Xenomonitoring of Lymphatic filariasis and risk factors for transmission on the Kenyan coast
Authors: Bartilol Brian , Babu Lawrence , Garama Karisa , Karisa Jonathan , Kamau Alice , Mwandawiro Charles , Wanjiku Caroline , Mbogo Charles , Maia Marta , Mwangangi Joseph , Rono Martin 
Published: January 2024

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 areAnopheles funestusandAnopheles gambiaein rural areas andCulex quinquefaciatusmosquitoes 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 ofW. bancroftiinfection 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 toCulex (n = 11,414)andAnopheles(n = 6,707) genera were collected. Morphological identification revealed that Anopheline mosquito were dominated byAn. funestus(n = 3,045) andAn. gambiae(n = 2,873).Wuchereria bancroftiinfection 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 areAn. rivulorum, An. funestussensu stricto andAn. arabiensis. The risk ofW. bancroftiinfection was significantly higher inAn. funestuscomplex (OR = 18.0; 95% CI 1.80-180; p = 0.014) compared toAn. 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 funestuscomplex 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 summary

Lymphatic filariasis (LF) in the African continent is mainly caused by a mosquito-borne nematode:Wuchereria bancrofti. In urban areas transmission is mainly byCulex quinquefaciatuswhereas in rural areas it is dominated byAnopheles funestusandAnopheles gambiaemosquitoes. We investigated the vectorial systems for LF in rural coastal Kenya and factors associated with the risk of diseases transmission in the region. We identifiedAn. funestussensu lato sibling speciesAn. rivulorumandAn. funestussensu 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.