Immunisation of calves against Eimeria alabamensis coccidiosis.

Twelve calves which had been immunised with a trickle dose of altogether 100,000 oocysts of Eimeria alabamensis 16 days before turnout and 12 uninoculated calves were monitored during their first 20 days of grazing on a pasture naturally contaminated with oocysts of E. alabamensis. Eleven of the uninoculated calves developed gruel-like diarrhoea 3-6 days after turnout and excreted more than 850,000 oocysts/g of faeces (OPG) a few days later. In contrast, none of the immunised calves developed clinical coccidiosis and most of them excreted only a few oocysts. They lost on average 18 kg less in bodyweight than the unimmunised control calves. On day 21 all the calves were rehoused and on day 27 they were challenged with 10 million sporulated oocysts of E. alabamensis and turned out onto the same pasture. Only insignificant clinical signs were observed in 2 of the immunised calves and in one of the control calves. It was concluded that immunisation is a promising control measure for E. alabamensis coccidiosis. However, fewer or attenuated oocysts must be used, as 9 of the 12 inoculated calves developed clinical coccidiosis before turnout as a result of the immunisation doses.