Anticoccidial vaccination: The absence or reduction of numbers of endogenous parasites from grosslesions in immune chickens after virulent coccidial challenge

Abstract

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 Eimevia acevvulina, Eimeviabrunetti, Eimevia maxima, Eimevia necatvix or Eimevia tenella. The relationship between the post-challengegrowth rate and specific gross lesion grades, matched in individual birds, was examined. The numbers ofendogenous parasites associated with graded lesions of each species were compared with those in challengednaive control birds. The vaccinated chickens were clinically immune judged by weight gains, but the naivecontrols reared under the same conditions were unprotected. The mean weight gains of vaccinated, immune birdshad lower coefficients of variation than those of unvaccinated, unprotected birds challenged with the samespecies. Whichever Eimevia 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); whereasall unvaccinated, unprotected birds had severe coccidial lesions, mostly graded 4. Innumerable endogenousparasites were associated with all lesions seen after challenging unvaccinated, naive birds, but 68% of the grosslesions in challenged, vaccinated, immune birds had no associated parasites, and the remaining 32% had veryfew. It is concluded that the use of lesion grades alone to assess an anticoccidial vaccine may under-rate itsefficacy, and that the occasional presence of gross lesions in commercially vaccinated chickens does not indicatevaccine failure unless performance is also adversely affected. Although in a naive bird gross lesions usuallyindicate disease, the presence of any gross lesions in a bird judged by performance criteria to be immune may beinterpreted as the host's successful repulsion of a parasite challenge.