Anticoccidial vaccination: the absence or reduction of numbers of endogenous parasites from gross lesions in immune chickens after virulen coccidial challenge

Male and female floor-reared chickens were immunized with a live, attenuated anticoccidial vaccine (Paracox) and then, 28 days after vaccination, were challenged with virulent strains of Eimeria acervulina, Eimeria brunetti, Eimeria maxima, Eimeria necatrix or Eimeria tenella. The relationship between the post-challenge growth rate and specific gross lesion grades, matched in individual birds, was examined. The numbers of endogenous parasites associated with graded lesions of each species were compared with those in challenged naive control birds. The vaccinated chickens were clinically immune judged by weight gains, but the naive controls reared under the same conditions were unprotected. The mean weight gains of vaccinated, immune birds had lower coefficients of variation than those of unvaccinated, unprotected birds challenged with the same species. Whichever Eimeria species the birds were challenged with, a proportion (5 to 75%) of the challenged, vaccinated, immune birds exhibited some coccidial lesions, mostly graded 1 or 2 (on a scale of 0 to 4); whereas all unvaccinated, unprotected birds had severe coccidial lesions, mostly graded 4. Innumerable endogenous parasites were associated with all lesions seen after challenging unvaccinated, naive birds, but 68% of the gross lesions in challenged, vaccinated, immune birds had no associated parasites, and the remaining 32% had very few. It is concluded that the use of lesion grades alone to assess an anticoccidial vaccine may under-rate its efficacy, and that the occasional presence of gross lesions in commercially vaccinated chickens does not indicate vaccine failure unless performance is also adversely affected. Although in a naive bird gross lesions usually indicate disease, the presence of any gross lesions in a bird judged by performance criteria to be immune may be interpreted as the host's successful repulsion of a parasite challenge.