The origins and biological significance of the coccidial lesions that occur in chickens vaccinated with a live attenuated anticoccidial vaccine.

The occurrence of vaccine-induced coccidial lesions in chicks vaccinated with a live attenuated anticoccidial vaccine is demonstrated. Healthy broiler chicks reared on litter to facilitate auto-reinfection, but isolated from extraneous coccidial infections, were vaccinated when 8 days old with Paracox. Coccidial lesions were found in chicks between 5 and 23 days after vaccination; 24% of the 87 chicks sampled during 29 days had intestinal lesions of Eimeria acervulina, Eimeria maxima, Eimeria tenella or of undetermined species, mostly (19 out of 21) scored as 1 or 2 on the Johnson & Reid scale of 0 to 4. The other two positive birds had E. tenella lesions, each scored as 3. The lesions observed up to 5 days after vaccination were identified as primary (i.e. a host response to the first vaccinal life cycle) and those observed from 6 days onwards were designated as primary or secondary (i.e. a host response to the second and subsequent vaccinal life cycles) in origin. In the absence of extraneous coccidial infections, none of the lesions observed could have been due to non-vaccinal coccidial infections. No adverse effects on the health or growth rate of the chicks exhibiting primary or secondary lesions in response to vaccination with Paracox were apparent.