RESUMO
1. Coccidiosis, caused by different Eimeria species, is believed to be a more prominent problem in loose-housed layers kept on litter than in battery cages. In this study, the impact and development of Eimeria infections were investigated in layers kept in litter-based, high stocking density systems for loose-housed hens. 2. Layers from 57 flocks on 26 farms were followed by necropsy of a representative sample of birds that died or had to be culled. Coccidiosis was diagnosed in 11 flocks (19.3%) from 9 (31%) of the farms. The outbreaks occurred when the birds were 19 to 32 weeks old. E. maxima was identified in 6 and E. tenella in 3 of the outbreaks. 3. Sixteen of the flocks were also monitored with faecal and litter samples collected at regular intervals. Oocysts were detected in samples from all these flocks. The pattern of oocyst excretion was similar in most of the flocks, with maximum counts at 4 to 8 weeks after introduction to the laying house. There was no significant correlation between the levels of oocysts in faeces and clinical coccidiosis. 4. Raising pullets without any coccidiostat, to increase their chance to develop immunity against coccidia, was not found to decrease the risk of coccidiosis during the production period when compared to the practice of giving amprolium and ethopabate during the rearing period.