Anthelmintic resistance in nematode parasites of sheep: learning from the Australian experience.

In recent years the problem of anthelmintic resistance has become of major importance in sheep and goat flocks throughout the world. In Australia, concerted efforts were initiated in the mid 1980s to halt the increase in resistance by the development and promotion of regional worm control programmes. This article outlines the genesis of these programmes, the way in which they were promoted and successfully adopted by farmers, and the concerns for effective parasitic control of sheep and goat flocks in the future. The principles outlined should be applicable in other countries where anthelmintics are relied upon for the control of nematodes in livestock.

Multiple anthelmintic resistance in Trichostrongylus colubriformis.

Following the failure of anthelmintic treatment to control an outbreak of trichostrongylosis in sheep, multiple resistance to levamisole and oxfendazole was confirmed in field strains of Trichostrongylus colubriformis at the CSIRO Pastoral Research Laboratory, Armidale. Resistance in Trichostrongylus spp to levamisole was also confirmed on an adjoining farm. From the results of an experiment where lambs were treated at the recommended dose rate with one or both anthelmintics, it was estimated that 32% of T. colubriformis were resistant to levamisole, 19% to oxfendazole, and 12% to both drugs. Simultaneous administration of levamisole and oxfendazole resulted in an additive anthelmintic effect. Naphthalophos (36.6 to 51.2 mg/kg) was 93% efficient against the multiple resistant strain. The similar histories of anthelmintic usage at the CSIRO Pastoral Research Laboratory and on the northern tablelands of New South Wales generally, suggest that multiple anthelmintic resistance in T. colubriformis may soon emerge as a problem on the northern tablelands.

Control of helminthosis in lambs by strategic treatment with closantel and broad-spectrum anthelmintics.

Treatment of ewes with broad-spectrum anthelmintic in August (pre-lambing) and early November, and of lambs in early November and early February, was effective in controlling infections with Trichostrongylus spp in lambs reared on contaminated pastures under set-stocked conditions. It was ineffective in controlling infections with Haemonchus contortus; 82% of lambs had to be withdrawn from the experiment because of severe haemonchosis. Treatment with closantel (7.5 mg/kg) at the same times was very effective against H. contortus but ineffective against Trichostrongylus spp; 25% of lambs had to be withdrawn because of severe trichostrongylosis. The same schedule using broad spectrum anthelmintic and closantel administered concurrently was effective against both parasites; no lambs had to be withdrawn and the bodyweight gain of lambs was higher than in lambs treated with broad-spectrum anthelmintic or closantel alone. The results provide a basis on which to develop a preventive anthelmintic treatment program to control haemonchosis and trichostrongylosis in sheep which will allow the current high frequency of treatment with broad-spectrum anthelmintics to be reduced. Such a program may retard selection for anthelmintic resistance in Trichostrongylus spp.

Development and distribution of Oesophagostomum columbianum in young lambs after oral or intraruminal infection.

There was no difference in the establishment or rate of development of Oesophagostomum columbianum in young lambs after infective larvae were administered either orally or by injection directly into the rumen. However, the linear distributions in the intestine of encysted third-stage larvae differed according to the route of infection. The distributions of fourth-stage larvae, after migration to the large intestine, did not differ. It is suggested that the distribution of parasitic third-stage O. columbianum is host-dominated, depending on the site of exsheathment and the rate of passage of ingesta, but that of fourth-stage and adult worms involves active site selection by the parasite.

Effectiveness of broad spectrum anthelmintics against selected strains of Trichostrongylus colubriformis.

A field population of Trichostrongylus columbriformis was divided into 4 lines for exposure to selection in the laboratory. The first line was selected with 50 mg/kg thiabendazole, the second with 4 mg/kg morantel tartrate, the third with 50 mg/kg thiabendazole followed by 4 mg/kg morantel tartrate and the fourth line was not selected for drug resistance. Following at least 9 generations of selection there was no difference in LD50 OR LD95 between the unselected and single selected strains of worms. The strain selected by both tiabendazole and morantel tartrate had a significantly higher LD50 against thiabendazole, morantel tartrate and levamisole than did the other three strains. The single selected strains had LD95's of 172, 21.5 and 2.3 mg/kg for thiabendazole, morantel tartrate and levamisole respectively, compared with corresponding values of 111, 17.3 and 2.4 in the unselected strain and 124, 15.5 and 3.0 in the double selected strain. The estimated efficiency of the recommended dose of each anthelmintic against the unselected field strain was;-thiabendazole (44 mg/kg) 50% efficient, morantel tartrate (8.8 mg/kg) 76% efficient and levamisole (7.0 mg/kg) 99.9%efficient.

The effect of anthelmintic treatment and other management factors on the incidence of breech strike in Merino sheep.

Repeated scouring and soiling of the breech region with faeces rendered mulesed, Merino sheep susceptible to breech strike. Anthelmintic treatment of ewes before and after lambing and of lambs 6 to 8 weeks old reduced intestinal trichostrongylosis, scouring and consequent breech strike. Wether lambs tended to be more susceptible than ewe lambs and sheep grazing long pastures were struck more frequently than those on short pastures.