[seminar blog] Surveillance of Invasive Mosquito Vectors in East Africa

written by: Michael Adu-Brew

When mosquitoes invade new regions, they leave behind a genetic record. Each invasion is preserved within their DNA, detailing stories of survival, adaptation, and their potential role in driving the emergence of disease outbreaks. This makes it crucial to look into the genes of these introduced or invasive mosquitoes. Dr. Tamar Carter's research delves into the genomes of malaria and dengue mosquito vectors in East Africa. Her work sheds light on their origins, movement patterns, and implications for disease transmission. The findings from her research establish a framework for the development of evidence-based strategies and provide actionable insights into malaria dynamics and effective control measures. During her presentation to the University of Maryland’s Department of Entomology, Dr. Carter highlighted the molecular ecology of these invasive vectors, tracing their invasion routes and their impact on disease transmission. 

Figure 1: Anopheles stephensi. Image from PMI Evolve

In recent years, the rapid emergence and spread of invasive mosquito species have reshaped global public health challenges. East Africa has witnessed the introduction and alarming expansion of two major vectors: Aedes aegypti and Anopheles stephensi. This development poses pressing concerns for disease control and stretches public health systems to their limits. Unlike many African Anopheles species that thrive in rural areas, Anopheles stephensi is particularly concerning due to its adaptation to urban environments. It possesses unique characteristics that  make it a challenging vector to control. It is able to breed in man-made water containers such as tanks and construction sites (as shown in figure 2),  and its resistance to pesticides complicates control efforts. This mosquito is capable of transmitting both Plasmodium falciparum and P. vivax, making it a significant driver of urban malaria resurgence across African cities. Similarly, Aedes aegypti, which originated in West Africa, has evolved into one of the most widespread mosquito vectors globally. This highly anthropophilic species is capable of transmitting arboviruses such as dengue and Zika. Despite its African origins, repeated reintroductions from non-African lineages into the continent have altered disease dynamics and intensified challenges for vector control efforts. ​

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Through her research, Dr. Carter collaborates with Ethiopian universities and neighboring ministries of health to understand these invasive species. Her work examines their invasion pathways, ecological impacts, and their roles in disease transmission. Carter's studies initially began in Jigjiga, Ethiopia, where she partnered with Dr. Solomon Yared, a researcher at Jigjiga University, to screen mosquito specimens. Initial assays classified them as Anopheles arabiensis. However, after refining their methods, they identified these mosquitoes as Anopheles stephensi—a species native to South Asia and the Middle East that is now spreading rapidly in Africa.

Figure 2: Examples of breeding sites of Anopheles stephensi. Image from Balkew et al. 2020.

To investigate the invasion history and population dynamics of Anopheles stephensi, Dr. Carter and her team utilized cutting-edge genomic techniques to uncover insights into the dispersal patterns of this mosquito species. Through methods such as mitochondrial gene analysis and genome-wide techniques such as double digest restriction-site associated DNA sequencing (ddRAD-seq), they mapped the genetic diversity and traced the expansion routes of Anopheles stephensi. Their research identified Dire Dawa, a major urban center in Ethiopia, as a pivotal hub for mosquito dispersal. This was largely attributed to the region's extensive transportation networks connecting cities like Jigjiga, Semera, and Kebridehar. Notably, genetic comparisons revealed that Anopheles stephensi populations in Yemen share greater genetic similarity with those in the Horn of Africa than with populations in South Asia. These findings emphasize the interconnectedness of these regions and challenge traditional assumptions regarding mosquito migration patterns, revealing these mosquitoes are being spread through human-mediated movement rather than natural migration. 

Figure 3: Map of study sites in Ethiopia

Understanding the interaction between invasive mosquitoes and pathogens is critical for controlling vector-borne diseases. The introduction of Anopheles stephensi into East Africa raises important questions about its ability to transmit local strains of Plasmodium falciparum, the primary parasite responsible for malaria in the region. To answer this, Dr. Carter’s research examined the Pfs47 gene, which plays a pivotal role in helping malaria parasites evade the immune response within the mosquito midgut. Her findings reveal that local parasite strains are either rapidly adapting to this new vector or are naturally compatible with it, emphasizing the significant threat posed by Anopheles stephensi. Moreover, genomic studies have identified specific adaptive traits of A. stephensi, including insecticide resistance and enhanced metabolic efficiency. These traits enable the species to survive and thrive in challenging urban environments, further complicating efforts to control its spread and mitigate the health risks associated with its presence.
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Dr. Carter's team continues to track Anopheles stephensi dynamics across Ethiopia through longitudinal studies in cities like Semera and Kebridehar. Their research aims to identify genetic traits and environmental factors driving the establishment of invasive species. Parallel studies on Aedes aegypti explore its coexistence and competition with Anopheles stephensi, providing further insights into their ecological niches and public health impacts. Integrating genomics, molecular ecology, and evolutionary biology into vector surveillance is proving to be a transformative approach. Genetic data reveals invasion patterns, tracks dispersal routes, and uncovers adaptive traits. Dr. Carter ended her talk by highlighting the importance of entomologists and epidemiologists working more closely together during invasion events—not just to better predict the behavior and spread of Anopheles stephensi, but to respond more effectively to emerging public health threats.

Author bio:
Michael Adu-Brew is a master’s student working in the Krishnan Lab. His research focuses on assessing the risk of pesticides on non-target insects, particularly monarch butterflies.

Reference:

1. Balkew, M., Mumba, P., Dengela, D., Yohannes, G., Getachew, D., Yared, S., ... & Carter, T. E. (2020). Geographical distribution of Anopheles stephensi in eastern Ethiopia. Parasites & vectors, 13, 1-8.
2. Liu, Q., Wang, M., Du, Y. T., Xie, J. W., Yin, Z. G., Cai, J. H., ... & Zhang, H. D. (2024). Possible potential spread of Anopheles stephensi, the Asian malaria vector. BMC Infectious Diseases, 24(1), 333
3. Olatunji, G., Kokori, E., Kwape, J. M., Olatunji, D., Anthony, C. S., Ogieuhi, I. J., ... & Aderinto, N. (2024). Anopheles stephensi and the impending challenge to malaria eradication in Africa. New Microbes and New Infections, 58, 101232.