Three groups of lambs, aged 3, 5 and 6 months respectively, were infected daily with 5000 infective larvae of Trichostrongylus colubriformis for periods of 15–20 weeks. Some of the lambs also received a single challenge infection at the end of week 16. The course of the infections was monitored by means of faecal egg counts, measurement of live-weight gains and regular post mortem worm counts.
There was an initial rapid increase in egg counts soon after patency, which was followed by an exponential fall in the counts in most lambs. This fall seemed to precede the expulsion of worms by a few weeks and was associated with reduced fecundity in the female worms. Furthermore, although the trend of the egg counts of some lambs suggested that a self-cure reaction had taken place, in a number of these lambs the low faecal egg counts did not reflect the size of their worm burden.
Heavy worm burdens were usually associated with clinical symptoms of trichostrongylosis, including anorexia, diarrhoea and loss of weight. As the lambs developed resistance to reinfection and expelled their worm burden a rapid clinical recovery occurred, except in a few lambs which appeared to have suffered irreversible damage from the earlier heavy worm burden.
The worm burden in most lambs appeared to be cumulative during the first 4–8 weeks before they developed a strong resistance to reinfection. There was considerable loss of the adult worm burden by 8–15 weeks after initial infection and some evidence of stunting in the remaining parasites.
The immunological control of T. colubriformis infections in these lambs was achieved by means of the development of resistance to reinfection, inhibition of the ovulation of the female worms, stunting of adult worms and expulsion of adult worms by resistant lambs. These may be separate immune phenomena requiring different thresholds of parasitic material in individual animals, depending on their age, the rate and duration of infection and their innate ability to develop resistance to the parasite.