The abundance of snail hosts and the rates of infection with
Schistosoma mansoni
were monitored monthly for four years in two representative localities subjected
to repeated chemotherapy of infected persons. Snail abundance varied from
1.0 to 4.4 collected per person/minute/station for
Biomphalaria straminea
and from 0.1 to 7.0 for
B.
glabrata
. Infection rates of snails in nature varied from 0% to 15% for the former
and from 0% to 70% for the latter species. Human infection increased from
35.5% to 61.9% in the locality occupied by
B.
straminea
, and decreased from 40.3% to 20.8% in that occupied by
B.
glabrata
. No relationship could be detected between human infection and the snail
variables. Despite seasonal variations, natural infection persisted throughout
the monitoring period in both snail species. It reached remarkably high
levels in
B.
straminea
when compared to those obtained by other authors probably because of differences
in methodology. It is recommended that longitudinal studies should be carried
out focally and periodically to avoid underestimating the prevalence of
schistosome infection in snails.
