention. Unlike natural breeding sites like swamps or rain puddles, which are difficult to manage on a large scale, water tanks are finite, identifiable, and often owned by individuals. This makes them a practical focus for community-based efforts to reduce mosquito populations. The study suggests that targeting these containers with larvicides or by covering them could disrupt the spread of the invasive mosquito before it becomes firmly established in new areas. The use of local health promoters to monitor these sites proved effective, demonstrating that a community-led approach can provide the detailed, repeated data needed to track an invasive species.

The researchers caution that the situation is not static. As the mosquito population grows and faces pressure from control measures, it may adapt and begin to use other types of habitats. The current preference for water tanks is a snapshot of the species at a specific stage of its invasion. Therefore, control programs must remain flexible and continue to monitor for signs that the mosquito is shifting its behavior. The study concludes that while the invasive mosquito is currently exploiting a niche that is underused by native vectors, this window of opportunity for containment depends on sustained surveillance and the willingness of communities to manage their water storage. By understanding exactly where this mosquito lives and how it differs from the native species, public health officials can deploy resources more effectively to protect communities from a new and evolving threat.