The acquisition of immunity to Eimeria species was studied in broilers rear
ed in floor-pens on new and used litter, and given different anticoccidial
drug programmes. Programmes included a single drug (synthetic compound or i
onophore) given in the starter and grower feed, or 'shuttle' programmes com
prising a synthetic drug followed by an ionophore (or vice versa) given in
the starter and grower feeds, respectively. None of the drug programmes pre
vented 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 l
itter that received no medication acquired immunity by 5 weeks, whereas bir
ds reared on new litter were not fully protected against a challenge inocul
um 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 u
sed 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 rap
idly to E, maxima than E, acervulina or E. tenella.