Abstract
An experiment was carried out to study the effects of vaccination with Paracox, a live, attenuated vaccine against avian coccidiosis, on broilers isolated from extraneous Eimeria parasites. The study involved 3200 broiler chickens raised in floor pens similar to commercial conditions, but in an initially Eimeria-free environment. Forty percent of the chickens were vaccinated at 3 days of age and given either a basal unmedicated feed or a feed supplemented with the feed antibiotic virginiamycin. Unvaccinated birds were given either the basal feed or feed supplemented either with virginiamycin or the anticoccidial ionophore narasin. At slaughter at 36 days of age vaccinated birds had a lower live weight than non-vaccinated birds. The difference was 4.6% in unmedicated, and 6.0% in virginiamycin medicated chickens. Feed conversion ratio at slaughter was 2.5% higher for unmedicated vaccinated birds, and 1.3% higher for virginiamycin medicated vaccinated birds, compared to respective non-vaccinated groups. There was no significant difference in overall performance of unvaccinated birds given narasin as compared to virginiamycin. At 10 days post vaccination vaccinated birds had a higher number of Clostridium perfringens in the caeca, but there was no difference thereafter. Throughout the experiment, caecal clostridial counts were considerably higher in vaccinated unmedicated birds than in unvaccinated birds given narasin. The number of oocysts shed in the vaccinated groups was very low, but during a subsequent challenge with E. maxima and E. tenella the birds' immunity was found to be satisfactory.