Trichostrongylus vitrinus: the influence of age of sheep and population size on the intestinal distribution.

Two age groups of parasite-free lambs, 4 months and 8 months, were infected with 50 000 infective larvae (L3) of T. vitrinus. A further group of 8 month old lambs were infected with 250 000 L3 T. vitrinus. The lambs were slaughtered at intervals after infection, and the size and linear distribution of their intestinal worm burdens examined and compared statistically. After the 50 000 L3 infections, it was found that a higher percentage of the initial inoculum had become established in the younger lambs but that there were no differences between the intestinal distributions of the population in either age group. After infection with 250 000 L3, the percentage of the larvae retained changed with time. Up to sixteen days post infection, 38% of the larvae administered were retained; thereafter the percentage fell to 22%. The linear distribution of the population in this size of infection showed that a significantly greater percentage was present between 4.8 and 7.2 metres distal to the pylorus when compared with that after infection with 50 000 L3.