Dave Leathwick PhD
AgResearch Grasslands
Palmerston North
New Zealand

Christian Sauermann PhD
AgResearch Grasslands
Palmerston North
New Zealand

This book was conceived through the authors’ realization that equine practitioners were not likely to achieve competence in evidence‐based parasite control (EBPC) by reading journal‐length articles or by attending a few hours of continuing education. Like any clinical skill set, parasite control must be grounded solidly in theory, practiced with thoughtful application, and continuously assessed and improved. Most new clinical skills, such as surgical procedures or diagnostic algorithms, represent variations of basic proficiencies or knowledge already held by practitioners, who can also turn to local mentors for advice and support. In contrast, the private sector harbors few, if any, experts in equine parasitology who can impart mastery of the principles of EBPC.

Evidence‐based parasite control is a relatively new development in equine medicine, but similar principles have been applied for decades by small ruminant practitioners in Europe and the southern hemisphere. In these locales, parasitic challenges to indigenous livestock are prevalent and extreme. Near‐total anthelmintic resistance by certain parasites (e.g., Haemonchus contortus) has rendered practical control of these highly pathogenic nematodes nearly impossible, with severe economic consequences for the sheep and goat industries on multiple continents. In comparison, equine cyathostomins (small strongyles) have demonstrated resistance to one or more anthelmintic classes for nearly four decades, but these nematodes are modest pathogens under most circumstances. The authors and other veterinary parasitologists have been disseminating EBPC recommendations for many years, but equine practitioners have been relatively unreceptive to these messages until very recently. The impetus for this changed attitude is uncertain, but it seems to be associated with the contemporary detection of anthelmintic resistance in some populations of Parascaris equorum. Mere demonstration of resistance in a second group of equine parasites is not likely the major threat perceived by practitioners. Rather, it could be the hard evidence that macrocyclic lactone anthelmintics, previously considered bullet‐proof panaceas in horses, are also vulnerable to nematode resistance.

Regardless of the motivation, equine practitioners now seem uniquely receptive to EBPC, and this book represents our attempt to address that interest and to fill the need with practical advice and logical recommendations. Most veterinary textbooks organize and discuss related facts, and then present recommendations for the logical application of that knowledge in clinical situations. This handbook has an additional objective that is far more daunting. The authors face the challenge of changing a mindset; of overcoming four decades of tradition, literally tens of millions of episodes of implementation, and competing recommendations from the marketing departments of every pharmaceutical company with a horse in the race. Change is painful but necessary, and progress in parasite control will be measured one practitioner and one horse owner at a time. As Darwin famously observed, “It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change.” The worms have been changing since the dawn of effective anthelmintic therapy; now it’s our turn.

We have not included an exhaustive collection of references in this book, because busy practitioners have neither the time nor the means to delve deeper into relevant literature. In addition, we readily acknowledge the irony that many of our “evidence‐based” recommendations have minimal scientific support at present. So until more definitive proof is published, some practices clearly represent “stop digging” advice. (This term is derived from the adage that when you find yourself in a hole, the first thing to do is stop digging.) It is often a greater management challenge to convince people to stop doing the wrong thing than it is for them to adopt correct measures.

Our primary goal is to teach, and we believe that training in EBPC is best done by vet‐side mentors. Accordingly, we beg the reader’s indulgence whenever our tone becomes informal or even casual. This merely reflects our teaching styles.

“Parasite control is confusing.” “There are so many opinions out there.” “What’s wrong with continuing to follow our historical practices?” Statements like these are commonly made by people who reach out to us with questions about this topic. While we understand these frustrations, they are really unnecessary and they are the main reasons for writing this book, which now emerges in its Second Edition. Equine parasitology is a very small research field with a limited number of scientists involved across the world. One would not expect a lot of new information to be developed in just a handful of years. Nonetheless, a substantial amount of new knowledge has been generated and relevant technology has emerged since the First Edition of this book was published in 2013. Thus, we found it timely to update the contents and publish a Second Edition.

