Studies on the effects of cercarial concentration and length of exposure on the infection of mice by Schistosoma mansoni

Abstract

Groups of mice were exposed to Schistosoma mansoni cercariae in a large volume of water. A factorial design was used to investigate the effects of cercarial concentration and length of exposure on infection rates and worm burdens. Low exposure times and cercarial concentrations gave no infections, but prolonged exposure to high concentrations gave high infection rates and worm burdens. Prolonged exposure to low concentrations gave low worm burdens and moderate infection rates: short exposure to high concentrations gave moderate infection rates and worm burdens. Statistical analysis showed that both infection rates and mean worm burdens were significantly related to the two main factors, but the relationship was complicated in each case by curvilinear effects and by a significant interaction between the main factors. Field cercarial concentrations are generally low on St Lucia, a fact suggesting that brief human water contacts, such as fording and filling domestic water containers, play little part in transmission. Prolonged contact during swimming, bathing and washing clothes may produce significant infection rates accompanied by low worm burdens. However, on the rare occasions when high cercarial concentrations occur, even the briefest exposure may produce significant infection rates and moderate worm burdens.