Clinical and bacteriological studies of El Tor cholera after ingestion of known inocula in Thai volunteers

Twenty-six healthy adult Thai volunteers were recruited for clinical and bacteriological studies of cholera induced by oral inoculation with Vibrio cholerae El Tor Inaba strain N16961. Vibrio dosages of 0.3 × 10 4, 1.6 × 10 5 and 1.9 × 10 6 c.f.u. were given to three groups of five volunteers, and 2.0 × 10 7 c.f.u. to 11 volunteers. Diarrhoeal attack rates correlated positively with the size of the inocula ( p < 0.01). It was estimated that a diarrhoeal attack rate of 90% (ED 90) would be achievable by inoculation of 1.3 × 10 7 c.f.u. of the organisms. There were no significant differences between the groups in the latent period to positive stool culture, maximum vibrio count per gram of stool and duration of stool positivity. The ED 90 of V. cholerae obtained may be used as a challenge dose in subsequent studies on protective efficacy of cholera vaccines in Thai adult volunteers.