Research note: Comparison of live attenuated and non-attenuated Eimeria vaccines on safety following overdose administration using the European pharmacopoeia monograph model.

Live non-attenuated and attenuated coccidiosis vaccines are available and induce a solid immune response to Eimeria spp. However, knowledge regarding the safety of live non-attenuated Eimeria vaccines is still limited. This fourteen-day, randomized, controlled, blind study was aimed to compare the safety profiles of a live attenuated and two live non-attenuated coccidiosis vaccines administered at overdose (10x) in chickens, following the standard model established in the European Pharmacopeia (Ph. Eur.) monograph. Fourteen-day old chickens (n=160) were randomized into four groups (1:1:1:1 ratio) balanced by weight to receive a commercial live attenuated vaccine, two commercial live non-attenuated vaccines, or PBS (control). Three birds died during the study, with only one death being attributable to inoculation of a non-attenuated vaccine. Few birds displayed clinical signs in the attenuated vaccine group, whereas those in the non-attenuated vaccine groups had bloody feces and gradually increased affected chickens. Animals receiving the attenuated vaccine had no or mild (grade 1) intestinal lesions compared to those receiving the non-attenuated vaccine with mild to moderate (grade 2) and for one of the vaccines also severe lesions (grade 3). Birds receiving the attenuated vaccine showed significantly increased mean body weight at day 14 (P < 0.001), without differences from the control group. Feed consumption was similar and lower in the control and attenuated vaccine groups, while was higher in the non-attenuated vaccine groups (from day 6 to day 14), indicating a more efficient feed conversion ratio (not significant). Birds receiving non-attenuated vaccines excreted more oocysts at later time points than birds receiving the attenuated vaccine. The attenuated Eimeria vaccine showed a favorable safety profile compared to non-attenuated vaccines regarding clinical signs and intestinal lesions, with no impact on body weight and feed conversion ratio under experimental conditions in SPF chickens, suggesting that this coccidiosis prevention strategy may contribute to decreasing coccidiosis costs.