The expansion of an invasive malaria vector: Anopheles stephensi detection in Arba Minch town in the southern Rift Valley of Ethiopia

doi:10.21203/rs.3.rs-3946371/v1

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The expansion of an invasive malaria vector: Anopheles stephensi detection in Arba Minch town in the southern Rift Valley of Ethiopia

https://doi.org/10.21203/rs.3.rs-3946371/v1

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published 27 Sep, 2024

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Urban areas are seeing a significant increase in malaria cases, and the establishment of an “exotic” urban malaria vector, Anopheles stephensi, is increasing the risk of urban malaria emergence. Immature stages of Anopheles mosquitoes were collected using the standard dipping technique from artificial breeding habitats and reared to adults. Adults aged 2–3 days were identified to species level using a morphological key and confirmed by species-specific polymerase chain reaction. The species distribution map was produced using the geographic coordinates of the breeding habitats. A total of 76 artificial containers (55 discarded tyres, 18 concrete water storage, and three plastic containers) in 21 locations in the town were visited to survey for container breeding Anopheles mosquitoes. Of the examined containers, 88% (67/76) had at least one Anopheles larva. Out of the emerged adults from immature stages, 32 were morphologically identified as An. stephensi, of which 26 (81%) were confirmed by molecular analysis. This is the first study to report An. stephensi from Arba Minch, one of the region's largest towns, highlighting the need for increased vigilance. The planned and ongoing study in and around Arba Minch will add to the continuing efforts to understand the bionomic and the role of An. stephensi in malaria transmission that will help develop a strategy to address the impending risk of urban malaria in Ethiopia.

Anopheles stephensi

Arba Minch

Entomological surveillance

Malaria transmission in Africa varies between urban and rural areas, with the latter having higher exposure risk for the native vectors and less adapted to urban settings (Hay et al. 2005). The increased mobility of people with rapid unplanned urbanisation leads to the "ruralisation" of urban settings, making them suitable for malaria transmission and increasing the risk of malaria in cities (Doumbe-Belisse et al. 2021). The invasion of Anopheles stephensi (An. stephensi), an efficient urban malaria vector originating from the Indian sub-continent and the Middle East (Sinka et al. 2011), could complicate the epidemiology of malaria in African cities.

The adaptability of An. stephensi to breed in artificial containers has made it a vector for malaria in urban settings (Moussa et al. 2023). For example, the introduction of this species to Djibouti was associated with a 30-fold increase in malaria cases (Santi et al. 2021). In Ethiopia, the species was initially identified in Kebri Dehar, a town in eastern Ethiopia, in 2016 (Carter et al. 2018). Since its detection, it has been spotted in multiple urban settings in the country, particularly in the eastern parts (Balkew et al. 2020). In addition, this species is permissive to both Plasmodium falciparum and Plasmodium vivax parasites in the wild and feeding experiments (Tadesse et al. 2021).

Arba Minch is a town in the Rift Valley in Ethiopia endowed with green vegetation due to its easy access to water from lakes, rivers, and streams. However, this also makes malaria transmission a year-round occurrence, with P. falciparum and P. vivax being prevalent (Getawen et al. 2018). Many species of Anopheles mosquitoes have been documented (Getawen et al. 2018; Akirso et al. 2023). The expansion of the construction sector, the unwise use of water resources in car washing businesses, and poor garage tyre disposal habits are creating more breeding sites. In recent surveys conducted in the town, we have not found An. stephensi (Akirso et al. 2023; Ashine et al. 2023). This study aimed to determine the possible occurrence of An. stephensi by examining more breeding sites.

This survey was conducted in January and early February 2024 across 21 locations in Arba Minch during the months with the lowest rainfall (Fig. 1). Arba Minch receives an annual precipitation of over 770 mm, with major rains peaking in April and small ones in October. The average minimum and maximum temperatures per year are 15.7 and 30.3°C, respectively. The survey involved searching for Anopheles mosquito immature stages in artificial containers, such as concrete and plastic water storage containers and discarded tyres. A standardised dipping technique was used to collect immature stages of Anopheles mosquitoes. The immature stages were transported to Arba Minch University Advanced Medical Entomology and Vector Control Laboratory and reared to adults for morphological identification. Emerging adults were kept in a rearing cage and fed on 10% sugar. Adult An. stephensi, aged 2–3 days, were identified using a taxonomic key (Coetzee, 2020). Further species confirmation was done at the Armauer Hansen Research Institute using An. stephensi-specific polymerase chain reaction targeting the internal transcribed spacer 2 region (Singh et al. 2023).

