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.