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The effect of loss of weight on maintenance of acquired immunity to Trichostrongylus colubriformis in Merino sheep was assessed in 2 experiments. Adult Merino sheep with previously acquired immunity to T colubriformis were housed in individual pens. Sheep were infected 3 times weekly with 2000 T colubriformis larvae. A diet based on chopped oaten and lucerne hay (experiment 1) or a pelleted diet based on lucerne hay and grain products (experiment 2), was offered in amounts to sustain a moderate gain in body weight, or was reduced step-wise to cause a loss in body weight. Worm egg counts were determined weekly. The reduction in feed on offer resulted in a mean weight loss of 4.0 kg over 9 weeks (experiment 1) or 4.8 kg over 7 weeks (experiment 2). Based on worm egg counts, there was no indication of a change in immunity to T colubriformis in either of the experiments. These results suggest that acquired immunity to T colubriformis in adult sheep that are exposed to infection is maintained during periods of moderate loss of weight associated with inadequate nutrition.

Effects of a controlled-release albendazole capsule (CRC) on nematode parasitism and production of Merino ewes and lambs were investigated in a replicated grazing experiment between August 1986 and July 1989. The experiment was conducted on a site where the naturally occurring parasites were benzimidazole-resistant, with double the recommended dose of oxfendazole reducing faecal egg counts of Trichostrongylus colubriformis by less than 50%. Two CRC treatments were compared with a minimal treatment control and the widely-used strategic control program, Wormkill. Egg counts and worm burdens of major parasite species of ewes and of lambs before and after weaning were significantly reduced by use of the CRC in ewes before lambing or in lambs at weaning. These parasitological effects were reflected in increased productivity of CRC ewes and lambs. Ewes given CRCs lost less weight and grew more wool during lactation than ewes given Wormkill or control treatments, while their lambs gained more weight and had heavier fleeces than lambs from Wormkill or control treatments. Where lambs were weaned by removing ewes from the lambing paddock, administration of the CRC at weaning to lambs whose dams had also been treated with the CRC did not result in improved production when compared with lambs from ewes dosed with CRCs and treated after weaning according to the Wormkill program. Some possible uses for the CRC in ewes and lambs were identified.

Three paddocks were contaminated with Haemonchus contortus eggs from early spring to mid summer by yearling sheep. In Paddock 1, all contamination was derived from worms that had been previously inhibited in their development. The contamination in Paddock 2 was produced only by infections recently acquired from pasture, and in Paddock 3 contamination was attributable to both sources. When the yearling sheep were replaced in mid-summer by newly weaned lambs, severe outbreaks of haemonchosis occurred on all paddocks. It was concluded that inhibited H. contortus larvae are capable of producing sufficient eggs, when they resume development, to initiate an outbreak of haemonchosis in susceptible sheep.

The prevalence of nematode infections in lactating dairy cattle was determined by faecal egg counts and cultures from over 350 cows selected at random from twelve herds. The mean egg count was 3 eggs per gram (epg) and 43% of the cows sampled were passing detectable concentrations of eggs (greater than 6 eggs) in their faeces. In June 1976 171 cows in 10 herds were treated with 7.5 mg/kg fenbendazole and their milk production compared over the following 2 months with that of 164 untreated cows in the same herds. Anthelmintic treatment had no significant influence on milk production in any of the herds. The contrasting results of this study and recent studies in the United States of America are discussed.

Treatments with mebendazole, cambendazole or febantel were ineffective in reducing the faecal egg count of a group of 40 horses infected with small strongyle species. Evidence was also obtained that this apparent resistance extended to fenbendazole. In horses dosed with morantel tartrate egg counts were reduced to zero by 7 days after treatment. The development of resistance is discussed in relation to previous treatment of the horses and the mode of action of febantel.