Anticoccidial vaccination of broiler chickens in various management programmes: Relationship between oocyst accumulation in litter and the development of protective immunity

Paracox anticoccidial vaccine was administered to a 7-day-old flock of comm ercial broiler breeder stock subsequently reared to point-of-lay in the sam e house. For comparison, three subgroups of another flock of broiler breede rs were also vaccinated with Paracox at 7 days of age, reared to 42 days an d then transferred to new litter on another farm until point-of-lay. The fi rst subgroup received no further treatment, but the second and third each r eceived a second vaccination with Paracox, either immediately after transfe r to the new litter or 42 days after transfer. Using an Eimeria necatrix mo del, protective immunity was demonstrated by virulent challenge of samples of birds from all groups by the age of 37-40 days (30-33 days after the fir st vaccination), and was maintained to at least 122-125 days of age, whethe r the birds remained on the same litter or were transferred to another farm , and whether they received one or two anticoccidial vaccinations. Therefor e, there is no disadvantage in transferring birds onto new litter 35 days a fter a single Paracox vaccination, nor is there any advantage in giving a s econd vaccination after such a transfer. Vaccinated birds seeded the new li tter with oocysts, despite being clinically immune to coccidiosis. A supple mentary laboratory experiment showed that birds vaccinated at 8 days of age passed almost no oocysts after a second vaccination at 43 days of age. Thi s indicated that they were not only protected against clinical coccidiosis, but were almost solidly immune to a homologous infection 5 weeks after a s ingle vaccination. Nevertheless, oocysts appeared in the litter of all four groups of commercial breeders throughout the trial, showing that wild-type heterologous infections occurred whether the birds were transferred to new litter or not, but these did not overwhelm the acquired protective immunit y and cause clinical coccidiosis.