Effects of water velocities on worm burdens of animals exposed to Schistosoma mansoni cercariae released under laboratory and field conditions

Abstract

1. 1. The infectivity of cercariae was reduced in exposed mice when water velocities attained 2.7 miles per hour. 2. 2. Intermittent drying intervals between animal exposures to schistosome cercariae showed no detectable difference in animal worm burdens. 3. 3. Cercariae-free flowing waters following animal exposure to cercariae did not reduce the worm burden. 4. 4. The animal worm burden acquired from cercariae-infested waters was inversely proportional to the distance cercariae were carried downstream. 5. 5. No loss in the number of released cercariae carried as far as 2000 feet downstream was detected by Rowan's cercariometer apparatus; thus, suggesting that cercariae lose their infectivity prior to loss of morphological integrity. 6. 6. Some cercariae carried up to 5000 feet downstream were capable of completing their life cycle in the vertebrate host. 7. 7. In a carefully controlled experiment with exposures between one to 120 minutes, mice exposed 30 minutes or longer not only acquired more worms per mouse but had a higher infection rate than mice exposed 15 minutes or less. 8. 8. Two important factors in limiting infections of schistosomiasis are: (1) velocity of water and (2) distance cercariae are carried downstream.