Spatial distribution of vector snails and their infection with Fasciola cercariae in different agro-ecological zones in Busia County, western Kenya

Background: Vector snails play vital role in transmission, epidemiology and control of trematode infections like Fascioliasis. Knowledge on their distribution, infection and preferred habitat can be used to determine transmission zones for trematodiases to enable prevention and control. Objectives: The study aimed to identify species of vector snails in different Agro-Ecological zones (AEZs) in Busia, determine their infection with cercariae and identify environmental factors that promote their survival. Methods: A cross-sectional survey was conducted in March-April, 2016 to determine the spatial distribution of vector snails. We harvested and identified freshwater snails based on their shell morphology using Mandahl-Barth key, and determined their infection with cercariae by microscopy. Descriptive statistics were used to determine the proportions of snails and infection in AEZs. Spatial analysis was done by use of Global Information System (GIS) software. We used correlation coefficient to determine the relationship between snail abundance and environmental characteristics. Results: A total of 1,678 vector snails was collected from 47 sampling sites. Snail types harvested included Lymnaeid (42.3%), Biomphalariae (23.4%), Bulinus (10.0%), Oncomelaniae (22.3%), and Melanoides (2.0%). Lower Midland (LM 1) AEZ harboured 44%, LM2 (33), LM3,4 (18%) and Upper Midland, UM3 (5%). Streams from springs had 41% of the snails. Cercariae were shed in 23.4% of the sites. About 26.5% of the snails shed cercariae. F. gigantica cercariae were shed by L.(Radix) natalensis, Bi. pfeifferi and Bi. sudanica. Lakeshore had both F. gigantica and S. mansoni cercariae shed by Bi. sudanica. Relationship between snail abundance and water pH had a correlation coefficient of 0.05 (p-value = 0.04). Grass was preferred by 54.4% of the snails. Conclusions: Lymnaeid snails’ presence and infection with cercariae were noted in all the AEZs. Streams with near-neutral water pH and grass were the most preferred habitats. Recommendation: Surveillance and public education on trematodiases should focus on all AEZs.