Measuring exposure to Schistosoma japonicum in China. III. Activity diaries, snail and human infection, transmission ecology and options for control

We used activity diaries and snail detection to relate water contact and Sc histosoma japonicum infection among a cohort of 178 residents on two island s in the Dongting Lake, China. Water exposure to each of 12 mapped water zo nes around the islands was calculated (m(2) min/day) for each subject. Infe cted Oncomelania hupensis hupensis snails in this area are focal and were f ound in only five of the 12 zones, with the highest rate being 5.7%. Thirty -one subjects (17%) were re-infected with a mean intensity of 63.2 epg. Mea n water contact was 7.4 m(2) min/day; 98% of water exposure was due to econ omic activity and only 2% due to swimming or bathing, washing and other nec essities of daily life, Males had more exposure and infection than females (P < 0.05). Infected subjects had more exposure (10.2 m(2) min/day) than th ose not infected (7.44 m(2) min/day) (P < 0.05). Compared with uninfected s ubjects, those infected had 2.9 times more exposure in infected-snail zones (P < 0.01). Also, human infection intensity (epg) correlated well with exp osure to infected snail zones (r = 0.552, P < 0.01). People < 20 years old had the highest re-infection (21.4%) and intensity (3.77 epg). Median expos ure for 20-49-year-olds (9.00 m2 min/day) was nearly double that of those a ged < 20 or > 50 years old (5.5 m(2) min/day). We conclude that map-referen ced water contact and snail evaluation boosts accuracy of activity-diary me asurements in large transmission foci for the Asian schistosome. Protecting against faecal contamination of snail inhabited sites, and against occupat ional exposure for island residents, should be a priority of future researc h. Potential strategies for migrating buffaloes and families living on visi ting fishing boats are explored. (C) 2000 Published by Elsevier Science B.V . All rights reserved.