Immunity of sheep to homologous challenge with dog-borne Sarcocystis species following varying levels of prior exposure

Ford G. E. 1985. Immunity of sheep to homologous challenge with dog-borne Sarcocystis species following varying levels of prior exposure. International Journal for Parasitology15: 629–634. Grazing sheep in which sarcocysts develop (sarcosporidiosis) survive the pre-cyst stages of the parasite infection which cause pathophysiological changes (sarcocystosis). A peak temperature rise occurs at 3–4 weeks post-infection in experimental animals. A maximum fall in packed cell volume occurs at 5–6 weeks post-infection. Groups of sheep maintained specific pathogen free were experimentally infected with sporocysts of dog-origin to determine the dose level required for protection or resistance, and 13 weeks later challenged with a pathogenic dose of 50,000 sporocysts. Prior exposure levels (PEL) were used of nil, 5, 50, 500 and 5000 sporocysts. All prior infections elicited an antibody response to a complement fixation test in all sheep. A peak antibody level was detected within 2 weeks of challenge, with a higher titre for all previously exposed groups than for the nil PEL controls. Resistance of the sheep to the challenge dose, based on reduction of counts of parasites developing (sarcosporidiosis), was shown by significantly lower counts for the 50 PEL group (85%), as well as for the 500 (92%) and 5000 PEL (98%) groups. The 5 PEL group was not resistant. The protection of sheep, based on reduction of the above pathophysiological changes (sarcocystosis), was highly correlated with the level of prior exposure (r = ?0.80, P < 0.001). The changes in body temperature and packed cell volume were significantly less in groups with 500 and 5000 PEL than with nil and 5 PEL. The 50 PEL group was intermediate.