Population dynamics of Trichostrongylus colubriformis in sheep: the effect of infection rate on the establishment of infective larvae and parasite fecundity.

The establishment of Trichostrongylus colubriformis was estimated in helminthologically naive 20-week-old Merino sheep given third stage infective larvae (L3) at rates of 2000, 632 or 200 L3 per day, 5 days per week. After varying periods of continuous L3 intake, a levamisole-susceptible strain of T. colubriformis was replaced with a highly resistant strain for 1 week. The animals were then treated with levamisole to remove the susceptible population, and establishment of the cohort of resistant worms was estimated. In previously uninfected sheep, approximately 65% of the L3 given in the first week became established as adults. This fell to low levels (less than 5%) after 7, 10 and 14 weeks of continuous L3 intake for the high, medium and low infection rates, respectively. At the low infection rate, establishment remained at maximum levels for the first 4 weeks, but then fell at a rate similar to that observed for the higher infection rates. This implied that a threshold of worm exposure was required before resistance to establishment developed. Parasite egg production, expressed as eggs per gram of faeces, was proportional to infection rate and is explained by higher worm burdens occurring at high infection rates. However, estimates of fecundity in eggs per female per day showed the opposite relationship with rate of infection. Fecundity stayed high (approximately 600) for 5 weeks at the low infection rate but only maintained this level for 3 weeks and 1 week at the medium and high rates, respectively. This suggests that fecundity, like establishment, was similarly affected at threshold levels of immunological recognition.