Assessment of recombinant bovine somatotropin as an immunomodulator during avian coccidiosis: immunization with living oocysts.

Coccidiosis, a disease of great economic importance to the poultry industry, is generally controlled prophylactically by additions of anticoccidial drugs to the feed. However, increasing development of drug-resistant coccidia species has stimulated searches for alternative control methods, one of which is vaccination. As part of this effort, recombinant bovine growth hormone (rbST) was tested as a possible immune stimulator in combination with live oocyst vaccination. At a dose of 0.045 mg per chick, given by s.c. injection at 1 d of age, rbST did not improve immunity developed by immunization with 500 or 2,500 oocysts of Eimeria maxima as judged by weight gain and lesion scores. At a single dose of 0.09 mg per chick given at 1 d of age in combination with IMMUCOX, rbST provided some protection against challenge infection with Eimeria tenella but not Eimeria acervulina as judged by reduction in lesion scores. Treatment with 0.09 mg rbST per chick alone at 1 and 3 d of age was protective against challenge with E. tenella but not E. acervulina or E. maxima as judged by reduction in lesion scores. These results strongly indicate that rbST can act as an immune modulator in chickens infected with coccidia, and provide a basis for further investigations of its use as a vaccine adjuvant.