Estimation of intestinal nematode prevalence: influence of parasite mating patterns.

Prevalence of infection with intestinal helminths is usually assessed through stool examination. There are limitations in using this technique to measure infection status, since some infected individuals have no patent infection. The relationship between infection assessed by stool examination and actual infection prevalence is investigated for the three major nematode species--Ascaris lumbricoides, Trichuris trichiura and the hookworms--using a model which describes the presence of non-egg producing worm combinations. The analysis demonstrates that stool examination under-estimates the actual infection prevalence, and that the degree of under-estimation is dependent on the level of infection, the nematode species and the parasite sex ratio. These findings have implications for the validity of epidemiological surveys and the evaluation of control programmes.