The relationship between schistosomes and their
intermediate hosts is an extremely intricate one with strains and
species of the parasite depending on particular species of snail,
which in turn may vary in their susceptibility to the parasites.
In order to gain a better understanding of the epidemiology of
the disease we have been investigating the use of molecular
markers for snail identification and for studying host-parasite
relationships. In this paper we will draw on examples concerning
schistosomiasis in West and East Africa to illustrate how a
molecular analysis can be used as part of a "total evidence"
approach to characterisation of Bulinus species and
provide insights into parasite transmission. Particular emphasis
is given to ribosomal RNA genes (rRNA), random amplified
polymorphic DNA (RAPDs) and the mitochondrial gene cytochrome
oxidase I (COI). Snails resistant to infection occur naturally
and there is a genetic basis for this resistance. In
Biomphalaria glabrata resistance to Schistosoma
mansoni is known to be a polygenic trait and we have initiated
a preliminary search for snail genomic regions linked to, or
involved in, resistance by using a RAPD based approach in
conjunction with progeny pooling methods. We are currently
characterising a variety of STSs (sequence tagged sites)
associated with resistance. These can be used for local linkage
and interval mapping to define genomic regions associated with
the resistance trait. The development of such markers into simple
dot-blot or specific PCR-based assays may have a direct and
practical application for the identification of resistant snails
in natural populations.
