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This article was published in 1981
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Update on Nematode Resistance to Anthelmintics
C.A. Hall, F.R.C.V.S., Veterinary Research Station, NSW Dept. of Agriculture, Glenfield
SHEEP
Since the last report, presented to you about 2 years ago, the major change recorded is a resistance to levamisole / morantel by Ostertagia sp. This is now seen more frequently and is widespread. Resistance to the benzimidazole anthelmintics in this species has an incidence of about 45% in the flocks surveyed.
The ad hoc committee consider that it is now time for labels on sheep anthelmintics to be worded BZ resistance in Haemonchus sp. and BZ: levamisole/morantel resistance in Ostertagia sp. This recommendation will be submitted to the registration authorities. They decided that anthelmintic resistance in Trichostrongylus sp., although present in the nematode population, is still relatively isolated to a few individual properties, and no action was warranted at this time.
GOATS
The T.C.V.D. recently decided to request that all BZ anthelmintics used in goats carry statements that they are only efficient against susceptible strains of Haemonchus, Ostertagia, and Trichostrongylus spp.
The ad hoc committee have recommended that this statement be extended to include all broad spectrum anthelmintics for goats.
The data obtained from anthelmintic use in goats suggests that it may be more appropriate if in the future dose rates and efficacy data are obtained in this specific host species. At present direct extrapolation has been made from sheep data.
CATTLE
No change to the previous information.
Additional Information - General
A population of nematode parasites, said to be originally from goats, but tested in sheep showed multiple resistance to BZ anthelmintics, levamisole/morantel and possibly other narrow spectrum drenches. At Glenfield we were able to verify the BZ resistance, but not the levamisole/morantel resistance. The D.P.I. Queensland and I.C.I. Melbourne reported that a dose rate of 15.0 mg/kg was about 40% efficient but at the V.R.S. Glenfield the same dose rate eliminated the total worm burden.
This type of situation, i.e. different responses between laboratories is not unknown, but satisfactory explanations are not forthcoming.
Information is also being obtained on the response of resistant parasites when present in a non-definitive host. Haemonchus contortus from sheep when passaged in calves showed an enhanced resistance which was not completely lost when passaged back into sheep. Similarly, a resistant isolate of several nematode species from goats as judged by egg counts and larval differentiation, appeared to be more susceptible when tested in sheep.
Even Ostertagia ostertagi of cattle will cycle quite effectively in goats. It may therefore be possible to select for resistance in this species should it become established in goats, which receive regular anthelmintic treatments. The effect of a subsequent passage into cattle remains unknown.
Resistance is an evolving situation and is slowly becoming the accepted norm against which future anthelmintics will need testing. Advice to graziers towards less dependence on anthelmintic treatments and better stock management to reduce parasitic infection should be the aim.