The alleviation of chronic copper toxicity in sheep by ciliate protozoa.

1. Rams, fauna-free from birth and initially of 48-65 kg live weight, were allocated to two groups of ten each and given a diet containing 14 micrograms copper/g dry matter; five additional rams were killed and their livers were analysed for Cu. 2. One group (faunated) was inoculated with a mixed population of ciliate protozoa, and contained between 60 x 10(5) and 195 x 10(5) protozoa/ml rumen fluid throughout the 184 d experiment. The other group remained fauna-free. Following blood sampling, three rams in each group were killed on day 63, two on day 125 and four on day 184. One sheep in each group died during the experiment. 3. Faunated rams showed higher weight gains and feed consumption than fauna-free rams. 4. Plasma Cu concentration (microgram/ml) increased from an initial 0.82 to a final 1.00 in faunated and 1.36 in fauna-free rams. Liver Cu concentration (micrograms/g dry matter) increased from an initial 745 to a final 962 and 1684 in faunated and in fauna-free sheep respectively, representing a 4.3-fold greater increase in the fauna-free than in the faunated group. The absorption and retention of Cu was 38-50% higher in the fauna-free than in the faunated rams. 5. It was suggested that rumen ciliate protozoa increased rumen production of sulphide (through increased breakdown of soluble proteins) which complexed part of the Cu, making it unavailable for absorption and utilization. Therefore, ciliate protozoa could determine susceptibility to chronic Cu toxicity in sheep.