A new protocol for a challenge test to assess the efficacy of live anticoccidial vaccines for chickens

The search for vaccines to control coccidioses caused by Eimeria species in chickens (Gallus gallus) is intensifying because of the increasing threat of drug resistance to anticoccidial agents. It is important, therefore, to develop a reliable standard method for the assessment of multivalent vaccine efficacy, because many criteria generally used to judge drug efficacy are not appropriate for vaccines. The lack of correlations between oocyst production, severity of lesions and bird weight gains is discussed. Furthermore, not all Eimeria species cause pathognomonic lesions. A new protocol for a vaccine efficacy test is described which uses growth rate of chickens after virulent challenge as the primary criterion and feed conversion ratio as the secondary criterion for protection against each of the separate coccidioses caused by the seven species of Eimeria that parasitize the chicken. The benefits to this protocol over previous ad hoc experimental designs are: (1) immunization is carried out with multivalent vaccines of Eimeria species up to the maximum of seven that may infect chickens; (2) assessments of immunity are carried out for each species separately so results can not be confounded; (3) the criteria of efficacy are those that are crucial to demonstrate commercial usefulness; (4) the possibility of drawing erroneous conclusions based upon inappropriate criteria such as oocyst production or lesion scores is avoided; (5) because the same criteria are used for each species, direct comparisons may be made amongst immunities to all of the species in the vaccine being tested. Results are presented from tests of three commercial batches of Paracox attenuated anticoccidial vaccine, showing that separate virulent challenges with all seven Eimeria species were controlled in vaccinated chicks.