Protective immunity against clinical sarcocystosis in cattle.

Five experiments involving 50 calves (Bos taurus) were conducted to determine if oral infection with sporocysts of the pathogen Sarcocystis cruzi or the non-pathogen Sarcocystis hirsuta would stimulate development of a protective immunity against illness or death from challenge infection with large numbers of S. cruzi sporocysts. Four experiments involved Holstein calves housed in individual pens in isolation buildings; one experiment involved Hereford calves on an open range. Calves infected with 50 000 to 100 000 S. cruzi sporocysts were protected from illness and death that would have resulted from challenge infections with 250 000 or 500 000 S. cruzi sporocysts given 70-252 days later. Calves treated with the anticoccidial drug amprolium from 21 to 35 days after being given an immunizing infection with S. cruzi to reduce clinical signs of the immunizing infection were also protected. Non-immunized calves and those given S. hirsuta sporocysts were not protected.