Demonstration of immunity against Isospora suis in swine.

Piglets naturally exposed or experimentally infected with Isospora suis oocysts were given challenge doses of oocysts to determine the extent of development of immune resistance. Piglets in both studies shed low numbers of, or no detectable oocysts, following challenge. Administration of methylprednisolone acetate failed to induce oocyst shedding in previously infected piglets. Piglets rechallenged with I. suis following steroid injections also failed to shed significant numbers of oocysts suggesting development of immunity to reinfection.