Heat tolerance of Boran and Tuli crossbred steers.

Experiments were conducted to evaluate the heat tolerance of the following breeds: Hereford (H), Brahman (B), H x B, H x Boran (H x Bo), and H x Tuli (H x T). Heat tolerance was evaluated in a climatically controlled room (Exp. 1) and under summer environmental conditions (Exp. 2) by comparing rectal temperatures (RT), respiration rates (RR), and sweating rates (SW). In Exp. 1, under extremely hot conditions (mean temperature-humidity index [THI] > 90), purebred B had significantly (P < .05) lower RT and RR than other genotypes, which may be indicative of greater surface area per mass to dissipate heat and a lower metabolic rate than other genotypes. Boran and Tuli crosses had RT (39.5 degrees C) that were intermediate to those of B (39.0 degrees C) and H x B (40.0 degrees C). The H genotype had the greatest RT at 40.3 degrees C. Among the breeds, trends in RR were similar to RR observed at THI < 77; B had the lowest RR, and H x B were intermediate. However, in these extreme conditions, RR did not differ among the purebred H and the Boran and Tuli crossbred steers, but H x B steers had lower RR than the other H crossbred steers. Sweating rates were significantly greater for the Bos indicus x Bos taurus crosses (H x B and H x Bo) than for the purebred genotypes (H and B) and the Bos taurus cross (H x T). In Exp. 2, mean RT for B, H x B, H x Bo, and H x T were very similar to those recorded under the moderate heat stress conditions found in Exp. 1. There were no differences in RT among B, H x Bo, and H x T genotypes. The RR increased over time for H only, and RR for other genotypes tended to be elevated only slightly over time. Among genotypes, SW was significantly greater for the H x Bo steers. The ability of the Bos indicus crosses to dissipate heat through enhanced SW and associated evaporative cooling was evident. However, the heat-tolerant nature of the Bos taurus cross (H x T) was not evident through enhanced RR or SW in either experiment. Compared with other genotypes, the lower RR of B steers was clearly evident and is assumed to be due to greater surface area and other skin characteristics that allow them to dissipate heat to maintain lower RT. These data suggest that the H x Bo and H x T are similar to H x B and intermediate to H and B genotypes in maintaining homeostasis when exposed to a high heat load.

Inhibition of bovine alpha-glucosidase by Castanospermum australe and its effect on the biochemical identification of heterozygotes for generalised glycogenosis type II (Pompe's disease) in cattle.

All 18 2-year-old Brahman bulls grazing in a paddock containing Castanospermum australe trees were diagnosed as heterozygotes for Pompe's disease by measurement of mononuclear cell alpha-glucosidase activity. However, removal of the bulls to a paddock free of C. australe and retesting 2 months later indicated that 15 were homozygous normal. An in vitro assay demonstrated that a crude aqueous extract of seeds from these C. australe trees contained a potent inhibitor of mononuclear cell alpha-glucosidase. Two Hereford steers were dosed with 0.6 g C. australe seed/kg bodyweight for 6 days. The alpha-glucosidase activity in blood mononuclear cells declined to 5% of normal within 48 h of commencement of dosing. It was therefore assumed that the bulls had consumed C. australe seeds. A means of differentiating true heterozygotes from animals consuming the toxic seed, using the ratio of plasma alpha-glucosidase activity at pH 5.6 to that at pH 3.7, is proposed.

The isolation of a field strain of Haemonchus contortus in Queensland showing multiple anthelmintic resistance.

Following the apparent failure of levamisole to control infections of Haemonchus contortus in sheep at Lawes in south eastern Queensland, a strain of this parasite was isolated at the Animal Research Institute, Yeerongpilly. This strain was used to infect sheep at Yeerongpilly and the Merrindale Research Station, Victoria where four experiments to classify the resistance pattern of the parasite were carried out. Resistance to thiabendazole was first suspected in 1969, and these experiments confirmed that resistance to this drug was still present. They also showed that a strong degree of resistance had been developed to both levamisole and morantel tartrate. Other benzimidazole anthelmintics and also the organophosphorus compound naphthalophos were only moderately effective against the original isolate but rafoxanide, nitroxynil and phenothiazine were almost 100% effective. Other highly effective chemicals were disophenol and closantel. After passaging the strain for four generations with both levamisole and albendazole, resistance to both naphthalophos and the newer benzimidazole anthelmintics increased dramatically. This is the first report of a field strain of H. contortus exhibiting resistance to benzimidazole, non-benzimidazole and organophosphorus anthelmintics.

Leucoencephalomyelitis of goat kids.

A leucoencephalomyelitis in 6 goat kids 2 to 5 months old is described. The disease was characterised by fever, ataxia, posterior paresis, circling and hyperaesthesia progressing to prostration. The neural lesion was confined to the white matter of the cerebellum and posterior brain stem in 4 kids, but in 2 others the cervical spinal cord was the main site affected. The lesion was characterised microscopically by dense perivascular cuffing with mononuclear cells, infiltration of the parenchyma with macrophages and a proliferation of glial cells and by a marked primary demyelination. In more advanced lesions, areas of the neurophil were replaced by a loose glial scar. There were associated pulmonary lesions of interstitial pneumonitis and hyperplasia of the peribronchiolar lymphoid tissue. Attempts to isolate an aetiological agent and to transmit the disease to young goat kids and lambs were unsuccessful. The disease has not been reported before in Australia but has distinct similarities to an infectious leucoencephalomyelitis of young goats which has been described in North America.