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This article was published in 1982
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Anthelmintic Resistance in Gastro-Intestinal Nematodes From Goats
C.A. Hall, NSW Dept. of Agriculture, Veterinary Research Station, GLENFIELD NSW
There are very few anthelmintics that have been specifically examined for efficacy against nematode parasites in the gastro-intestinal tract of goats by controlled experimental trials. It has been generally accepted that any dose rate recommended for use in sheep would be equally efficacious when administered to goats. This assumption may have led to both under or over dosing against different nematode species. It should not be unexpected if further experimentation showed that this practice has been one factor leading to the high incidence of anthelmintic resistance in goats.
Other explanations could be the poor management procedures practised by goat owners by rearing large numbers of animals on small pasture areas. This creates the necessity to treat animals at regular and frequent intervals to maintain health and production. There is also no doubt that anthelmintics have been purchased more for economic reasons than efficacy against the parasites present in the intestinal tract.
The incidence of properties where goats show nematodes resistant to anthelmintics is relatively high compared with sheep. The nematode species from goats that are known to show resistance are Haemonchus, Ostertagia and Trichostrongylus spp. to the benzimidazole anthelmintics and to levamisole for the latter two species. At least two populations of nematodes from goats have shown multiple resistance to all the available anthelmintics i.e. Gatton, Queensland and Chain Valley Bay in N.S.W. The anthelmintics involved were the benzimidazoles, levamisole/morentel, rafoxanide, naphthalophos and phenothiazine. However, when the Gatton strain was transferred to the V.R.S. Glenfield a dose rate of 15 mg (1 kg/2x) of levamisole removed all the parasites on two separate occasions. At Gatton the same dose rate was reported as 38% efficient.
Published work from trials at the V.R.S. Glenfield, showed that the level of resistance can be changed by passaging the parasite through an alternative host. This data also allows the suggestion to be made that probably the highest level of resistance is shown in goats followed by calves and finally sheep. Unfortunately after the next passage into the definitive host this level of increased resistance is often maintained. Therefore where it is possible for goats and sheep to graze pastures together nematode resistance could become a major problem.
In most cases it has been possible to manage drenches so that a satisfactory reduction in egg count was obtained with a minimal disturbance to production. Rotation of drenches pre-swabbing with copper sulphate and a mixture of two anthelmintics each at their recommended dose rate have proved successful.
The ivermectin should not at present be considered a new and completely different anthelmintic. It is said to have a paralysing effect for removal of parasites which is a similar method to levamisole/morantel and therefore a side resistance may exist.
In any case, extension advice should be such that in the future no drench is used on 80% of the animal population for 100% of treatments. This may only lead fairly quickly to an additional resistance.