Published online by Cambridge University Press: 05 June 2009
Old lambs (8 months of age) infected with 50,000 L3 of Nematodirus battus had larvae developing normally in their tissues on day 4 post-infection (P.I.), but by day 8 P.I. there were only 4105 ± 1044 worms left in the alimentary tract. Some of these worms contained crystals in their intestine. Eight-month-old lambs treated with dexamethasone and infected with 50,000 L3 of N. battus contained a mean worm burden of 7878 ± 1262 on 18 days P.I. Untreated 8-month-old lambs similarly infected were virtually worm free by day 18 P.I. Peripheral eosinophilia became elevated in the untreated lambs over the course of infection and, at post-mortem, the tissue mast cell and eosinophil counts were much higher than in the dexamethasone treated group. Although the phenomenon of age resistance is thought to have a strong immunological component, there may also be other physiological factors, resulting in fewer nematodes and lower fecundity of the worms.