Population dynamics of Trichostrongylus colubriformis in sheep: the effect of host age on the establishment of infective larvae.

Sheep, reared worm-free in pens and aged 12-36 weeks, were infected each weekday with 2000 infective T. colubriformis larvae (L3). Establishment was measured at various times during the course of infection and the rate of development of resistance to new infection was found to be faster in older than in younger hosts. In addition arrested development at the exsheathed L3 stage was found to be less marked in older hosts. Sheep which had experienced natural infection on pasture up to 20 weeks of age before exposure, in pens, to the same experimental infections as their worm-free counterparts showed similar immune responses. However, at 36 weeks of age, pasture-reared sheep had acquired a high level of resistance to infection with T. colubriformis.