During a survey to identify container-breeding Anopheles mosquitoes in Arba Minch town, 76 artificial containers were visited. These containers comprised 55 discarded tyres, 18 concrete water storage containers, and three plastic containers found in 21 locations within the town (Fig. 1). Upon examination, it was observed that 88% (67/76) of the containers had at least one Anopheles larva. Of the 18 concrete water storage containers used for car washing, 56% (10 out of 18) were positive for Anopheles mosquitoes. It was also revealed that 25% (14 out of 55) of the surveyed discarded tyres were positive for at least one Anopheles larva. On the other hand, none of the three plastic containers surveyed for Anopheles immature showed positive results.

Two hundred thirty-one Anopheles larvae were collected, with 131 coming from discarded tyres and a concrete water storage container in one site. Of these 131 larvae, 32 adults emerged and were morphologically identified as An. stephensi. Out of these, 26 (81%) were confirmed by molecular analysis, showing an amplification band of 438 bp (Fig. 2).

Another two sites also tested positive for An. stephensi, however, confirmed morphologically using the typical morphological structures like speckled legs, a maxillary palpus with two apical broad pale bands, speckling on the third palpus segment, and two pale breaks on the second main dark region of the wing vein 1 (Fig. 3).

We detected An. stephensi in Arba Minch, one of the largest development corridor and tourist destinations in the country's southern region, has good air and land transportation links. Anopheles stephensi was primarily detected in discarded tyres and concrete water storage containers for car washing in the months with the lowest rainfall. This adaptation of An. stephensi enables the species to contribute to malaria transmission year-round, potentially eliminating the seasonality of malaria. Since its report in the eastern part of the country in 2016 (Carter et al. 2018), the species has been expanding its geographic distribution and quickly adapting to new environments (Balkew et al. 2020). One possible reason for its quick adaptation to the new environment could be common urban practices such as poor tyre disposal and brick-built water storage materials for car washing and building (Yared et al. 2023). These practices create breeding habitats similar to the species' original areas, allowing An. stephensi to adapt itself quickly to the new environment.

In 2022, CD light traps and Prokopack collections were used to sample adult mosquitoes in Arba Minch town. The immature stages were collected from the breeding habitats within the compounds of 60 households where the adult collection was done. In that study, only An. rhodesiensis was found in the manmade containers inspected (Akirso et al. 2023). Over the past two years (2021–2023), additional study to investigate the presence of An. stephensi also did not detect the species (Ashine et al, 2023). The selected sites for the study and the methodology used, such as purposive and repeated sampling of breeding habitats within a selected quadrant, might partially account for the failure to detect the species (Ashine et al, 2023). Thus, the current study covered as many breeding habitats as possible and detected positive sites for the species.

Our ongoing and planned activities include conducting an extensive entomological and parasitological study in Arba Minch and other regional towns to ascertain whether the transportation and related services are contributing to the rapid spread of the species and devising mechanisms to contain it. Alongside this, we will continue to look into the relative contribution of An. stephensi to transmission and strategies to minimise the challenge that might be posed by this mosquito in urban settings.

Acknowledgements We acknowledge the owner of the water storage containers for permitting us to collect larval samples. We thank the Armauer-Hansen Research Institute for providing us with a laboratory facility for molecular analysis.

Authors contribution F.M, E.G. conceived the idea and designed the study; F.M. conducted fieldwork, identified the species in the field, and wrote the draft manuscript; T.A. assisted species identification; F.M,B.L,T.A, and E.G. revised the manuscript thoroughly; N.N.conducted laboratory analysis; E.H.coordinated laboratory analysis; N.E.assisted the fieldwork and sample collection; T.T.M.conducted a mapping and climate data analysis. All authors read and approved the final version.

Funding The Norwegian Program for Capacity Development in Higher Education and Research for Development project (No. 59360-Senuph II) provides funds for fieldwork.

Data availability The data is presented in the manuscript

Ethical consideration Not applicable

Competing interests No competing interests to declare

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No competing interests reported.

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Journal Publication

published 27 Sep, 2024

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Editorial decision: Revision requested
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You are reading this latest preprint version

The expansion of an invasive malaria vector: Anopheles stephensi detection in Arba Minch town in the southern Rift Valley of Ethiopia

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