In the April 2017 issue of Equine Disease Quarterly, Dr. Gene Lyons (PhD) provides a brief review of the history and current status of anthelmintic treatment of important equine gastrointestinal parasites. No new anthelmintics with newer modes of action have been introduced since the early 1980s, and levels of anthelmintic resistance are ever increasing in cyathostomin and Parascaris spp parasites. While resistance is slow to develop, work by Dr. Lyons has clearly illustrated that once it appears in a given parasite, it is there to stay.

Today, we can expect resistance to at least one drug class to be present in every equine operation across the world, and an overwhelming majority will feature multi-drug resistance. With only three classes to choose between, we are running out of treatment options. A pertinent question to ask is how to tackle this emerging crisis and what to expect in the future.

The first step is to acknowledge the extent of the problem. Despite recommendations given during the past couple of decades, a majority of individuals in the industry continue to use old-fashioned parasite control programs based on frequent treatments given year-round without any consideration of treatment efficacy, parasites present, and climatic conditions. If no diagnostic testing is done, resistance will not be identified.

For the long term, we need new anthelmintic drug classes with new modes of action. It is important to learn from the past, however, and realize that no drug class is going to remain effective indefinitely, and reverting back to treatment regimens of the past would be a complete mistake