Effect Of Seasonal Rainfall And Other Environmental Changes, On Snail Density And Infection Rates With Schistosoma mansoni Fifteen Years After The Last Snails\' Study In Kigungu, Entebbe, Uganda

Abstract

Background: The last study on snail population density in relation to rainfall pattern in Kigungu canoe landing and recreational sites on Lake Victoria shore was earlier
carried out about fifteen years ago. This study also reviewed the influence of other
environmental factors on the snails\' infection rate.
Objective: To reassess the density dynamic of Biomphalaria (B) choanomphala and
Biomphalaria (B) pfeifferi, which act as the intermediate host for S. mansoni and
Bulinus (B) globosus, and Bulinus (B) tropicus, which act as intermediate host for S.
haematobium.
Design: Retrospective study.
Setting: Busy canoe landing sites along Lake Victoria in Kigungu fishing village were
selected for the snail sampling.
Results: Nine thousand one hundred and ninety four B. choanomphala were collected
over the study period. The numbers of B. choanomphala collected in each year was 4742
(51.6%) and 4452 (48.4%) in 2004 and 2005 respectively. Of the 4742 B.Choanomphala
collected in 2004, 82 (1.7%) shed human cercariae and 329 (6.7%) shed non-human
cercariae. Whereas in 2005, out of 4452 B. choanomphala collected 302 (6.85%) shed
non-human cercariae and 82 (1.8%) shed human cercariae. Similarly, 4173 B. pfeifferi
were also collected in the same period. Out of which 2224 (53.3%) were collected in
2004 and 1949 (46.7%) in 2005. For B. pfeifferi, 42 (1.9%) out of 2224 snails collected in
2004 shed human cercariae and 246 (11.1%) shed non-human cercariae. While in 2005,
33 out of 1949 snails (1.7%) shed human cercariae and 159 (8.2%) shed non-human
cercariae. Other snails of medical importance collected included 292 B. globosus
and 3094 B. tropicus. None of the Bulinus spp. collected shed any human cercariae
but 37 (2.1%) and 30 (2.3%) B. tropicus shed non-human cercariae in 2004 and 2005
respectively. In 2004 and 2005, the area received, 1729mm and 1959mm of rainfall
respectively. The mean rainfall during the year was 144.05 mm and 163.3 mm in 2004
and 2005 respectively. There was a negative correlation between rainfalls and snail
density dynamic.
Conclusion: We have found in this study that in spite of the bush clearing of the
papyrus swamps which originally was the major habitats for B. choanomphala, B.
pfeifferi and the Bulinus spp the intermediate host for schistosome at all canoe landing
sites at Kigungu, these snails are still present. Moreover, that their population density
dynamic and infection rate are inversely proportional to the rainfall pattern.

East African Medical Journal Vol. 85 (11) 2008: pp. 556-563