The development of immunity to Eimeria species in broilers given anticoccidial drugs.

The acquisition of immunity to Eimeria species was studied in broilers reared in floor-pens on new and used litter, and given different anticoccidial drug programmes. Programmes included a single drug (synthetic compound or ionophore) given in the starter and grower feed, or 'shuttle' programmes comprising a synthetic drug followed by an ionophore (or vice versa) given in the starter and grower feeds, respectively. None of the drug programmes prevented parasite multiplication, since oocysts were found in the intestines of birds and in the litter. Birds were challenged at weekly intervals with a mixture of species (E. acervulina, E. maxima and E. tenella) in order to establish whether protective immunity had developed. Birds reared on used litter that received no medication acquired immunity by 5 weeks, whereas birds reared on new litter were not fully protected against a challenge inoculum until 7 weeks of age. Apart from birds given an ionophore followed by a synthetic drug, medicated birds developed immunity by 7 weeks, irrespective of the type of drug programme, or whether they had been reared on new or used litter. Birds reared in commercial houses (where one and seven previous flocks had been raised) and given a synthetic drug followed by ionophores, also developed immunity to Eimeria species. Immunity was acquired more rapidly to E. maxima than E. acervulina or E. tenella.