Equine anthelmintics by prescription only?

Dear Sir, At a recent meeting in Denmark, reports were presented describing resistance to macrocyclic lactones in small strongyles (1–3). This closes the circle, with resistance having now been demonstrated to all three classes of anthelmintics in cyathostomes (mac-rocyclic lactones, benzimidazoles, and pyrantel). Furthermore, it closely follows the recent demonstration of resistance to both macrocyclic lactones and pyrantel in equine ascarids (4,5). Alarmists might describe this as the post-anthelmintic era. Of course, this is hyperbole, but alarm bells should still ring. Resistance in equine helminths is not yet so dire an issue as the appearance of multi-resistant bacterial “superbugs,” but the same behaviors led us to both points. These are reliance on drugs to do something we could achieve by other means, improper use, and use without determining need or appropriateness. Some may argue that our reliance on anthelmintics reflects a lack of attention to pasture management to minimize parasite burdens, and that this is an issue of limited resources. However, these constraints can be overcome by strategic, as opposed to year-round use of anthelmintics, by regular monitoring of parasite egg counts in feces, and by leveraging knowledge we already have of parasite life cycles. If we were to use these strategies to reduce our drug use we would very likely decrease the rate of selection for anthelmintic resistance in horse parasites (6). In short, for horses that reside permanently in Canada, we can manage without year-round anthelmintic treatments by adopting simple management strategies. Eliminating unnecessary use of anthelmintics in horses would of course be challenging, and some may see the effort as providing limited returns and too much aggravation. Others may feel that parasite control can be left in the hands of owners. However, both approaches are clearly dangerous, and almost certainly have been associated with the increase in prevalence of anthelmintic resistance and clinical parasitism (7). It is therefore time to acknowledge that parasites in horses are not being controlled with the ready availability of anthelmintics. We need to recognize that we cannot continue to depend on a drug-based approach for parasite control. The writing is on the wall, we must minimize dependence on quick fixes and look to management to control equine gastrointestinal parasitism. This means broad education, and banning the sales of equine anthelmintics other than by prescription, as has been carried out in Denmark (1999), Sweden (2007), and The Netherlands (2008). This may invoke criticism and be seen as an attempt by veterinarians to “corner the market,” while pharmaceutical companies are likely to be less than happy with the proposal. However, if we are to retain the ability to control gastrointestinal nematodes effectively, economically, and in the face of varying management circumstances, it is essential that we reduce our dependence on anthelmintics and guard these agents as jealously as we should antibacterials. We call on our profession to start the necessary dialogue.

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