It has been our ambition from the very beginning to make this a practical book, written in straightforward language, and we have attempted to retain this style in the Second Edition. The case scenarios near the end of the book serve as a testament to this ambition. Having said that, we realize that the text may appear somewhat academic to some, and we make extensive use of scientific‐style references throughout. Although the majority of readers are unlikely to look up references and read scientific papers, we know from experience with the First Edition that a proportion will do this. Therefore, we have expanded the reference lists with papers published within the last five years. Veterinarians who are members of American Association of Equine Practitioners (AAEP) or British Equine Veterinary Association (BEVA) will notice that the authors have published several review papers in the journal Equine Veterinary Education, which is distributed to the membership of both associations. Thus, there is direct access to more in‐depth information there. We consider the citation of published references as a healthy and objective exercise, which helps us to avoid making unsupported and sometimes misleading assumptions. In a world where any kind of information can be disseminated globally in a matter of seconds, we believe that basing recommendations on credible and peer‐reviewed evidence is the only responsible approach for a publication such as this. Veterinary textbooks and the scientific literature are full of examples of statements that are repeated through generations. In the end, no one remembers where the statement originated or why a specific practice was initiated. When the literature is searched meticulously, it is often found that a statement has been misconstrued or that it was never based on objective data to begin with. Along those lines, revisiting the literature helped us to identify some misleading and even erroneous content that appeared in the First Edition. Yes, we are also guilty of making unsubstantiated assumptions.

While we have not experienced the launch of a new anthelmintic class for equines since the First Edition was published, we have enjoyed remarkable advances on the diagnostic frontier. As a result, the diagnostic chapter (Chapter 9) has grown more than any other portion of this book. A parasitologist cannot imagine a world without our beloved fecal egg counts, and despite their old school nature, we can conclude that they are here to stay. In fact, they remain the foundation of good evidence‐based parasite control. The diagnostic chapter contains an expanded discussion of interpretation of fecal egg counts, with special emphasis on accuracy and precision. Furthermore, this edition has devoted an entire chapter to anthelmintic resistance. We continue to see more and more resistance across the world, and this chapter features two sets of heat maps summarizing all reported findings of anthelmintic resistance in cyathostomin and Parascaris spp. parasites.

Some exciting developments have evolved since 2013. One of these is the use of computer modeling to predict the dynamics of equine parasite infections and anthelmintic resistance development. These tools allow us to investigate the principles of these complicated biological phenomena without the need to involve live animals in tedious, expensive, and frequently inconclusive research. This book also includes contributions from two leading scientists in the field of computer modeling: Dr. Dave Leathwick and Dr. Christian Sauermann from AgResearch, New Zealand. They have generously provided model simulation outputs which illustrate important biological principles in equine parasitology.

Other unique features in this Second Edition include the addition of new clinical case scenarios near the end of the book. These are all based on actual cases that we have encountered through the years. The book also features a glossary of technical and scientific terms that appear in the book. Hopefully, this glossary will increase the understanding of readers who are not veterinarians or parasitologists. Finally, a large number of new images have been added.

We realize that we just represent another opinion about parasitology; we might even be adding to the confusion about these topics. Nonetheless, our ambition is the exact opposite, and our opinions at least are based on the best possible evidence available at this time. Undoubtedly, we will need to revise some of the content again in a few years. Until then, enjoy.

We are deeply grateful to fellow scientists, veterinarians, horse owners, and farm managers from all over the world for asking us challenging questions about equine parasite control. They serve as an invaluable source of inspiration for this book. Sincere thanks to our friends, colleagues, and collaborators in New Zealand, Drs Dave Leathwick and Christian Sauermann, for their insightful contributions to several chapters. We warmly acknowledge Dr. Tetiana Kuzmina, Dr. Stine Jacobsen, Dr. Paul Slusarewicz, Dr. Alan Loynachan, Ms. Shaila Sigsgaard, Ms. Holli Gravatte, Mr. Jamie Norris, Ms. Maci Stephens, Ms. Faith Miller, Ms. Jennifer Bellaw, Ms. Maria Rhod, and Ms. Tina Roust for providing high quality photographs. Last, but not least, we are deeply indebted to Mr. Jamie Norris, a scientific illustrator in the making, for preparing beautiful life cycle figures and for his tireless help with digitally optimizing image quality.