Integrated Parasite Control - How to Strike That Balance

Parasites have evolved and methods of controlling them must also evolve. A “silver bullet” anthelmintic no longer exists, but all formulations still have a place in equine parasite control. Parasite surveillance information, generated via quantitative fecal egg count, is essential for monitoring the efficacy of parasite control programs. Fecal egg counts should be used in guiding drug selection, treatment frequency recommendations, and classifying the pasture contamination potential of individual horses.

Baseline treatment recommendations for adult horse coincide with other preventive health measures, such as spring vaccinations and fall dental examinations. Veterinarians who provide direct parasite control services along with other preventative health measures are more vertically integrated into the total health care needs of their patients. Ongoing work is underway to investigate the relationship between inflammatory reactions to parasite burden and anthelmintic treatment as well as the possible interaction with host response to vaccination. One study in horses and one study in cattle suggest superior vaccine response in horses and cattle with low parasite burdens. Author’s address: M.H. Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546-0099; e-mail: martin.nielsen@uky.edu.

1. Introduction

Parasite control is an integrated part of general equine management and is generally viewed as routine annual care along with vaccinations. Traditional approaches for deworming have largely resembled vaccination schedules; medication is applied to all horses with regular intervals year-round and often with a degree of rotation between products.^1–4 However, mounting evidence of ever-increasing anthelmintic resistance is forcing the equine industry away from this approach, and surveillance-based strategies are recommended instead. Equine parasite control has become the art of treating just enough and choosing the right drug for each treatment. This is a challenge that requires veterinary involvement to design a structured approach to strike the balance between overand under-treatment.

It is often stated by parasitologists that no onesize-fits-all program exists for equine parasite control, and each strategy has to be based on conditions on the given farm. While this is true, it does not provide any guidance to the end-user. Instead of pointing to the complexity of parasite control, it is more helpful to outline the general features that characterize equine parasites and identify the elements that should be considered essential in a successful parasite control program. This paper will attempt to outline some of the important elements in an integrated parasite control program. 

Parasite Species Resistance Patterns

Equine parasite control is complicated by the magnitude of different parasite species always infecting horses in coinfection. These differ in biology, pathogenicity, and in levels of anthelmintic resistance, so many control recommendations may be appropriate for one parasite category but not others. Despite this, it is striking how uniform the equine parasite fauna appears to be across the world. Despite vast climatic and geographic differences, equine operations all appear to face the same classical intestinal parasite categories: the group of cyathostomins (small strongyles), tapeworms (Anoplocephala perfoliata), and ascarids (Parascaris equorum). These can be considered truly ubiquitous and present in any establishment. The only exception to this pattern is the group of large strongyles that have declined dramatically in prevalence during past decades, and occurrence now appears to be dependent on the parasite control strategy used.⁵

In addition to the mere presence of parasites, the occurrence of anthelmintic resistance in these appears to be strikingly uniform. Of the four parasite categories mentioned above, two of them have developed widespread resistance to commonly used anthelmintic drug classes. Cyathostomins are now widely resistant to benzimidazole-type drugs, and pyrantel resistance is commonly reported both in Europe and North America.^6,7 Furthermore, it is of great concern that recent studies have documented reduced egg reappearance periods after ivermectin and moxidectin treatment^8,9; this has been associated with resistance in immature stages of these parasites.^10-12 A few studies have documented reduced fecal egg count reductions at two weeks post ivermectin treatment.^6,13,14 Resistance in Parascaris equorum is also widespread, with a situation complementary to what is described with the cyathostomins: Resistance to ivermectin and moxidectin is reported across the world¹⁵ while only a few studies have reported resistance to pyrantel salts.^16,17 Although anecdotal evidence suggests resistance to benzimidazoles in P. equorum, no confirmed reports exist in the peer-reviewed literature.

The possible existence of anthelmintic resistance in large strongyles has been discussed,¹⁸ but the general consensus is that this group of parasites has maintained susceptibility to all anthelmintic drug classes. This explains the rare occurrence of these parasites in managed horse populations. As long as treatment intervals are kept frequent enough to break the life cycle, transmission appears to be effectively interrupted. Recent work performed in Denmark has illustrated that if treatment intensities are lowered below two treatments per horse per year, Strongylus vulgaris can occur at up to 80% of farms.⁵ It remains unknown if this can be associated with a higher risk of parasitic disease. 

The equine tapeworms have not been documented resistant to any of the anthelmintics labeled for tapeworm treatment, but this probably reflects the lack of established methods and protocols for determining anthelmintic resistance in this parasite category. Certainly, resistance is biologically plausible with any parasite category.

Educated Anthelmintic Use

Despite the widespread occurrence of anthelmintic resistance, none of the currently marketed formulations can be considered completely useless. As outlined above, they all have issues of resistance in some equine parasites, but all also maintain good efficacy against others. Essentially, the equine anthelmintics have all gone from being broad-spectrum to being more narrow-spectrum. Thus, the large majority of cases should offer at least one good and efficacious treatment option for each of the important equine parasites mentioned above. However, the real challenge is to make the right treatment choice. This can only be carried out with some degree of parasite surveillance. This is best exemplified with the question of whether a foal or weanling is primarily infected by ascarids, strongyles, or both. Fenbendazole and possibly pyrantel are likely to represent good treatment options for ascarid treatment, but cyathostomins may very well have developed resistance to one or even both of these drugs. On the other hand, macrocyclic lactones would probably be drugs of choice for cyathostomin treatment in most cases but is unlikely to be efficacious against ascarids on many farms across the world. So in order to choose the right anthelmintic, it is necessary to perform fecal egg counts that will provide the needed information. If these samples are followed up with post-treatment egg counts, the veterinarian will gain useful knowledge about treatment efficacy and possible resistance on the given farm.

2. Fecal Egg Counts

Quantitative fecal egg counts are the cornerstone of equine parasitological diagnostics. However, it is important to understand the nature of egg counts in order to make the appropriate interpretation of the results. There are numerous egg counting techniques available and even more different modifications of these. Classic egg counting techniques that are widely used include Stoll, Wisconsin, and McMaster. In essence, all methods are based on the same principle, which is flotation of eggs to allow them to separate from the fecal matter making them available for microscopy and counting. The two most important features characterizing any given egg counting technique are the detection limit and the level of variability between repeated counts. The detection limit is synonymous with the multiplication factor for the technique, i.e., the smallest egg count detectable with the method. The level of variability is considerable with most egg counting methods, and it is important to take this into account when results are interpreted. [...]