The Great Equine Worming Debate -
Susan Corning BA MSc BVSc MRCVS FRIPH

INTRODUCTION

Why do we even bother to worm our horses? Hopefully, the answer is straightforward: to keep our horses healthy. However, for any animal owner, the decision as to which anthelmintic product to use is based upon a number of issues - including its efficacy, spectrum of worm species controlled, cost-efficacy, and worming programme simplicity.

And, just when you thought that you had considered all of these issues and decided upon a worming strategy that best suited your own horse’s situation, the Great Equine Worming Debate begins yet another round on the hot topic of worm resistance! Confronted with various hypotheses about different strategies that might “delay” the onset of resistance, horse owners may be confused as to how to reconcile these theories with their own more immediate goal of their horse’s best health.

However, the issue of worm resistance is indeed an important one, as experience tells us that no matter what the active ingredient, the “worms will win” in the long run. So the Great Equine Worming Debate does have ongoing relevance, particularly as it provides an opportunity for horse owners to make informed decisions on the basis of not only a number of different theories, but also based upon up-to-date factual evidence. This means that horse owners can feel more confident in their choice of a consistent worming strategy that is proven both scientifically and in the field to protect their horse.

MAIN FACTORS IN THE DEVELOPMENT OF RESISTANCE

Although there are a number of factors which may be involved in the development of resistance, two important factors are (1) initial efficacy in removing parasites and (2) frequency of anthelmintic use. In the first instance, if a product is highly effective in removing parasites, there will be few left to produce offspring. However, if a product has moderate efficacy, those parasites surviving treatment are least affected by the anthelmintic, and therefore may be likely to produce offspring with inherited resistance. Similarly, underdosing will result in surviving worms passing on their “immunity” to subsequent generations.

With regard to frequency of use, the more times a population is exposed to an anthelmintic, the more frequently selection pressure is applied, and the more rapidly resistance is likely to appear in that population.

In fact, it has been suggested that the most important factor in delaying the onset of resistance is the anthelmintic’s initial kill of resident resistant worms, and the second most important factor is the frequency of use of anthelmintic.1 This means that the ideal anthelmintic is a highly potent one to which there is no resistance, and which can be given relatively infrequently as possible whilst providing continual protection.

EQUINE NEMATODE RESISTANCE IN THE UK

In the UK, there is known equine nematode resistance to benzimidazoles and pyrantel. Benzimidazole resistance has been widespread for over a decade.2 Cyathostome (small redworm) resistance to benzimidazoles is a particular problem, with reports of efficacy as low as 12.6%, even after a five-day course.3 Approximately five years ago, pyrantel resistance was also identified in the UK, in both cyathostomes and large strongyles.4

This means that, to date, cyathostomes have developed resistance to every major anthelmintic group except the macrocyclic lactones (MLs). In 1999, it was hypothesised that, because of its persistency and efficacy against encysted cyathostomes (which are otherwise left “untouched” by ivermectin), moxidectin might enhance selection for ML resistance.5 However, moxidectin has been used in horses for nearly ten years, during which time this predicted resistance has in fact not occurred. Today, there is still no confirmed equine nematode resistance to either of the ML sub-groups of moxidectin or ivermectin.

Therefore, a current concern is to determine a strategy to best ensure that resistance to the ML group is delayed for as long as possible.

Rotation as a strategy to delay resistance?

The rotation of wormer groups from year to year has been suggested as a strategy to delay the onset of resistance. Originally proposed as a sheep farming strategy; rotation has not yet been proven in practice to make a significant difference in the length of time for resistance to occur. In fact, computer modelling has suggested that no drug rotation or frequent drug rotation make no difference to the onset of resistance.6

Furthermore, the rotation strategy does not transfer well to the equine situation, where historically, several different drug classes all had to be used in the same year in order to protect against different parasites or their developmental stages. An example of one such “complete control programme” recommended that every year the horse owner should use a double dose of pyrantel for tapeworms, ivermectin for bots, and a five-day course of fenbendazole for encysted cyathostomes.7

Moreover, the rotation strategy could leave little room for choice in the light of confirmed pyrantel and benzimidazole resistance. The only anthelmintics without reported resistance, and which have a high efficacy and relatively long dosing intervals, are ivermectin and moxidectin (8-10 week dosing interval for ivermectin, 13 weeks for moxidectin).

Choice of macrocyclic lactone as a strategy to delay resistance?

Another current issue is whether one of the ML sub-groups (ie ivermectin or moxidectin) will hasten the onset of equine nematode ML resistance more quickly than will the other. Recently published data has shown that, in sheep nematodes, although selection for resistance by either compound is relatively slow, moxidectin is likely to select for resistance more slowly than ivermectin.8 This result is paralleled by actual field experience in Australia, where sheep nematode resistance to ivermectin occurred after only 4 years of use, compared with 10 years before isolated cases of moxidectin resistance were reported.9 Because selection by one compound increases resistance to both (“side resistance”), it appears that in order to delay the onset of resistance, moxidectin should be the first choice when using an ML.

Extending dosing intervals as a strategy to delay resistance?

Monitoring Faecal Worm Egg Counts (FWECs) to extend dosing intervals is another strategy proposed to delay the onset of resistance. This method can be used in order to only treat animals when they are producing eggs above a specific level (for example, in excess of 200 eggs per gram of faeces). However, FWECs do not give an indication of the larval burden of the animal, and this could mean that a horse is harbouring an unknown burden of the potentially dangerous encysted inhibited cyathostomes. The only compounds which have a registered license claim to control encysted cyathostomes are fenbendazole (a 5-day treatment), and moxidectin (single treatment).

It should be further noted that there is a lifelong susceptibility to cyathostomes and that they can cause clinical disease in any age of horse during any season.10 They can also cause life-threatening illness at the time that they invade the horse’s gut.11 This means that a “once-a-year encysted cyathostome dosing strategy” may leave a horse vulnerable to clinical cyathostominosis.

A SIMPLE SOLUTION

The Great Equine Worming Debate undoubtedly brings to light a number of hypotheses about how to delay the onset of nematode resistance. For the horse owner, this information can be important to their more immediate concern of their horse’s health. The main message for horse owners is to use a highly effective anthelmintic with no resistance, which can be given as infrequently as possible, whilst still protecting the health of your horse.

The following simple worming programme, which requires no annual rotation, is based around giving moxidectin (EQUEST®, Fort Dodge Animal Health) every 13 weeks, and includes a praziquantel product (EQUITAPE®, Fort Dodge Animal Health) twice yearly in line with tapeworm treatment recommendations.12 Praziquantel has been shown to have activity against all three species of equine tapeworm with no reported resistance.

Protecting their horse’s health should always be the primary objective of any horse owner. This worming programme offers horse owners a simple and effective answer to the control of parasites both in their horse and on the pasture.


For Further information visit:-
www.wormingyourhorse.info