Unravelling the distribution of vectors of major vector-borne diseases in Koshi Province of Nepal: A concern of expansion in diverse geo-ecological and climatic regions.

BACKGROUND: Vector-borne diseases (VBDs), including malaria, visceral leishmaniasis, lymphatic filariasis, Japanese encephalitis, and dengue, are major public health concerns and are either slated for elimination or projected for control in Nepal. One of the major challenges in controlling these VBDs is halting their emergence and expansion in diverse geo-ecological and climatic regions. In this study, we collected vectors of major VBDs to assess their distribution, diversity, and associations with ecological variations and to provide an updated understanding of the current situation.

METHODOLOGY/PRINCIPAL FINDINGS: A descriptive cross-sectional survey was conducted in five districts of Koshi Province, eastern Nepal, during May and June 2023 to collect vectors across three distinct geo-ecological and climatic regions: mountains, hills, and lowlands, situated at altitudes ranging from 98 to 1,274 meters. Adult mosquitoes and sand flies were captured using CDC miniature light traps, BG-Sentinel traps, and manual aspirators. We fitted generalized linear models (GLM) with a negative binomial distribution to assess the association between vector abundance and geo-ecological and climatic variables for two vector species (Culex quinquefasciatus and Phlebotomus argentipes). We found the malaria vector, Anopheles annularis, the lymphatic filariasis vector, Cx. quinquefasciatus and the visceral leishmaniasis vector, Ph. argentipes, across all three geo-ecological regions. Other vectors of the malaria parasite, An. pseudowillmori and An. willmori, and Japanese encephalitis vector Cx. tritaeniorhynchus were recorded only in hilly districts. Mean temperature and rainfall had a positive effect on Cx. quinquefasciatus density, but a deleterious effect on Ph. argentipes. Culex quinquefasciatus and Ph. argentipes were captured in higher abundance at the household level in the hills (IRR = 1.23 and IRR = 13.00, respectively) and mountains (IRR = 1.96 and IRR = 4.00, respectively) compared with the lowlands.

CONCLUSION: Two major vectors, Cx. quinquefasciatus and Ph. argentipes were indiscriminately present in all geo-ecological regions. Climatic variables seemed conducive to vector survival, distribution, and growth across diverse altitudes from the lowlands to the high hills and mountains. Our findings highlight the need for the VBDs control programme to implement regular monitoring, strengthen existing surveillance systems, and support evidence-based planning and implementation of vector control interventions across wider geo-ecological regions to prevent disease transmission.