The histopathology of the reactions of calves to experimental infection with the nodular worm, Oesophagostomum radiatum (Rudolphi, 1803). II. Reaction of the susceptible host to infection with a single dose of larvae

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Abstract
The histopathological aspects of the host response to Oesophagostomum radiatum were studied in calves, reared worm-free, following a single dose of 5000–10,000 infective larvae. The penetration of larvae caused mainly productive changes in the gut tissue and was not followed by overt clinical symptoms. Five to seven days after infection, obliteration of small blood-vessels by endothelial proliferation and perivascular accumulation of histiocytes and lymphocytes occurred, \ particularly in the vicinity of the histotropic larvae, resembling a delayed type hypersensitivity reaction. The ensuing inflammation appeared to facilitate the termination of the histotropic phase by disorganizing the tissue produced around the third-stage larva during its development. The migration of fourth-stage larvae to the colon and subsequent moult to the fifth stage was accompanied by infiltration of eosinophil leucocytes into the gut tissue with accumulation of these cells around and within the mucosal glands, particularly in the colon. The resultant crypt abscessi, which resembled the active lesions found in ulcerative colitis, showed protracted healing, with formation of granulomas containing giant cells. Plasma cell production characteristic of a secondary response occurred only in the lymph nodes draining inflamed sections of the large intestine where adult worms were located. Healing of the acute enteritis resulted in fibrosis and hyperplasia of the gut wall.