The use of a molluscicide in conjunction with chemotherapy to control Schistosoma haematobium at the Barombi Lake foci in Cameroon. II. Urinary examination methods, the use of niridazole to attack the parasite in man, and the effect on transmission from man to snail.

The use of niridazole, in conjunction with snail control, in an attempt to control the transmission of Schistosoma haematobium at 2 Cameroon crater-lake villages is described. Quantitative examinations of urines for S. haematobium eggs were made on the whole population before control began, at intervals during the maintenance of control, and again after control ceased. The methods and difficulties are described, and their accuracy is discussed. After each examination all accessible egg-passers were treated or re-treated with niridazole. The drug was well tolerated and effective. Most treated patients ceased to pass live eggs, and most of those who later became positive again were thought to represent reinfections rather than relapses. During the maintenance of snail control, niridazole treatment of the 'whole' community reduced the total load of S. haematobium eggs passed to less than 1 per cent of the pre-control figure; and repeated subsequent treatments maintained a level of the same order. Despite this, there remained at different times 0.6-50.9 per cent of persons passing small quantities of live eggs, and these were an important source of continuing transmission. The possibility that more than one strain of S. haematobium was involved is discussed. An index of transmission from man to snail is proposed and discussed in relation to determining the level of the "break-point" in transmission. Although combined niridazole/molluscicide control reduced transmission from man to snail by about 3 log cycles, the "break-point", which is thought to lie 4 or more log-cycles lower, was not reached. When control was stopped, transmission built up again, with the prevalence rate in man rising more rapidly than the intensity of infections.