Anthelmintic resistance: fact or fiction? (Proceedings)

The ability to use anthelmintics to prevent and eliminate parasite infections or disease has been the cornerstone for many parasite control programs. Unfortunately, in many instances, what was a cornerstone has become the whole program. With many products no longer under patent and available over-the-counter, it has become cheaper in the minds of many owners to just buy the dewormer than to think about the total control program.

Anthelmintic resistance in sheep and goat nematodes is still considered to be the number one problem facing small ruminant producers worldwide. Within the US, anthelmintic resistant nematode populations have been well-documented throughout the southeastern, mid-central, mid-Atlantic, and northeastern US. The primary culprits are the abomasal parasites Haemonchus contortus and Teladorsagia circumcincta. The status of anthelmintic resistance in small ruminants living in areas of the US outside theses areas is not well documented nor understood. However, cases of H. contortus resistance have been documented in goats and sheep in Colorado, indicating this problem is likely not restricted to any one section of the country.

Reports of anthelmintic resistance in cattle started surfacing in the mid-1990s; however, reports were rare and the phenomenon was considered to be of minor importance. As time passed, though, reports became more and more frequent and by 2005, anthelmintic resistance was documented in cattle in the US. To date, anthelmintic resistance has been reported in cattle in Brazil, Argentina, Belgium, Netherlands, the UK, India, Bangladesh, Nigeria, New Zealand, Australia, and the US. These reports tend to be in cattle on intensively grazed permanent pastures with most cases in the stocker class. Cases have been documented in such diverse areas as California and Wisconisin.

About 40 years ago, a novel approach to equine parasite control was introduced. Called the interval dose system, owners were advised to treat horses every 6-8 weeks to prevent maturation of Strongylus vulgaris, thereby minimizing pasture contamination. As part of the movement towards strategic parasite control, this approach not only addressed treating parasites within the animal but, also, addressed the prevention of future infections and disease. Widely accepted, this approach is credited towards the dramatic reduction in equine colics associated with S. vulgaris. By the 1980s, S. vulgaris was becoming relatively uncommon and a shift in parasite populations occurred with small strongyles (cyathostomes) accounting for the majority of strongyle egg output in grazing horses. Unfortunately, the efficacy of the program in controlling S. vulgaris, combined with the availability of safe and inexpensive anthelmintics, led many owners to adopt a zero tolerance towards the presence of any strongyle egg. Consequently, rather than focus on control, over the years, the focus became total eradication. The goal, then, became to keep fecal egg counts (FEC) at or as near zero as possible and rote deworming by the calendar became so ingrained in many management schemes that a lot of owners absolutely refuse to consider skipping a scheduled treatment, whether it is needed or not.

At the time the interval dose system was introduced, cyathostomes were considered to be of minor consequence, especially when compared to the highly pathogenic S. vulgaris. However, as S. vulgaris-induced colics decreased, the pathogenic effects of cyathostomes became evident and, today, they are considered to be the primary parasitic pathogen of horses. Unfortunately, the suppressive use of anthelmintics has contributed significantly to the increased prevalence of anthelmintic resistant-cyathostomes around the world. Resistance to benzimidazoles is widespread and resistance to pyrantel salts (tetrahydropyrimidines) is becoming more common. Macrocylic lactone-resistant cyathostome populations have yet to be definitively identified, although resistant Parascaris equorum populations have emerged. Studies within the US suggest that anthelmintic resistance is a serious situation with 40% of farms in one study harboring anthelmintic resistant-cyathostome populations.

For the most part, anthelmintic resistance has been an issue of livestock and equine production systems. Recently, however, lack of efficacy reports are starting to surface for canine heartworm. Reduction in efficacy has been experimentally documented, but, translation of this information to the field is highly controversial. An update on this emerging issue will also be presented.