On the longevity of Schistosoma mansoni in man on St. Lucia, West Indies.

Data have been collected for at least five years in areas where transmission of Schistosoma mansoni has been controlled in the West Indian island of St. Lucia. Faecal egg output, as determined by Bell's technique, was periodically determined in 625 people and the geometric mean egg loads by year and age-group (in 1970) are presented. Assuming that egg output is linearly related to worm load, that flukes exhibit exponential survival, and survival is independent of worm load, the mean life-span of flukes within each host is calculated. It is valid to pool results for hosts ten years of age and older and a mean life-span of 3.3 years (with approximate 95% limits of 2.7 to 4.5 years) for the flukes is obtained. Results for younger hosts were heterogeneous and no pooled estimate is determined but it is suspected that the mean life-spans are greater for these hosts. The validity of some of the assumptions is discussed.