Malaria Transmission Dynamics in Sentinel Sites and Mixed Crop Irrigation Scheme in Western Kenya

Abstract

Knowledge of the effects of environmental changes on malaria vector bionomics and surveillance of malaria transmission is important in understanding malaria epidemiology. This study aimed to provide information on the effects of a recently established irrigation scheme in Kenya on the bionomics and transmission of malaria vectors. In addition, establish long-term changes in malaria transmission profiles and map hotspot transmission regions in western Kenya. Entomological and parasitological surveys were undertaken in Kombewa, Iguhu, and Marani, whereas only entomological surveys were conducted in irrigated and non-irrigated agroecosystems in Homa bay in western Kenya. Malaria vector speciation, blood meal sources, and sporozoite infections were analyzed using polymerase chain reaction (PCR). Malaria parasite species and gametocytes were identified microscopically. Maps of hotspots for malaria transmission were generated using ArcGIS software. The predominant malaria vector species was Anopheles arabiensis in Homa Bay County in western Kenya. The mean indoor and outdoor densities of An. arabiensis were higher in the irrigated study zone compared to the non-irrigated zone. The An. funestus mean indoor resting density was higher in the irrigated zone, while only a few An. funestus were collected in the non-irrigated zone. In the irrigated zone, the outdoor human blood index (HBI) of An. arabiensis was almost twofold higher (outdoor, 5.20%; indoor, 3.90%) than indoor, whereas in the non-irrigated zone, the blood meals were of bovine origin. The indoor HBI of An. funestus (49.43%) was 14-fold higher than that of An. arabiensis (3.44%) in the irrigated zone, whereas in the non-irrigated zone, none of the An. arabiensis tested positive for human blood. The annual entomological inoculation rates (EIRs) for An. arabiensis in the irrigated zone were 0.71 infective bites per person per year (ib/p/year) and were higher indoors (0.41 ib/p/year) than outdoors (0.30 ib/p/year). Malaria transmission was not detected in the non-irrigated zone. Asymptomatic malaria

parasite prevalence decreased in Kombewa, Iguhu, and Marani from 2005 to 2008. However, since 2011, parasite prevalence has resurged by >40% in Kombewa and Marani. Malaria vector densities have similarly shown reductions from 2005 to 2008 in Kombewa, Iguhu, and Marani; thereafter, they rose steadily until 2014 before decreasing. A greater infection risk was observed in Kombewa compared to Iguhu and Marani. The median time and probability of non-infection during malaria episodes were lowest in Kombewa compared to Iguhu and Marani. A gender bias towards males in infection was observed. During 2018 to 2019, the annual EIRs were 5.12, 3.65, and 0.50 (ib/p/year) at Kombewa, Iguhu, and Marani, respectively. Cold spots and hotspots of spatially clustered elevated vector abundance and Anopheles larval densities were detected in Marani. The results show that irrigated agriculture may cause an upsurge in malaria transmission. Malaria prevalence remains high or has resurged in some sites in western Kenya despite continuous intervention efforts. Targeted control of those at risk of asymptomatic infection who serve as human reservoirs might effectively decrease malaria transmission and prevent resurgences. Incorporation of additional vector control tools such as larval source management to complement the long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) in the irrigation scheme is recommended. Further studies on socio-behavioural factors and biological sex differences in malaria infection is advised. Geospatial technology is recommended for effective vector-borne disease prevention, control, and eventually elimination.