Anthelmintic resistance involving particularly the gastrointestinal nematodes of small ruminants is escalating globally, to the extent that in certain countries, such as South Africa, it has already reached alarming proportions, and is affecting practically all the anthelmintics. In this paper it is argued that the high levels of resistance in nematodes of veterinary importance indicate that the drugs have been used incorrectly. It is suggested that the phenomenon of refugia plays a much more important role in the selection of anthelmintic resistance than other phenomena that are more frequently investigated and recommended for counteracting it, such as reduced drenching frequency and avoiding under-dosing. While refugia is commonly mentioned in passing in most papers on anthelmintic resistance, it is, almost without exception, not incorporated in the final control/ management proposals. On the strength of the conclusions arrived at in the present paper strategies such as the drench-and-move system in which all the animals in a flock are drenched before they are moved to pastures containing few or no worms in refugia, and the system of strategic drenching on safe pastures should be condemned and never recommended. If such strategies are indeed unavoidable, the farmer should be warned that the farming system would probably not be sustainable even in the short term, in view of the generally high levels of resistance already present in most of the important sheep-producing regions. Farmers should be educated to consider refugia above all else when designing worm management programmes. Finally there seems to be too much complacency concerning the possibility that anthelmintic resistance may also escalate in cattle, eventually to reach the proportions that it has in sheep.

Refugia--overlooked as perhaps the most potent factor concerning the development of anthelmintic resistance.

https://pubag.nal.usda.gov/download/10367/PDF

Validation of the FAMACHA eye color chart for detecting clinical anemia in sheep and goats on farms in the southern United States.

Influence of sex and reproductive status on susceptibility of ruminants to nematode parasitism

Strongylid gastrointestinal nematodes are an important cause of disease and economic loss in small ruminants. These parasites are particularly important in the eastern United States where the bloodsucking parasite Haemonchus contortus is the predominant species. Sheep and goats are infected with gastrointestinal nematodes while grazing and the biology of the infective larvae on the pastures is important in planning effective control programs. Fecal egg counts can be used to monitor control programs and drug efficacy and evaluate levels of pasture contamination.

Gastrointestinal nematodes of small ruminants: life cycle, anthelmintics, and diagnosis.

Publisher Summary The historical succession of one drug class after another has resulted in part from sequential improvement in drug characteristics (spectrum, safety, convenience), but also in part from the emergence of drug resistance. The phenomenon of resistance, so characteristic of the treatment of infectious diseases in general, is a prominent feature of the new era in antinematodal therapy. Anthelmintic use may vary to some extent in various parts of the world, but it rests largely on a common set of chemical compounds. An aspect of the older drugs is the development of new delivery technology. Administration of the injectable product gives high efficacy against many nematode and arthropod parasites, and these are disucssed in the chapter. The drug is active against gastrointestinal and pulmonary nematodes, against immature as well as mature worms, and against inhibited fourth-stage larvae. The susceptible arthropods include ticks, lice, mites, and grubs. Drug resistance is a heritable reduction in the sensitivity of a parasite population to the action of a drug, the reduction being expressed as a decrease in the frequency of individual parasites affected by exposure to the drug. Drug resistance generally results from decreased drug uptake, increased metabolism of the drug, or changes at the drug-receptor site.

Chemotherapy of nematode infections of veterinary importance, with special reference to drug resistance.

Resistance of exotic and domestic lambs to experimental infection with Haemonchus contortus.

A comparison of the periparturient rise in fecal egg counts of exotic and domestic ewes.

Resistance of nonlambing exotic and domestic ewes to naturally acquired gastrointestinal nematodes.

In studies comprising a total of 117 naturally infected crossbred lambs, it was found that lambs turned out to spring pasture were exposed to immediate infection with overwintered Ostertagia, Nematodirus, and Trichostrongylus spp, but did not acquire Haemonchus contortus infection until July. With all species, there appeared to be only one major disease-producing generation of worms per year; it was responsible for serious worm burdens and clinical disease from July onward. High frequencies of hypobiosis (arrested development) were demonstrated in H contortus (98% to 100%), Ostertagia circumcincta (89% to 96%), and Nematodirus filicollis (77% to 90%) by late November; Trichostrongylus spp comprised most of the adult (egg-laying) worm population at that time. The onset of hypobiosis appeared to be induced by seasonal stimuli rather than by host-immune responses. The results of these studies showed benzimidazole resistance to be a major problem. In the first year, fenbendazole (5 mg/kg) reduced worm burdens by 46% and 20% in September and December, respectively. In December of the second year, levamisole (8 mg/kg) reduced worm burdens by 99%, thiabendazole (44 mg/kg) by 55%, fenbendazole (5 mg/kg) by 47%, and fenbendazole (15 mg/kg) by 11%. Levamisole was highly efficient (99%) against adult worms, hypobiotic worms, and benzimidazole-resistant worms. Ostertagia circumcincta was strongly resistant to the benzimidazole drugs at all dosages tested, while H contortus and Trichostronglylus spp were partially resistant. There was no evidence of reversion of resistant worms to susceptibility on this farm, even though benzimidazole drugs had not been used for 9 years.

Control of hypobiotic and benzimidazole-resistant nematodes of sheep.

Control of periparturient rise in worm egg counts of lambing ewes.

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