EXTENSION OF THE VOLUNTEER CHALLENGE MODEL TO STUDY SOUTH-AMERICAN CHOLERA IN A POPULATION OF VOLUNTEERS PREDOMINANTLY WITH BLOOD-GROUP ANTIGEN-O

The volunteer challenge model was used to study the virulence of strai ns of Vibrio cholerae O1 El Tor recently isolated from cases of choler a in South America. Fifteen of the 24 volunteers (62%) were of blood g roup O, the group most prevalent in South America and the group at inc reased risk of more severe cholera. Two El Tor Inaba strains and 2 El Tor Ogawa strains were given to volunteers at a dose of 1-2 x 10(6) co lony-forming units. All 4 strains caused diarrhoea in 67-83% of volunt eers. Volunteers with blood group antigen O had an increased attack ra te for diarrhoea (P=0.015) and a marginally increased mean diarrhoeal stool volume (P=0.08) after challenge. One-third of the volunteers wit h blood group O, and none of the volunteers with other blood groups, d eveloped severe diarrhoea (>5 L) (P=0.01). This study established a mo del of South American cholera that can be used to predict field effica cy of candidate vaccines among populations with a high prevalence of b lood group antigen O.