1.0 INTRODUCTION
Freshwater snails are organisms that form an important component of food chain and food web of most freshwater ecosystems. Some of the freshwater snails that inhabit freshwater bodies such as lakes, ponds and dams are known to be of medical and veterinary importance. It is estimated that about 350 species are of medical and veterinary importance. The intermediate hosts of human schistosomes belong to three genera;Biomphalaria, Bulinus and Oncomelania. Bulinus species causes urinary schistosomiasis, Biomphalaria species causes intestinal schistosomiasis and Oncomelania species causes fascioliasis or liver rot in animals (Melo et al. , 2012). The distribution and abundance of freshwater snail populations are influenced by temperature, food supply, predators, parasites, rainfall and water composition. Sunlight, flowering aquatic weeds, abundance of micro-flora and high dissolved oxygen also contribute to the abundance of freshwater snails (Hosea et al., 1998). Snail abundance, distribution and density vary significantly with season (WHO, 1993). Biotic factors such as availability and density of aquatic macrophytes have also been reported to play vital roles in the distribution of freshwater snails in different parts of Africa (Ofoezie, 1999).
Physico-chemical factors of the water body that are considered as most important factors in the aquatic environment particularly for freshwater snails are temperature, pH, turbidity, dissolved oxygen, calcium, magnesium and phosphate (Abbasi et al. , 2011). Various physico-chemical and biological circumstances must be simultaneously taken into consideration for understanding fluctuation of biological populations in water body (Dalas, 2004).
Warwade dam, an earth dam is a “weir” constructed in the 1970s solely for the purpose of irrigation is playing a significant role in the socioeconomic activities in the lives of communities sorrounding it, especially Warwade village. Other activities observed by Dogara et al. (2019) at the dam sites include fishing, washing of clothes, swimming and bathing. Since Warwade dam as a “weir” is a typical standing water that is likely to harbour vectors that transmit diseases such as malaria, schistosomiasis and lymphatic filariasis. Reports of surveys from Jigawa State Ministry of Health revealed that the state is endemic for schistosomiasis (Abdurrahaman, 2016). The same report showed that school aged children of Warwade town have a prevalence of 7.1% for urinary and intestinal schistosomiasis.
The global effort to eliminate schistosomiasis is enshrined in World Health Organization Schistosomiasis plan with a vision of a “World Free of Schistosomiasis” with its end goal to eliminate the disease as a public health problem by 2025 (WHO, 2010). The control of schistosomiasis is based on large-scale treatment of at-risk population groups, access to safe water, improved sanitation, hygiene education, and snail control (WHO, 2020). Snail control is a key component in the global and national programmes for the elimination of schistosomiasis. However, its success largely depends on the understanding of the ecology of freshwater snail vectors. Despite the widespread use of earth dams used in irrigation in Nigeria, little is known of the consequences for mollusks of medical or veterinary importance, and hence for the importance of control. Preliminary study in August, 2018 of freshwater snails of the dam by Dogara et al. (2019) revealed the presence of four species in order of increasing abundance and distribution;Lymnaea natalensis , Bellamya unicolar, Melanoides tuberculata and Bulinus globosus . The physicochemical factors measured includes temperature, conductivity, acidity and alkalinity were found to favour the survival of the freshwater snails. This information is not enough to give a realistic picture that could be used in planning and executing freshwater snails control programme that are known to transmit schistosomiasis in Warwade dam. There is therefore the need to understand how intermediate snail hosts abundance will be affected by seasonal changes which mainly influence the prevailing abiotic and biotic factors of their habitats. Again, the period of highest relative abundance must be known if control measures are to be successfully implemented given that the development of an effective control strategy requires the study of population dynamics of the intermediate hosts and its relation to environmental factors (Hussein, et al., 2011). There is also the need to understand the intra-specific competition between the different freshwater snail species in the dam. This work is aimed at studying the distribution and abundance of the freshwater snails of the dam over a longer period of time in relation to important physicochemical factors that affect them.