Chronic infection with Trichostrongylus vitrinus in sheep. Pathological changes in the small intestine.

Eight four-month-old sheep were infected daily for 90 days with 2500 Trichostrongylus vitrinus larvae. Infections were patent by week 3; mean faecal egg counts reached a peak of 645 epg at week 5, then subsequently declined. Mean worm burden at slaughter was 6100 (range 60--25,080). Two sheep had visible lesions in the small intestine at slaughter. In both sheep, numerous shallow red depressed areas extended for 5 to 7 metres distal to the pylorus while, in one, more extensive irregular depressed areas occupied the first 1.5 metres. Microtopographical examination and scanning electron microscopy showed that these affected areas were devoid of villi and contained numerous worms embedded in the mucosa. Histological examination revealed many similarities with the lesions caused by T colubriformis. Subtotal villous atrophy and crypt elongation were present in the intestines from five of the remaining six sheep, and in these the mucosa contained numerous mononuclear cells and intraepithelial globule leucocytes.