Mankind has known about worms in horses for centuries, but have had effective strategies to deal with them for a much shorter time.
Blood-letting was once considered an effective cure, as was making a horse drink its own blood. In fact, blood-letting would cure just about anything in what was clearly a triumph of marketing over substance.
People were on a better track when they decided that dosing a horse with certain things by mouth would overcome a worming problem.
Without doubt, literally hundreds of different agents have been tried over the centuries and most shared a number of attributes. At best they were ineffective and harmless, but many were poisonous and caused terrible health problems in horses, even death.
History tells us that animal offal was given as a worming agent and even highly toxic mercury.
Some of these remedies would undoubtedly have delivered a killer blow to the parasites in a horse’s gut, but unfortunately harmed the horse in the process. What proved toxic to the parasites proved similarly toxic to the horse.
The key to identifying drenching agents has focused on identifying substances that are toxic to the parasites but do little or no harm to the horse.
Herbal remedies rose in popularity but many proved toxic to horses.
By the early 1900s horses were regularly being dosed with either carbon disulfide or carbon tetrachloride.
They both killed parasites but, like so many of the drenches that had gone before, both proved toxic.
Another worming agent from the period included tobacco (apparently a variety called Virginian shag was considered very effective). It was given internally for parasites, and a plug of tobacco in the anus was considered effective against pinworms.
Horse owners were delivered what could rightly be called the first dewormer of the modern era in the 1940s. It was called phenothiazine.
It was an effective treatment against strongyles although its toxicity to horses remained a concern. It remained popular for a good 20 years and scientists noted what would become the bette noire of all drenches – growing worm resistance to the chemical.
Resistance to phenothiazine was first recorded in the late 1950s.
But by this time horse owners suddenly had a choice. They could use piperazine and it was promoted as being effective against a range of parasite types. Unfortunately, it couldn’t work its magic against large strongyles.
Later still, some organophosphates were used but proved very toxic, and not just for horses.
Some of these early wormers were marketed in combination and proved quite effective. The combination also meant the active agents could be given in lower doses, which was better for horses.
One of the biggest breakthroughs in worming came in the 1960s and 70s when the benzimidazoles came on the scene. They were highly effective against a wide range of parasites and the dose rate required was so low that they were considered very safe.
First out of the starting blocks was thiabendazole. This chemical not only delivered never-before-seen levels of effectiveness, it delivered another important change.
It truly brought deworming to the masses. Before then, drenches were largely given by veterinarians. The margin of safety was such with the benzimidazole family that they could be sold over-the-counter in easy-to-use pastes.
Other members of the family quickly followed: cambendazole, oxfendazole, fenbendazole, oxibendazole and mebendazole. Some of these can still be found in modern drench formulations.
These were grim times for your average equine parasite. The war, it seemed, was won.
Horse owners took to these safe and effective drenches in their droves. The parasites, never before exposed to such an effective agent, died in countless numbers.
What could possibly go wrong?
Scientists had already seen worm resistance in phenothiazine, and the benzimidazole family was to be no exception.
Parasites had not lived for millions of years without learning a trick or two. Worms began developing a resistance and the effectiveness of these agents began to decline.
By the 1970s resistance was well documented, but the show was far from over.
Next up was pyrantel, which was new, effective and, like the benzimidazoles, safe to use. Parasites proving resistant to the benzimidazoles were dispatched with this new weapon in the war against worms.
But this was proving very much like a game of cat of mouse. Horse owners had a range of deworming agents available to them but the worms kept developing resistance.
Could the cycle be broken?
The real star in the Deworming Hall of Fame arrived in the early 1980s. Ivermectin was effective to an almost unbelievable extent. It worked on a wide range of parasites and proved very safe.
Once on the market in paste form, it quickly became a market leader.
For a time, it seemed parasites were not developing resistance to ivermectin, but it was not to be. Resistance may not have been as quick to develop against ivermectin as it was against other deworming agents, but develop it did, although admittedly not to the same extent as other drench families.
Next on the world stage was moxidectin, which, like ivermectin, is a member of the drench family called the macrocyclic lactones. It is broad spectrum like ivermectin, but, crucially, proved effective against small strongyles in their encysted larval stage. More on this later.
Today, a range of deworming chemicals is available to horse owners but all have fallen victim – at least in part – to worm resistance.
Perhaps scientists will develop new drenches in the fight against parasites. Perhaps these drenches will work on parasites in a way that reduces or even eliminates the development of resistance.
It would be dangerous for the horse community to assume that something bigger or better is just around the corner.
Prudent horse owners need to work with the cards they have been dealt and, today, that comprises a quite limited range of deworming chemicals with a range of properties.
Some are effective against some parasites, but not all, and others are effective only against parasites at certain life stages. All have seen worm resistance develop against them, some more than others.
The battle, on the evidence so far, is not about to be won.
Horse owners must therefore use all the tools available to them, using horse and paddock management techniques that minimise exposure to parasites, as well as rotating the drenches available to them to reduce the chances of resistance.
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