History of Equine Parasite Control

This image from the May/June 1981 issue of Horse Illustrated shows a vet administering anthelmintic drugs via nasogastric tube.

Veterinary care for horses has progressed by leaps and bounds since the days when equines were working animals. Today’s horses live longer, healthier lives and are active in later years. Vaccines have helped to prevent unnecessary death from diseases that were once a serious threat. But one battle veterinary researchers are still fighting is equine parasite control.

The Dark Ages

Administering paste dewormer can be an unpleasant experience now if you have an uncooperative horse, but it’s a walk in the park compared to what horses in the Renaissance were subjected to. At that time, it was believed that parasites would be killed if some of the horse’s blood was drained, then fed back to him.

Another unpleasant remedy was offal, or the intestines of animals such as chickens. Chicken eggs were also said to be a remedy. Mercury, which we know now is highly toxic, was also a remedy. It may have succeeded in eliminating some worms, but it may have taken the equine hosts along with them.

As recently as the early 1900s, parasites were still viewed with a bit of superstition. The book Veterinary Notes for Horse Owners1 suggested the following protocol for eliminating worms outside of the horse’s system:

All the worms which are observed to come away from the horse, whether naturally or by the influence of medicine, should be thrown into a fire or into boiling water, so as to cut short their power of evil.

Plants and Herbs

Less shocking than blood, guts and poison, but probably no more effective are the variety of herbs and other plants that were considered at one time to be effective parasite control agents. Aniseed, licorice and flaxseed were all used, and were probably much easier to administer than prior methods as horses tend to like the taste of those herbs.

Thymol, an extract of thyme, was frequently suggested as a treatment. Tobacco leaves were used for a time, and horses tended to eat them readily. Unfortunately, the amount required to kill parasites would also be toxic to the horse and could make a horse sick after treatment.

Better Living through Chemistry

By the 1900s, veterinarians had begun to use chemical treatments to kill parasites. However, those early formulas shared the same primary problem as their herbal predecessors: in order to kill the parasites, a horse owner would have to administer a dose that was large enough to be toxic to the horse. The treatments were also considerably more complicated than what horse owners are accustomed to today.

Veterinary Notes for Horse Owners suggested the following treatment for roundworms:

“…the most effective plan is to give ½ drachm [equivalent to 1/16 fluid ounces in today’s measurements] each of tartar emetic and sulpate of iron in the food four times a day for a week, and then to administer a purgative in the form of a ball of aloes or a drench of a quart of linseed oil; keeping the animal during that period rather “short” on bran (dry or in a mash) and hay or grass. Although this diet will weaken him for the time being, it will also affect the parasites, and will induce them the more readily to quit their abode…Or we may give 3 ounces of turpentine in a quart of linseed oil. Turpentine causes death to the parasites on contact…As turpentine has a strongly stimulating effect on the kidneys, the large dose of 3 ounces should not be repeated.”

The same guide also advises “anointing the inside of the anus with a little mecurial ointment” to rid a horse of threadworms.

In the mid-1900s, better anthelmintic (anti-parasite) drugs came on the market. Phenothiazine was one of the early drugs that was effective against strongyles. Piperazine was the first drug that could be called “broad-spectrum,” meaning it worked on several different parasites, in this case ascarids, small strongyles and pinworms. These and other early deworming drugs still had a high risk of toxicity if dosed incorrectly. They often had to be mixed and administered by a veterinarian to achieve effectiveness against all parasites without harming the horse.

Emerging Classes

The deworming agents we know today began emerging around the 1970s. The broad-spectrum class of benzimidazoles helped reduce the risk of toxicity as they could be effective at a lower dose and a wider margin of safety. This opened the door to the creation of the paste dewormers that horse owners could administer on their own rather than calling out the vet for every treatment. Pyrantel, which is still around today as Strongid, came about in the 1970s as well and was an alternative to the benzimidazoles.

One of the most commonly used dewormers for horses and other animals is ivermectin, which hit the market in the 1980s. Ivermectin remains popular as it is not only effective against multiple species of parasite in various stages of life, but carries almost no risk of toxicity.

Resistance

Reports of dewormer resistance are as old as chemical deworming. That first chemical treatment, Phenoziathine, was introduced in the 1940s, and veterinarians had found evidence of resistance by the 1960s. Benzimidazoles were beginning to suffer from resistant parasites by the time pyrantel was introduced to help reduce the problem.

One of the primary challenges faced by veterinarians and researchers is that there have not been new deworming agents discovered in recent years, and there are no promising drugs on the horizon. This may be due in part to the widespread popularity of ivermectin; because it was so easy, effective and popular, research may have stagnated for a time. The most recent addition to the deworming arsenal is moxidectin, which is similar to ivermectin. Moxidectin was first developed in the 1990s.

Get in the Routine

In the mid-20th century, most horse owners didn’t follow a specific deworming routine; if they believed their horses to be infested, they would call out the vet to administer a cocktail of deworming agents, typically through a nasogastric tube. Starting in the late 1960s, the protocol of deworming every two months and rotating classes gained popularity. With the switch from preventative deworming rather that reactive (waiting for symptoms of infestation, then treating) parasites seemed to be under control. Where infestation had once been the leading cause of colic, it dropped significantly to where parasites became a minor issue rather than a serious threat to horse life.

However, the false sense of security modern horse owners have with a standard rotation, resistance has become a concern once again. Deworming horses that are not affected by certain parasites was generally not considered a big problem with the horse-safe deworming drugs in today’s parasite-control arsenal, but that excessive chemical treatment may be increasing resistance. Veterinarians and researchers now believe that the best routine is to deworm only the horses carrying a parasite load, and only for those parasites with which they are infested. This can be determined through a fecal egg count and fecal egg reduction test, available through equine veterinarians or through the mail. For more information, see The New Deworming Rules.

Parasite control has come a long way since the days of superstition and toxic treatments. With continued education of horse owners and a progressive view of treatment strategies, intestinal parasites will remain a controllable health problem for horses.

1. Hayes, Matthew Horace; Veterinary Notes for Horse Owners, Hurst and Blackett, January 1903.

Leslie Potter is Managing Editor of horseillustrated.com. Follow her on Twitter: @LeslieInLex.