Epidemiology of trichostrongylidosis in cattle: development of infective larvae in the second part of the grazing period and their contribution to infections in the same year

Abstract

A grazing experiment was performed in the Swiss midland region with 22 first-season calves which were grazed together from May to mid July when they were allocated to two equal groups, one receiving a morantel sustained-release trilaminate bolus (Paratect Flex®) bolus, Pfizer, Zurich, Switzerland) and the other one remaining as an untreated control. Each group was moved to a clean pasture on 21 July. The larval contamination on the pasture with the control animals reached 4652 third stage larvae (L 3) per kilogram of dry matter (L 3 kg −1 DM) after 10 weeks. Parasitic gastroenteritis was observed in two calves of the control group in August, and mean serum pepsinogen levels exceeded 4000 mU tyrosine in this group in September. Eggs from Ostertagia sp. and Cooperia sp. that were excreted by the animals at the beginning of July and the end of August showed similar hatching rates of 94% and 99%, respectively. In the calves of the bolus group the egg output was reduced by 95% within 14 days of administration of the bolus. Larval contamination on the pasture of the bolus group was zero until the beginning of September when a slight increase occurred, reaching 793 L 3 kg −1 DM in October. At the end of the trial the bolus-treated calves had a mean weight gain advantage of 18 kg ( P<0.05) compared with the controls. The results confirm that a second generation of larvae developed on pasture in August and September and caused parasitic gastroenteritis in first-year grazing calves in late summer. The relevance of the results for the prophylaxis of parasitic gastroenteritis in calves in mixed grazing systems with calves and older cattle is discussed.