Evaluation of an experimental irradiated oocyst vaccine to protect broiler chicks against avian coccidiosis.

ABSTRACT

The current study investigates the use of irradiated oocysts to protect broiler chicks, raised on litter, from infection with multiple species of Eimeria. In order to determine the optimum radiation dose for each Eimeria species, 1-day-old chicks were immunized with oocysts of Eimeria maxima, Eimeria acervulina, or Eimeria tenella exposed to gamma radiation ranging from 0-500 Gy. The litter oocyst counts at 7 days postimmunization, and the effect on weight gain following a challenge infection, decreased with an optimum dose between 150-200 Gy. Based on this finding, broiler chicks were immunized with a mixture of E. maxima, E. acervulina, and E tenella that had been exposed to 150 or 200 Gy. This resulted in more than a 100-fold reduction in litter oocyst counts and significant protection from a challenge infection, as measured by improved weight gain and feed conversion ratio (FCR). Immunization of birds with oocysts receiving 200 Gy was less effective in providing protection from a challenge infection. An additional formulation of vaccines containing two different oocyst doses of the three species that had been irradiated with 150 Gy were evaluated in their ability to attenuate oocyst output and convey protection to challenge. Results were similar with both high and low numbers of irradiated oocysts. Immunized chicks shed less oocysts at 7 days postimmunization and were protected from negative effects of challenge infection as measured by FCR, changes in weight gain, lesion scores, and measurement of body composition. However, the level of protection was somewhat less than that achieved by immunization with nonirradiated oocysts. The overall conclusion is that an irradiated oocyst vaccine developed in this study can effectively protect chicks that are raised on litter from challenge infection with multiple species of Eimeria, comparable to vaccines with virulent or precocious strains.