The invasion of Anopheles stephensi in Ethiopia may lead to an increase in malaria incidence in the region. Therefore, understanding the insecticide resistance profile and population structure of Anopheles stephensi recently detected in Fike, Ethiopia is crucial to guide vector control to stop the spread of this invasive malaria species in the country. Following entomological surveillance of Anopheles stephensi in Fike, Somali Region, Ethiopia, we confirmed the presence of Anopheles stephensi in Fike at the morphological and molecular levels. Characterization of larval habitats and insecticide susceptibility testing revealed that A. fixini was most commonly found in artificial containers and was resistant to most adult insecticides tested (organophosphates, carbamates, pyrethroids) except pirimiphos-methyl and PBO-pyrethroid. However, immature larval stages were susceptible to temephos. Further comparative genomic analysis was conducted with the previous species Anopheles stephensi. Analysis of the Anopheles stephensi population in Ethiopia using 1704 biallelic SNPs revealed genetic linkage between A. fixais and Anopheles stephensi populations in central and eastern Ethiopia, especially A. jiggigas. Our findings on insecticide resistance traits as well as possible source populations of Anopheles fixini may help in developing control strategies for this malaria vector in the Fike and Jigjiga regions to limit its further spread from these two regions to other parts of the country and across the African continent.
Understanding mosquito breeding sites and environmental conditions is critical to developing mosquito control strategies such as the use of larvicides (temephos) and environmental control (elimination of larval habitats). In addition, the World Health Organization recommends larval management as one of the strategies for direct control of Anopheles stephensi in urban and peri-urban settings in infestation areas. 15 If the larval source cannot be eliminated or reduced (e.g. domestic or urban water reservoirs), the use of larvicides can be considered. However, this method of vector control is expensive when treating large larval habitats. 19 Therefore, targeting specific habitats where adult mosquitoes are present in large numbers is another cost-effective approach. 19 Therefore, determining the susceptibility of Anopheles stephensi in Fik City to larvicides such as temephos may help inform decisions when developing approaches to control invasive malaria vectors in Fik City.
In addition, genomic analysis may help develop additional control strategies for the newly discovered Anopheles stephensi. In particular, assessing the genetic diversity and population structure of Anopheles stephensi and comparing them with existing populations in the region may provide insight into their population history, dispersal patterns, and potential source populations.
Therefore, one year after the first detection of Anopheles stephensi in Fike town, Somali region, Ethiopia, we conducted an entomological survey to first characterize the habitat of Anopheles stephensi larvae and determine their sensitivity to insecticides, including the larvicide temephos. Following morphological identification, we conducted molecular biological verification and used genomic methods to analyze the population history and population structure of Anopheles stephensi in Fike town. We compared this population structure with previously detected Anopheles stephensi populations in eastern Ethiopia to determine the extent of its colonization in Fike town. We further assessed their genetic relationship to these populations to identify their potential source populations in the region.
The synergist piperonyl butoxide (PBO) was tested against two pyrethroids (deltamethrin and permethrin) against Anopheles stephensi. The synergistic test was performed by pre-exposing mosquitoes to 4% PBO paper for 60 minutes. Mosquitoes were then transferred to tubes containing the target pyrethroid for 60 minutes and their susceptibility was determined according to the WHO mortality criteria described above24.
To obtain more detailed information about the potential source populations of the Fiq Anopheles stephensi population, we performed a network analysis using a combined biallelic SNP dataset from Fiq sequences (n = 20) and Genbank extracted Anopheles stephensi sequences from 10 different locations in eastern Ethiopia (n = 183, Samake et al. 29). We used EDENetworks41, which allows network analysis based on genetic distance matrices without a priori assumptions. The network consists of nodes representing populations connected by edges/links weighted by the Reynolds genetic distance (D)42 based on Fst, which provides the strength of the link between pairs of populations41. The thicker the edge/link, the stronger the genetic relationship between the two populations. Moreover, the node size is proportional to the cumulative weighted edge links of each population. Therefore, the larger the node, the higher the hub or convergence point of the connection. The statistical significance of nodes was assessed using 1000 bootstrap replicates. Nodes appearing in the top 5 and 1 lists of betweenness centrality (BC) values (the number of shortest genetic paths through the node) can be considered statistically significant43.
We report the presence of An. stephensi in large numbers during the rainy season (May–June 2022) in Fike, Somali Region, Ethiopia. Of the more than 3,500 Anopheles larvae collected, all were reared and morphologically identified as Anopheles stephensi. Molecular identification of a subset of larvae and further molecular analysis also confirmed that the studied sample belonged to Anopheles stephensi. All identified An. stephensi larval habitats were artificial breeding sites such as plastic-lined ponds, closed and open water tanks, and barrels, which is consistent with other An. stephensi larval habitats reported in eastern Ethiopia45. The fact that larvae of other An. stephensi species were collected suggests that An. stephensi can survive the dry season in Fike15, which is generally different from An. arabiensis, the main malaria vector in Ethiopia46,47. However, in Kenya, Anopheles stephensi… larvae were found in both artificial containers and streambed environments48, highlighting the potential habitat diversity of these invasive Anopheles stephensi larvae, which has implications for future entomological surveillance of this invasive malaria vector in Ethiopia and Africa.
The study identified the high prevalence of invasive Anopheles malaria-transmitting mosquitoes in Fickii, their larval habitats, insecticide resistance status of adults and larvae, genetic diversity, population structure and potential source populations. Our results showed that the Anopheles fickii population was susceptible to pirimiphos-methyl, PBO-pyrethrin and temetafos. B1 Thus, these insecticides can be effectively used in control strategies for this invasive malaria vector in Fickii region. We also found that the Anopheles fik population had genetic relationship with the two main Anopheles centres in eastern Ethiopia, namely Jig Jiga and Dire Dawa, and was more closely related to Jig Jiga. Therefore, strengthening vector control in these areas may help prevent further invasion of Anopheles mosquitoes into Fike and other areas. In conclusion, this study offers a comprehensive approach to the study of recent Anopheles outbreaks. Stephenson’s stem borer is being expanded to new geographic areas to determine the extent of its spread, assess the effectiveness of insecticides, and identify potential source populations to prevent further spread.
Post time: May-19-2025