An experiment on six pairs of twin lambs earned out in 1946–6 was repeated in 1946–7, comparable results being obtained in the two years. One of each pair of twins was given 10,000, 15,000 or 20,000 infective larvae of Trichostrongylus axei when one month old; and four months later all the lambs were given 40 daily doses of 4,000 infective larvae of Trichostrongylus axei. Egg counts and weight gains were recorded weekly and when they were nine months old the surviving lambs were killed and examined post-mortem. The results show that the smaller doses of larvae given to lambs one month old increased their resistance to the second infestation but that the high doses (20,000 or more) decreased the resistance. There was an intermediate dosage level at which no effect, either of increased or decreased resistance, was observed.

The results of this experiment are discussed in relation to the work of other authors on resistance in animals to superinfection with parasites. It is suggested that the resistance to parasitic gastritis developed by sheep in the field may be partly a result of the intake of small numbers of larvae early in life and that it may actually be beneficial to expose young lambs to light infestations. It is, however, dangerous to expose them to a high level of infestation at an early age and because of the difficulty of assessing the dose of larvae necessary to stimulate resistance in a particular lamb, even by artificial infestation, it is impossible to make precise practical recommendations.