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Observations on the pathogenic effects of Nematodirus helvetianus in dairy calves.

Six Holstein-Friesian calves 3 to 5 months old, were used in a small-scale experiment to determine the pathogenic effects of the thread-necked strongyle Nematodirus helvetianus. Two calves were kept as controls and 4 were infected, 2 with single doses of 25,000, and 2 with 50,000 N helvetianus larvae. Observations made during the 8 weeks of the experiment found that N helvetianus is capable of producing severe pathologic changes in the intestine of previously healthy calves. At 38 days after infection, the calves infected with 25,000 and 50,000 larvae gained 32.9% and 72.4% less weight respectively, than their controls. An increase in total lymphocytes was observed with no significant change in hemoglobin concentration. Adult worms were found to penetrate the intestinal mucosa with considerable destruction, including tunneling. Rectal temperatures were elevated in infected animals. The prepatent period for N helvetianus in single infection was 18 to 19 days. A spontaneous expulsion of the adult parasites was observed in 1 of the calves exposed to 50,000 larvae. Worms recovered in this animal numbered 565, whereas the number of worms recovered in another calf infected with 50,000 larvae was 21,860.