Physiological and parasitological responses to nematode infections of fattening cattle in the Western Pampas of Argentina

The epidemiology of nematode infection was studied in fattening grating cat tle from weaning (April 1994) to market at the end of their second autumn ( July 1995). Sixty Aberdeen Angus calves of seven months of age were randoml y allocated by weight to two groups: GT, treated every three weeks with dor amectin (200 mcg/kg); and GI, an infected group, only treated with fenbenda zole (7.5 mg/kg) at weaning and on the Ist of October. The two groups were grazed together on contaminated lucerne pastures until July, on 'clean' oat pastures until October and again on contaminated lucerne until the end of the trial. Fecal egg counts (epg), herbage larvae (Li), serum pepsinogen (P ep) and blood eosinophils (Eo) were evaluated monthly. Eight steers were sl aughtered for worm recovery, three in July 1994, three in December 1994 and two in July 1995. Grazing feed intake was estimated by fecal output (chrom ic oxide method)/l-diet digestibility and to measure non-specific response, Brucella antibodies were detected at 11 and 40 days post-vaccination in ea rly winter, Fecal egg counts, Pep and Eo of GI increased (P<0.01) from Apri l to July when there was a moderate-to-high level of infection. Ostertagia, Trichostrongylus and Cooperia were the predominant genera. By late winter, all parameters decreased on oat 'clean' pastures and increased again when cattle returned to moderately infected lucerne. During summer, the paramete rs measured reflected the negligible numbers of Lj On pastures until early autumn. At this time, increased numbers of L-3 were followed by a moderate rise (P<0.01) of epg, Pep and Eo values. During winter, GT showed higher (P <0.04) Brucella antibody IgG titers while feed intake of GI was 24.9% depre ssed (P<0.02). There were total cumulative weight-gain (WG) differences (P< 0.001) between groups (GT=263.1 kg; GI=214.3 kg). During the second autumn, the mean WG of GT steers was 16.6 kg greater (P<0.04) than that of GI. Vac cination titres against Brucella suggested non-specific depression of immun ity, while higher Pep and Eo levels in second-year steers may have reflecte d hypersensitivity reactions. (C) 1999 Elsevier Science B.V. All rights res erved.