It’s not often that a traditional horse-keeping routine gets completely upended, but that’s exactly what has happened with equine parasite control over the last two decades. After years of faithfully deworming horses every eight to twelve weeks, rotating among products of different chemical classes, horse owners in the mid-1990s were told that approach was misguided and even potentially harmful.

Here’s the problem: Each time an anthelmintic (worm-killing) chemical is used, most of the targeted pests in a horse’s gut die, but a few—those least susceptible to the chemicals—survive. Those remaining parasites go on to reproduce, and when they do, they can pass on their ability to withstand the deworming agent to the next generation. Over time, with each successive dose, a population of parasites that no longer responds to deworming chemicals grows. It’s like a genetic arms race, but for the time being we’re stuck with the weapons we already have. No new classes of dewormers are on the horizon right now.

We have not yet reached the stage where we cannot control parasites in horses. But to put that day off as long as possible, it’s important to use dewormers more judiciously than before. That means adopting a three-pronged approach to deworming: targeting the horses most affected by parasites, timing treatments appropriately, and taking measures to limit the exposure of all horses to internal parasites.

Your veterinarian can help you develop an individualized parasite control program for your horses. Still, it’s good to know which organisms you are targeting and the best ways to protect against them.

Four troublesome internal parasites

Many parasitic worms and larvae can affect horses, but four species pose the most serious threats to equine health:

• Small strongyles (cyathostomins). More than 40 species of these worms may affect horses. They spend a portion of their life span encysted in the intestinal wall. Small strongyles may do little harm, but in very large numbers, they can cause enough cumulative damage to disrupt a horse’s digestion and absorption of nutrients, causing him to lose weight and be generally unthrifty. When large numbers of encysted larvae emerge simultaneously, they may cause colic and, rarely, death.

• Ascarids (Parascaris equorum) affect mainly foals. Intestinal infections may cause unthriftiness, slowed growth, a dull coat and low energy. Heavier infections may block the intestines, causing life-threatening ruptures and colics. Adult horses tend to develop immunity to ascarids as they mature.

• Tapeworm infection can cause anemia and general unthriftiness. In some cases, tapeworms may lead to serious colics and issues such as perforations of the intestines and peritonitis 0.

• Large strongyles, also called bloodworms, are rare in horses today but can cause significant harm. Several species can appear in horses, but the most damaging, Strongylus vulgaris, migrates through the walls of the abdominal arteries, weakening them and leaving them prone to rupture. Their activity may also create blood clots, which may inhibit circulation to the intestines. Other species of large strongyles may damage the liver or other internal organs.

Other parasites that cause problems

These parasites tend to be less of a worry—because they are rare, they do less harm, or because horses develop immunity as they mature. Some, however, may cause problems:

• Botflies (Gasterophilus spp.) are not worms, but the larvae of a species of fly that lays its eggs on horses. When horses lick the eggs, the larvae hatch and embed themselves in the tissues of the mouth, before they emerge and are swallowed. Then they attach themselves to the wall of the stomach, before turning into grubs that pass out with the feces. The larvae may cause damage where they attach to the stomach wall.

• Filarids (Onchocerca spp.) are not intestinal parasites. The larval stage of the worm, transmitted by biting midges, migrates under the skin and may cause itchy dermatitis on the face, neck, chest, withers, forelegs and abdomen. The adults burrow into the neck (nuchal) ligament, where they may cause inflammatory reactions.

• Pinworms (Oxyuris equi) do little harm to the intestine, but when the females lay their eggs near the anus, they produce severe itching that causes the horse to rub bare patches on the rump and tail. Most horses seem to develop immunity to pinworms as they mature.

• Threadworms (Strongyloides westeri) mainly affect foals and may cause severe diarrhea. Most horses develop immunity to threadworms by the time they are 1 year old.

Deworming agents

The products most commonly used to deworm horses fall into three major categories.

Avermectins and milbemycins are still largely effective against a wide range of parasites, but resistance, especially in ascarids, is growing. This class includes:

• Ivermectin, which works against most of the common equine parasites except for tapeworms. However, it does not work against small strongyles while they are encysted.

• Moxidectin, which is similar to ivermectin but also acts against encysted small strongyles.

Benzimidazoles are effective against many adult parasites, but their efficacy against larval stages and all small strongyles is declining. This class includes:

• Fenbendazole, which kills large strongyles, pinworms and ascarids.

• Oxibendazole, which works against large strongyles, pinworms, ascarids and threadworms.

• Oxfendazole, which kills large strongyles, roundworms and pinworms.

Pyrimidines, also called pyrantel salts, have several applications. Resistance to chemicals in this class is growing among small strongyles. This class includes:

• Pyrantel pamoate, which works against large strongyles, pinworms, ascarids and tapeworms in a standard dose. When used at a double dose it kills 85 to 95 percent of tapeworms.

