Prevalence and spatial distribution of Trematode cercariae in Vector Snails within different Agro-Ecological Zones in Western Kenya, 2016

Introduction Freshwater vector snails' distribution, infection with cercariae, preferred habitat and possible trematodiases transmission foci is not well known in Western Kenya. We sought to determine the distribution and prevalence of infection of snails per agro-ecological zone and environmental factors in vector snail habitats. Methods We conducted a cross-sectional survey from March, 2016 - May, 2016, harvested and identified snails using shell morphology, determined their infection with trematode cercariae using microscopy, used descriptive statistics to estimate the prevalence of infection and relationship between snail abundance and environmental factors. Results We sampled a total of 1,678 vector snails from 47 sampling sites of which 42% were Lymnaeid, 23% Biomphalaria, 10% Bulinus, 22% Oncomelaniae and 2% Melanoides. Lower Midland I Ago-Ecological Zones had 44% of the snails and streams from springs had 41% of the snails. Overall, 26.5% (445/1678) (95% CI: 24.4 - 28.6) of the snails shed cercariae. Cercariae were found in 11 (23%) of the sites and in all zones. F. gigantica cercariae were shed by L. natalensis, B. pfeifferi, B. sudanica. Lakeshore had both F. gigantica and S. mansoni cercariae shed by B. sudanica. About 72% (1,202/1,678) of snails were found in water with a pH 6.5 - 7.5. Grass habitat had 54% (912/1,678) of the snails. Conclusion Lymnaeid snails were present in all the zones, while streams from springs and near neutral habitats had most of the snails. Infection with trematode cercariae was noted in all the zones. Trematodiases control should be focused on all zones especially in freshwater streams and lakeshores.


Introduction
Freshwater vector snails of medical and veterinary importance act as obligate intermediate hosts for parasitic trematodes [1,2]. Vector snails play a vital role in the transmission, epidemiology and control of trematode infections, such as fascioliasis, a zoonotic food-borne trematode infection caused by parasitic trematodes, Fasciola hepatica and Fasciola gigantica [3] and Schistosomiasis that is caused by Schistosoma mansoni and Schistosoma haematobium [4][5][6]. Both Fascioliasis and Schistosomiasis are considered as neglected tropical diseases by the World Health Organization (WHO) since they affect some of the poorest of the poor populations in the world [7]. The chronic nature of trematode infections in the human and animal hosts makes it difficult to determine when and where transmission actually occurs [1,3,8,9]. Investigation of freshwater vector snails provides vital information on active transmission foci for trematode infections [9,10]. In Kenya, trematode infections are prevalent mostly in domestic livestock where they cause economic losses especially when the infected liver prevents trade and exportation of products of animal origin [11]. In Western Kenya it was estimated that total economic losses due to liver cirrhosis for a 10-year period was US$ 0.8 million [11,12]. There is little information on trematode infections and the distribution of the snail vectors in Western Kenya and other parts of Africa too [3]. Busia County in Western Kenya has the conducive habitat for proliferation of vector snails and the trematode parasites that use them as intermediate hosts [13,14], however, effective surveillance systems are lacking in veterinary and medical disease surveillance system in the region [15]. This study aimed at estimating prevalence of infection by identifying species of snails infected with trematode cercaraie in different Agro-Ecological Zones (AEZs) in Busia Kenya and to identify the environmental and physico-chemical factors that promote the presence of these snails in their habitats. By determining the distribution of the vector snails, our study findings could be used to identify possible disease transmission zones or foci for the trematode parasites. The knowledge on geographic distribution of the intermediate host snails could help improve efficiency of allocation of the available limited resources in control interventions to specific transmission foci. regional representation. We then randomly selected water bodies per AEZ depending on snail concentration in each site. The identified sites were mapped using Global Positioning System (GPS) and the geo-coordinates recorded appropriately. The snails were harvested from sites using dip scoop in areas with large water bodies or hand collection in areas with shallow water bodies.
Sampling was extended to pastures near the water bodies.
Sampling time was fixed at 15 minutes per location and performed between 8.00am and 11.00am. The presence and number of each snail species was recorded. proportionately based on species and sampling site, in groups of not more than 20 snails in flat-bottomed transparent vials containing water drawn from the ponds where snails were harvested. They were exposed to indirect sunlight for a maximum duration of four hours for cercariae shedding. The vials in which cercariae were not shed on the first exposure were re-exposed on the subsequent days until the fifth day. From the water in which snails were placed, 200 µl (0.2ml) of cercariae suspension was drawn. The suspension was mixed gently to allow for even distribution of cercariae before withdrawing aliquots of 5-10 of cercariae suspension from each vial.
The aliquots were then placed in a petri dish and a sufficient amount of tap water added to cover the bottom of the petri dish.

Results
A total of 1,678 vector snails of medical and veterinary importance and 559 prosobranch snails of little public health significance were  found in swamps near the lakeshore. This was in agreement with a study done by Opisa and others who found that B. sudanica were mostly found along the shores of lake Victoria and few in the inland sites [14]. Almost all lymnaeid snails were found in streams that originated from springs located away from the lakeshore. This was in line with similar findings in a study done in Venezuela where L.
truncatula was found to prefer higher altitudes [20]. Howell and others also found a similar distribution in which populations of L.
natalensis were high at lower altitude but decreased towards higher altitudes while L. truncatula was found mostly at higher altitude [15]. However, the findings in our study were in contrast to those from a study conducted on host snails in land and lake habitats of western Kenya that found ponds to be preferred by host snails for schistosomiasis [13]. In this study, Biomphalariae and Lymnaeid species were found to be infected with trematode cercariae. The B.
sudanica species found in the swamp near the lakeshore were