A survey of Angiostrongylus cantonensis was carried out to investigate the mode of transmission from molluse to rat in a fixed study area of Yoron Island from 1979 to 1982. Rattus rattus was found to be infected with a small number of worms in spite of heavy infection with third-stage larvae in Achatina fulica and an abundance of this snail in the area. Natural infection and/or susceptibility with A. cantonensis were confirmed in three small snail species. Bradybaena circulus, Fruticicola despecta and Luchuena reticulata. Young A. fulica was found to be infected with fewer third-stage larvae than mature A. fulica. It was concluded that molluscs which were infected with a small number of third-stage larvae of A. cantonensis play an important role in maintaining the life cycle of A. cantonensis. The percentage of rat stomachs containing molluse tissue was relatively low, and the incidence and infection was low in rats. Infection with A. cantonensis did not occur very often in R. rattus in nature.

In an operational Schistosoma mansoni field-study in an area of about 20 km (population approximately 8000), transmission detection by simple snail sampling was compared with cercariometry. Between 1985 and 1987, 62 field sites were sampled at fortnightly intervals. Of a total of 2758 field observations, 89.8% gave full snail data; 64.4% full cercarial data; and 61.7% complete data for both methods. The complete data sets showed significant but not strong correlations between Biomphalaria pfeifferi (total and infected with S. mansoni or other trematodes) and cercarial (S. mansoni and non-human) recoveries. Non-human (but not S. mansoni) cercarial recovery decreased with deteriorating cercariometry filter quality. Both snail and cercarial recoveries diminished significantly with increasing water flows at the time of collection. Many samples yielded infected snails or cercariae, but not both, and neither method detected significantly more transmission sites. The method of choice for detecting transmission in a large-scale field-study depends on logistical and financial considerations. Relatively simple snail sampling allows quick, cheap and widespread data collection adequate for most purposes but more complicated cercariometry is still valuable for specific, small-scale studies.