Relationship of gastrointestinal histology and mucus antiparasite activity with the development of resistance to trichostrongyle infections in sheep.

Changes in the numbers of globule leucocytes, mast cells, eosinophils and goblet cells in the gastrointestinal mucosa were examined in relation to the development of resistance and elimination of nematodes in grazing sheep in their first year of life. Sheep immunised against Trichostrongylus colubriformis, and sheep treated with dexamethasone were also examined. A strong association between resistance to infection and the presence of globule leucocytes was found. In contrast, the numbers of mast cells or goblet cells were not correlated with resistance. Globule leucocyte and eosinophil numbers were also correlated with antiparasite activity in mucus. Immunising infections of T. colubriformis given to 10-month-old sheep, their duration limited by thiabendazole treatment, gave rise to considerable immunity to homologous challenge infections. Larvae that developed to the 4th stage were as effective at stimulating immunity as those that developed to the 5th stage. Dexamethasone treatment abrogated resistance to trickle challenge infection with T. colubriformis and reduced mucosal globule leucocyte and mast cell numbers. After cessation of drug treatment, the re-establishment of resistance and adult worm elimination were associated with repopulation of the mucosa with large numbers of globule leucocytes and high antiparasite activities in mucus.