Effect of age and initial infection intensity on the rate of reinfection with Trichuris trichiura after treatment.

The study examines the rate of re-acquisition of Trichuris trichiura infection after treatment in two populations, one of mixed age and the other of children with known pre-intervention infection intensity. A population living in a Caribbean village was treated with mebendazole and the rate of reacquisition of infection of four age classes (2-4, 5-10, 11-30, 30+ years) monitored over a 20-month period. The reinfection rate was higher in the child age-classes indicating either that children are more exposed to infection or that adults slowly develop a partially effective acquired immunity. A cohort of children (mean age 4.5 years) was separated into 3 intensity categories on the basis of expelled worm burdens and their rate of reacquisition of infection monitored over a 12-month period. The rate of reinfection was directly and positively associated with initial infection status. This may indicate that children with low intensity infections are consistently less exposed to infection or that they have effective immune responses. The latter conclusion, however, would imply that they had acquired this immunity early in life, and so appears to contradict the suggestion that resistance is only slowly acquired by adults. Reconciling these two conclusions may require more sophisticated immunological models than have been suggested previously for geohelminthiases.