Morantel resistance by Haemonchus placei in cattle

Anthelmintic resistance in nematode parasites of cattle: a global issue?

Acceptable performance of grazing cattle frequently depends on the availability of effective broad-spectrum anthelmintics to remove, or prevent infection with, gastrointestinal nematodes. This control is increasingly threatened by populations of nematodes resistant to the most commonly used anthelmintics. Although this appears to have developed more slowly than in nematodes infecting small ruminants, the number of reports in the literature over the past five years suggests a rapidly escalating problem. This review discusses this literature, several issues unique to cattle parasitism and anthelmintics, and how previous research in small ruminants can improve the management of anthelmintic resistance in cattle.

Endoparasite Control

The gastrointestinal nematodes are considered to be the most important group of internal parasites. Gastrointestinal parasitism is a major constraint to animal health, productivity, and profitability in grazing livestock production systems. The clinical manifestations of infection may range from no apparent effect to death. Despite years of research, assessment of production losses in many regions of the United States and of the potential impact of gastrointestinal nematode control remains to be determined.

Probable multigeneric resistance to macrocyclic lactone anthelmintics in cattle in New Zealand

AIM

To establish the efficacy of topical formulations of eprinomectin and abamectin against naturally acquired abomasal and small intestinal nematode infections in cattle purchased from a North Island bull-beef property.

METHODS

A controlled slaughter trial, involving eighteen 6-8-month-old mixed breed calves, was conducted in May 2002.The animals were randomly allocated on the basis of faecal egg count to one of three equal-sized groups (n=6), consisting of an untreated control group and two treatment groups. One of the treatment groups was treated with a topical formulation of eprinomectin, the other with abamectin. Both anthelmintics were administered as a single topical treatment on an individual liveweight basis, at the manufacturer's recommended dose rates of 0.5 mg/kg. All calves were housed in separate groups with no access to pasture throughout the entire trial and were slaughtered 7-10 days after treatment.

RESULTS

Both anthelmintic treatments were highly effective(worm count reduction >98%) against Ostertagia ostertagi, Trichostrongylus axei and Cooperia punctata, but were not effective at reducing worm counts significantly of either Cooperia oncophora or Trichostrongylus longispicularis. Against these latter two parasites, worm count reductions of only 72% and 79%, and 81% and 76%, respectively, were recorded following treatment with eprinomectin or abamectin, respectively (all p>0.05).

CONCLUSIONS

These results demonstrate evidence of resistance to macrocyclic lactone anthelmintics by C. oncophora and probably T. longispicularis also.

CLINICAL RELEVANCE

As well as perhaps providing the first record of resistance to any anthelmintic by T. longispicularis, the present findings may also represent the first case of resistance to macrocyclic lactone anthelmintics exhibited by more than one parasite species at a time in cattle in New Zealand.

Anthelmintic resistance: past, present and future

Anthelmintic resistance continues to increase in geographic range, in the number of species affected and the range of drugs involved. Several aspects of resistance have emerged as important issues. They include lack of genetic reversion, presence of side resistance and lack of universality. Furthermore, resistant isolates recovered from the field may have different characteristics to those selected in pen passage. Research into anthelmintic resistance has not progressed far beyond the stage of descriptive research. Some progress has been made in developing control strategies and in diagnosing resistance, especially in the development and adoption of in-vitro tests. However, these still need improvements in their ability to detect resistance to closantel and avermectin/milbemycin anthelmintics. Less progress into understanding the basis of resistance has occurred. Research priorities include improvement of diagnostic tests and the development of molecular tests, particularly for resistance to levamisole and the avermectin/milbemycins. Resistance itself, as a selectable marker for genetic transfection in parasites, is a potential tool for investigating parasite biology.