Effect of dexamethasone on the ability of sheep to resist reinfection with nematodes

Five groups of wethers were placed on pasture known to be infected with nematode larvae. Groups I to IV were reared on pasture, and group V was drawn from pen-reared, ‘worm-free’ stock. Groups III and IV and tracers (group V) were drenched at the beginning of the experiment, but groups I and II were not. From day 0, when placed on the infected pasture, groups II and IV were treated twice weekly with dexamethasone trimethylacetate. Faecal egg counts and blood lymphocyte counts were recorded throughout the experiment. On day 56, five animals from each group were slaughtered and their worm burdens counted.Faecal egg counts and worm burdens of dexamethasone-treated sheep were much higher than the respective controls; the tracer animal counts were intermediate. Lymphocyte counts of groups II and IV were significantly lowen than for group I by day 50. Some animals were vaccinated with killed Salmonella typhimurium on day 21; these animals produced antibodies despite treatment with dexamethasone. Titres were, however, lower than for non-dexamethasone-treated animals.