The Future of Parasite Control? Commentary from “Equine Disease Quarterly”

In this issue, Dr. Gene Lyons 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 that reverting back to treatment regimens of the past would be a complete mistake. The pharmaceutical industry is not anticipating developing any equine products in the foreseeable
future. Recent pharmaceutical trends are aimed at combination deworming products, i.e. formulations where two or more dewormers targeting the same parasites are combined into the same product. Research in the sheep industry has also highlighted the importance of reducing treatment intensity to avoid development of  multi-drug resistance. A recent project by the author highlighted the importance of high starting efficacy of the given combination. If combination treatment efficacy is markedly less than the desired 95 percent or above, resistance may develop quickly. The author’s laboratory is also testing a bacterial dewormer. Strains of naturally occurring Bacillus thuringiensis produce crystal proteins capable of killing worm parasites. If successful, this could become an anthelmintic product in the future. Perhaps the most important element in future parasite control programs is utilization of good diagnostic tools. Fecal egg counts will remain a cornerstone of control programs, but they have limitations in not providing information about larval stages and specific types (species) of parasites present. Recent collaborations have led to several new diagnostics, including species- and stage-specific serum ELISAs for important strongyle parasites, and an ultrasonographic technique for determining ascarid burdens in foals. One current project is making use of DNA-sequencing technologies to identify all parasite species present in a horse. Most recently, we have developed an automated smartphone-based fecal egg-counting system, which allows easier, quicker, and more precise fecal egg counts to be determined. Taking these diagnostic approaches collectively, the goal is to enable veterinarians and their equine clients to make more informed decisions about parasite control. The road to effective and sustainable parasite control is evidence-based, with veterinarians playing a central role.
Martin K. Nielsen, DVM, PhD, DipEVPC
(859) 218-1103
Maxwell H. Gluck Equine Research Center
University of Kentucky
Lexington, KY