Infection and re-infection patterns were evaluated in a recent Schistosoma mansoni focus in northern Senegal, by determining concentrations of serum circulating anodic antigen (CAA), as a measure of worm burden, and counting eggs in faeces before, 6 or 12 weeks and 1 year after praziquantel treatment in two subsequent cohorts (cohort A and B). No differences in egg counts and CAA concentrations or their relationship were found between the cohorts, which were examined 2 years apart. Within both cohorts, CAA concentrations showed the same, typical, age-related patterns as egg counts, with a peak in children and a strong decline in adults. These trends were apparent both before and 1 year after treatment. The results indicate that an age-related resistance to infection and to re-infection has been firmly established, at a steady level, in the recent S. mansoni focus investigated, with no indication of a gradual development of immunity or anti-fecundity immunity over a period of 2 years. Both shortly and 1 year after treatment, the decrease in egg counts was stronger than that in CAA concentrations, indicating that that there had been a reduction in worm fecundity after treatment. The possibility that praziquantel may induce anti-fecundity immunity has important implications for the use and interpretation of the results of (egg-count-based) re-infection studies designed to follow the development of naturally acquired immunity.
Polman, K., Stelma, F. F., Le Cessie, S., De Vlas, S. J., Falcao Ferreira, S. T., Talla, I., Deelder, A. M., & Gryseels, B. (2002). Evaluation of the patterns of Schistosoma mansoni infection and re-infection in Senegal, from faecal egg counts and serum concentrations of circulating anodic antigen. Annals of Tropical Medicine and Parasitology, 96(7), 679-689. https://doi.org/10.1179/000349802125001708