Cysteine may play a role in the immune response to internal parasites in sheep

To investigate the potential role of cysteine in the increased parasite susceptibility of sheep selected for increased wool production, ten 2-year-old rams from both fleece-weight-selected (FW) and randomly selected (C) Romney lines received either 2 g of supplemental cysteine per day (+CYS) or saline (+SAL), via abomasal infusion. Following drenching to remove any pre-existing parasites, all sheep were dosed with Haemonchus contortus and Trichostrongylus colubriformis infective larvae. Sheep were also subjected to non-parasitic immune challenges: inoculation with ovalbumin and a commercial clostridial vaccine containing the tetanus toxin. Cysteine infusion elevated plasma cysteine concentrations by an average of 65% (P < 0.0001), but no difference in cysteine levels was observed between selection lines. Off-pasture faecal egg counts (FEC) were higher in FW sheep (645 v. 200 eggs/g, P < 0.01). FEC were unaffected by selection line or infusion treatment following experimental infection, although FEC of +SAL sheep were at least double those of +CYS sheep. Total parasite numbers in the abomasum and small intestine at slaughter were unaffected by selection line or infusion treatment. Cysteine infusion tended to result in greater peripheral eosinophilia over the experimental period (P < 0.10), and higher counts of globular leukocytes in the abomasum at slaughter (P < 0.05). Anti-parasite antibody responses showed no response to experimental infection, tending to drop over time; previous parasitic infection may have confounded results. There were no effects of selection line or infusion treatment on IgG responses to tetanus toxin, although data suggested that a swifter response occurred in +CYS sheep. Anti-ovalbumin IgG responses tended to be greater in +CYS sheep (Day 52, P < 0.10). This study supports the greater susceptibility of FW sheep in natural parasitic challenge, as indicated by off-pasture FEC. Cysteine appears to influence certain aspects of immunocompetency in sheep, although the exact role of cysteine in the relationship between wool production and parasite susceptibility requires further elucidation.