OBSERVATIONS ON WORM POPULATION-DYNAMICS IN CALVES NATURALLY INFECTEDWITH SCHISTOSOMA-MATTHEEI

The evolution of faecal egg output, worm burdens and tissue egg counts in young calves was monitored during the first year of natural exposu re to Schistosoma mattheei infection on a Zambian farm. According to t he duration of their stay on the farm, these calves were classified in to 2 groups of 14 temporary tracers (TT calves) which were introduced on a 2-monthly basis for residential periods of 2 months, and 12 perma nent tracers (PT calves) introduced either at the beginning of the exp eriment (Group A) or 2 months later (Group B) and gradually removed af ter residential periods of 2, 4, 6, 8, 10 and 12 months on the farm. W orm counts in the TT calves showed that infection occurred throughout the year on the farm and that levels of infection acquired during each period of 8 weeks correlated well with the respective infected snail densities observed at the main transmission site. Marked differences i n worm population dynamics were recorded between the 2 groups of PT ca lves. In Group B animals which apparently were initially exposed to he avy transmission, according to the results from TT calves, much higher worm counts and greater susceptibility to reinfection were observed t han in Group A animals initially exposed to lighter exposure. These re sults suggest that the development oi resistance to natural infection with S. mattheei may depend on the initial exposure to the parasite. L ow initial exposures may lead to resistance whereas high initial expos ures may result in decreased immune responses resulting in susceptibil ity to infection.