The effects of cold exposure, food allowance and litter size on immunity of periparturient sheep to Teladorsagia circumcincta and Trichostrongylus colubriformis

Sixty twin- and 60 single-bearing ewes were used in a 2 X 2 X 2 factorial design incorporating nutritional and shearing-cold exposure treatment (no. = 15 per subgroup) to study the effects on periparturient immunity against nematode infection. From week -8 to week -4 relative to parturition, animals grazed ryegrass-white clover pastures at allowances designed to provide either 0.8 (low allowance) or 1.2 (high allowance) of estimated metabolizable energy requirement for a ewe carrying 1.5 lambs. During week -4, half the sheep from each reproductive effort and nutritional group were shorn and exposed to artificial wind and rain for 4 h on each of 4 days consecutively. During the same week, all animals were challenged twice with 1.5 X 10(4) Teladorsagia circumcincta and 1.5 X 10(4) Trichostrongylus colubriformis infective third stage larvae to supplement the natural infection from pasture. From week -3 to week + 5 relative to parturition the sheep were run together and offered pasture at rates estimated to enable them to meet their nutrient requirement. Shearing and cold stress reduced ewe body weight (by up to 4.66 kg per sheep), and temporarily reduced the levels of IgA against T. circumcincta (by proportionately 0.24) and T. colubriformis (0.34), and raised faecal egg count (1.6 to 4.4 fold) but did not affect the parasite-specific total antibody levels in the serum. The low allowance during pregnancy was associated with a 6.09-kg decrease in ewe body weight, decrease in the levels of total antibody and of IgA against both T. circumcincta and T. colubriformis, and increased faecal egg count (by 1.5 to 10.6 fold) during pregnancy and lactation. Faecal egg count was consistently higher (up to 8.8 fold) in twin-bearing and rearing ewes than in single-bearing and rearing ewes except during the first 4 weeks of the experiment. A significant effect of litter size on antibody levels was relatively small when compared with short-term changes ( reductions) in antibody levels immediately around parturition. There were significant negative correlations between faecal egg count during late pregnancy and ewe body weight in lactation and significant negative correlations between the levels of total antibody against T. colubriformis and faecal egg counts near the end of the experiment. We conclude that litter size is likely to have greater influence on the resistance of ewes to nematode infections in the periparturient period than either pasture allowance or cold stress within the range of parameters used in this work.