The natural regulation of Trichostrongylus spp. populations in young grazing sheep.

The dynamics of Trichostrongylus axei and intestinal Trichostrongylus spp. were studied in grazing lambs over a 2-year period. Monthly comparisons were made between worm counts of experimental lambs continuously exposed to infection from 3 to 7 months of age, and tracer lambs which were allowed to graze on the experimental plots for successive 4-week periods. It was found that irrespective of the level of larval intake, experimental lambs were unable to regulate their worm burdens until at least 5 months of age. When larval intake was high, resistance to establishment of larvae apparently occurred at approximately 6 months of age followed by rejection and resistance to reinfection. In conditions of low larval intake, worms accumulated with no apparent control operating in lambs up to 7 months of age. These results suggest that a rapid turnover of worm populations does not occur in Trichostrongylus spp., and that in young lambs, worms accumulate until sufficient exposure to infection stimulates resistance to larval establishment and ultimately rejection of adults.