• Pyrantel tartrate, which is used for daily feed-through dewormers. It controls large strongyles and ascarids.

• Praziquantel, which works specifically against tapeworms.

Modern parasite control

To help keep our chemical weapons functioning longer, it’s important to create individualized treatment plans for each horse in your care. Here are the guiding principles of your new strategy:

• Drop the idea of eliminating parasites. Remember that your overall goal is to keep all your horses healthy—but that doesn’t mean parasite free. Few of the worms your horse will pick up will do him serious harm in small numbers.

• Identify the high shedders. Research has shown that adult horses shed strongyle parasite eggs at different rates. In fact, in some herds, only a small number of horses may be responsible for most of the strongyle eggs that are shed onto the pasture.

The only way to identify which horses are high shedders is to conduct fecal egg counts—tests that analyze the manure to look for parasite eggs. If you identify high shedders in your herd, targeting your deworming treatments to those individuals will greatly reduce the number of eggs on your pasture. Low-shedding horses in your herd may require no more than twice-annual treatments to control large strongyles.

• Choose the right agent. Another benefit of fecal egg counts is that they will identify the types of parasites present on your farm. Then you can choose chemicals that are proven to work against those worms.

Fecal egg counts can also help you determine whether the parasites on your farm are developing resistance. An initial count is performed before a horse is dewormed and then the test is repeated 10 to 14 days later. Ideally, the number of eggs would be reduced by 90 percent or more. If the count is reduced by less than 80 percent, then you need to investigate. First make sure that you are using the anthelmintic according to instructions: Check the expiration date, make sure you have stored it properly and administered the correct dosage. If all of those requirements have been met, the parasites on your farm are becoming resistant to that chemical, which means you’ll need to switch to a different chemical, both to slow the progress of resistance and to effectively treat your horse.

Note that because tapeworms shed eggs sporadically, they may not appear in a routine fecal egg count. Tapeworm infection can also be identified using a blood test to look for antibodies. A similar test that looks for tapeworm-specific antibodies in the horse’s saliva is now available in Europe but not yet in the United States.

• Deliver the right dosage. Giving a horse too small a dose of a dewormer is one of the risk factors that can increase resistance. You can calculate your horse’s weight in pounds with the help of a tailor’s tape: Measure his girth circumference in inches, square it, then multiply by his body length (measured from the point of his shoulder to the point of his buttocks), then divide that number by 330.

• Time treatments for best effect. Ironically, the greater the percentage of worms you kill in one deworming on your farm, the faster resistance will develop. That’s why it’s a good idea to time your treatments for when a higher percentage of parasites are in “refugia” or “out of reach.” That is, while the dewormer is working inside one horse, the chemicals would have no effect on eggs already on the pasture or in other low-shedding horses. Those eggs, for parasites that might be susceptible to the chemicals, could then be picked up and would “dilute” the number of resistance genes found in the next generation of worms. However, administering dewormers when the refugia population is low reduces this effect—for example, eggs do not survive long on the ground during very cold winters or in hot, dry summer weather. Frequent treatment of the whole herd at once also reduces refugia.

Generally, the experts recommend that each horse be given two annual deworming treatments, with high shedders receiving an extra treatment. Your veterinarian can help you develop a deworming plan that will work best for your horses.

How to reduce exposure to parasites

Killing worms and larvae that have infected your horse is an important goal, but it’s a better idea to limit the numbers of parasites your horse picks up in the first place:

• Allow ample turnout space. When horses have enough room, they tend to segregate their pastures into eating areas and toileting areas, called “roughs.” This way, they are less likely to graze over the areas when the parasite eggs have been dropped with the manure. Ideally, you’d have about two acres per horse.

• Pick up manure regularly from turnouts. When allowing two acres per horse isn’t possible, it’s important to pick up manure at least weekly. Compost manure thoroughly before spreading it on fields; the high heat will kill any eggs.

• Harrow only during hot, dry weather. Breaking up manure balls and spreading them thin can help kill parasite eggs faster, but do this chore only on hot, dry days when the sun will dry it out more quickly.

• Rotate grazing spaces. Periodically moving horses into different enclosures will allow for parasite eggs to die off before they can be picked up by grazers. If you have only one large pasture, consider using temporary fencing to limit the horses to one section at a time.

• Protect feeds from manure. Ground-level grazing is best for a horse’s respiratory health, but dropping hay on the ground in crowded turnouts may expose him to more parasites. Use ground-level feeders to keep hay clean.

• Quarantine new arrivals. Keeping a new horse separate from the main herd for a time is wise for many reasons, such as identifying any illnesses he may be carrying. While the newcomer is in isolation, it’s a good idea to conduct a fecal egg count. If he turns out to be a high shedder, you will want to administer a dewormer before he is turned out with the herd. 

This article first appeared in the May 2017 issue (#476) of EQUUS magazine