Xenomonitoring of Lymphatic filariasis and risk factors for transmission on the Kenyan coast

Lymphatic filariasis (LF) is an infectious neglected tropical disease caused by a mosquito-borne nematode and is a major cause of disability. In 2022, it was estimated that 51 million people were infected with LF. In Kenya filariasis is endemic along the entire coastal strip. The main vectors are Anopheles funestus and Anopheles gambiae in rural areas and Culex quinquefaciatus mosquitoes in urban areas. In 2022, mosquitoes were collected from Kilifi, Kwale and Taita-Taveta counties which are located within the LF endemic region in Kenya. Subsequently, genomic Deoxyribonucleic acid (DNA) was then extracted from these mosquitoes for speciation and analysis of W. bancrofti infection rates. The impact of socio-demographic and household attributes on infection rates were assessed using generalized estimating equations. A total of 18,121 mosquitoes belonging to Culex (n = 11,414) and Anopheles (n = 6,707) genera were collected. Morphological identification revealed that Anopheline mosquito were dominated by An. funestus (n = 3,045) and An. gambiae (n = 2,873). Wuchereria bancrofti infection rates were highest in Kilifi (35.4%; 95% CI 28%-43.3%, n = 57/161) and lowest in Taita Taveta (5.3%; 95% CI 3.3%-8.0%, n = 22/412). The major vectors incriminated are An. rivulorum, An. funestus sensu stricto and An. arabiensis. The risk of W. bancrofti infection was significantly higher in An. funestus complex (OR = 18.0; 95% CI 1.80-180; p = 0.014) compared to An. gambiae (OR = 1.54; 95% CI 0.16-15.10; p = 0.7). Additionally, higher risk was observed in outdoor resting mosquitoes (OR = 1.72; 95% CI 1.06-2.78; p = 0.027) and in homesteads that owned livestock (OR = 2.05; 95% CI 1.11-3.73; p = 0.021). Bednet (OR = 0.39; 95% CI 0.12-1.32; p = 0.13) and poultry ownership (OR = 0.52; 95% CI 0.30-0.89, p = 0.018) seems to provide protection. Anopheles funestus complex emerged as the primary vectors of lymphatic filariasis along the Kenyan coast. These findings also highlight that a significant portion of disease transmission potentially occurs outdoors. Therefore, control measures targeting outdoor resting mosquitoes such as zooprophylaxis, larval source management and attractive sugar baits may have potential for LF transmission reduction. Author summaryLymphatic filariasis (LF) in the African continent is mainly caused by a mosquito-borne nematode: Wuchereria bancrofti. In urban areas transmission is mainly by Culex quinquefaciatus whereas in rural areas it is dominated by Anopheles funestus and Anopheles gambiae mosquitoes. We investigated the vectorial systems for LF in rural coastal Kenya and factors associated with the risk of diseases transmission in the region. We identified An. funestus sensu lato sibling species An. rivulorum and An. funestus sensu stricto as the dominant vectors of lymphatic filariasis along the Kenyan coast. We also show that a higher proportion of transmission is likely to take place outdoors necessitating the implementation of vector control strategies that target exophilic mosquitoes such as zooprophylaxis and larval source management. Factors associated with transmission of LF include ownership of livestock and houses made of natural materials such as thatched roof and mud walls. Bednet and poulty ownership were associated with protection. We also highlight the importance of molecular xenomonitoring in the surveillance of lymphatic filariasis, because of its non-invasive nature and potential for incriminating new vectors of lymphatic filariasis.