In this edition, let’s chat about internal parasites and how times have changed with the way we deworm horses. The good news is that deworming horses should be simpler and cheaper if we adopt current expert advice.

As background, until the mid-20th century worms were difficult to control, and treatments were potentially toxic. Most horse owners didn’t follow a routine for deworming; if they thought the horse had parasites they called the vet to drench the horse, often with some terrible chemical with little efficacy.

It was only in the 1960’s that the first of the modern deworming chemicals were developed, the benzimidazoles or ‘BZs’. These drugs were revolutionary; safer for horses, and effective against worms such as large strongyles which were thought to be a main contributor to colic cases. The new drugs were widely used and led to dramatic reductions in the number of horses getting sick and dying from parasites. The BZs are still commonly used today and are most effective against roundworms (ascarids) in foals.

The “’mectins” were developed in the late 1970’s, beginning with ivermectin. The last ‘mectin to appear was moxidectin in the late 1990s, and these are still the dominant drug group in use today, being most effective against small and large strongyles.

An unfortunate by-product of these amazing drugs was that they were used more frequently. Every 8 weeks was the initial recommendation, along with ‘rotation’ amongst chemicals to cover all the relevant parasites of concern. Happily the impact of the dreaded large strongyles receded, but. rotation was continued once the ‘mectins arrived, this time to try and avoid drug resistance. Today we have reached a point where small strongyles have replaced large strongyles as the ‘parasite of concern’ for adult horses, but rotation has not prevented the emergence of resistance. A 2014 study in Australia of over 100 horse farms revealed that all farms were infested with small strongyles, whereas less than 8% of the farms had evidence of large strongyles; and resistance has been well documented both here and overseas.

Cyathostomins (small strongyles) are a nasty group of parasites, in that larval (immature) stages encyst, or ‘hide’ in the horse’s gut lining – with most wormers being ineffective against this developmental stage. We generally measure the number of eggs shed in the manure to assess worm burden, however encysted stages do not produce eggs, so there is no reliable way to know if a horse has encysted small strongyles or how many.  More concerning is a syndrome where all of the larval worms emerge from their cysts in the gut wall at once – this can cause severe colic and is fatal in up to 50% of cases. The reasons for mass emergence are largely unknown so using a wormer that kills encysted worms is a useful tool in worm management. Moxidectin as a single dose or fenbendazole at double dose for 5 days consecutively are the only options at present.

I mentioned the failure of rotation to prevent resistance, and sadly resistance exists in variable degrees to all available wormers nowadays. If you are rotating,you could potentially be rotating from one ‘resistant drug’ to another, or you may be rotating from a more effective to a less effective drug, which seems counter-productive.

Unfortunately, there are no new and exciting wormer classes in development that are safe, broad-spectrum and without resistance.  The high cost associated with the discovery of new drugs combined with the small size of the equine parasiticide market potentially limits drug development. In addition, any new drugs are likely to be much more expensive than the options we currently have. So, what to do? Well, we need to make the best use of what we currently have – and we need to start thinking strategically.

In essence, you need to treat as infrequently as possible, but as much as required.

In the next edition we will talk more about how we deworm our horses ‘smarter, not harder’.

Stay tuned for next week, when we present part 2 of this article.