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Modern worm control for adult horses

With the ever increasing accounts of drug-resistant parasites, in Australia and overseas, it seems a pertinent time to address this issue. This article will present some of the current information and advice regarding equine worm control and the ever-growing issue of anthelmintic resistance, which can be used to transition towards more modern and sustainable practices based on evidence and logic. The specific recommendations that have been put forward by equine parasitologists centre on treatment options for horses in different age and reproductive categories. In this article we will focus on strategies for adult horses.

Before ploughing into the latest information, it’s a good idea to strip back out-dated and preconceived notions about worm control in the horse and to re-examine underlying goals. Undoubtedly the ultimate goal is to promote good health and prevent disease in the horse, but this really simmers down to limiting the passage of eggs into the environment, thereby limiting transmission. It is imperative to acknowledge the difference between “limit” and “prevent”. It is neither achievable nor desirable to completely prevent or attempt to eradicate gastrointestinal parasites of horses. There needs to be some tolerance for low levels of infection, which horses have evolved to endure without considerable risks to their health and welfare. A second, and equally important goal, is to use practices that do not promote the development of resistance.

For more mature horses (approximately 2-3 years and older), broad-spectrum worm control primarily targeting cyathostomins (also known as small strongyles or redworm) can still be achieved with mectin drugs. Treatment intervals of six weeks, usually timed for convenience to match farrier visits or other management procedures (common practice on many horse farms) should be discouraged. The evidence-based approach suggests using FEC’s to identify low-egg shedders which can subsequently be treated less frequently. These low shedders may only require 1-2 treatments per year in comparison to high-shedders which may still require treatment every eight weeks or so.

Pasture contamination with cyathostomin eggs is not generated equally by all mature horses in a herd. The “80/20” rule suggests that the majority of parasites (80%) are harboured by a minority (20%) of individual horses. Further, studies have suggested that the number of eggs shed by individual horses remains fairly consistent throughout their life, and this is particularly true for horses that are classified as “low shedders”. The commonly used thresholds for ranking horses on faecal egg counts (FEC’s) are:

  • <200 epg = low egg-shedder
  • 200-500 epg = moderate egg shedder
  • >500 epg = high egg-shedder

(where epg is eggs per gram of faeces tested)

The most commonly used threshold for treatment is 200 epg. FEC monitoring can be used to identify horses in these categories allowing treatments to be targeted more towards the moderate to high egg shedders. The optimum time to sample a horse to determine its contaminative potential is 3-4 weeks after the effects of the previous anthelmintic treatment have worn off.

Many horse owners adopt some form of drug rotation strategy in an attempt to delay the onset of anthelmintic resistance, however, it is a topic on which there is a great deal of conflicting advice. Unfortunately there are no specific equine studies on rotational drug use, so recommendations can only be extrapolated from sheep and goat computer modelling studies. The consensus among researchers is that long rotations (i.e. annual) between drug classes are more effective at delaying resistance than short rotations (i.e. rotating with each treatment). The three broad-spectrum drug classes and their relative active ingredients that encompass all registered equine worming products available on the Australian market are presented in Table 1:

Table 1.Classes of drugs available for treatment of horse worms and the corresponding standard egg reappearance period.
DRUG CLASS Active Ingredient
BENZIMIDAZOLES (BZ’s) Oxibendazole
Oxfendazole
Fenbendazole
TETRAHYDROPYRIMIDINES (THP’s) Morantel (Tartrate)
Pyrantel (Embonate)
MACROCYCLIC LACTONES (mectins) Abamectin
Ivermectin
Moxidectin

 

One glaringly obvious dilemma in implementing rotational strategies for horses is the lack of drug classes to rotate between. Of the three available classes of broad-spectrum anthelmintic, one (the BZ’s) is virtually useless against cyathostomins, and another (the THP’s) with unknown resistance status, has only one product available on the shelves. Studies from overseas make up the bulk of available data on drug-resistant equine parasites as comparatively few studies have taken place in Australia. Resistance among cyathostomins to the BZ family of drugs is now extremely common and widespread, worldwide. Unless recent testing confirms that BZ’s are still effective on a property, they should not be used in isolation for control of cyathostomins. To date there are no reports of BZ resistance among other equine parasites of importance.

The mectin drugs form the basis of most worm control programs, which means when it’s time to rotate, the combination drugs must be utilised. Strategy-T © is often used in rotation with ML drugs, as it is the only product that contains actives from the two other drug classes. Whilst in the non-mectin phase of the rotation, it is important to remember that a mectin drug must still be incorporated for the control of bots. One treatment during late autumn or early winter (after the first frost) should be sufficient to kill all “overwintering”  larvae.

There are some great online resources that offer horse owners a comprehensive foundation of information and advice on many facets of equine gastrointestinal parasites, their treatment and control.

Article courtesy of Dr Anne Beasley PhD, School of Veterinary Science, University of Queensland.

 

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