Deworming Young Horses

Target this important age group by working with your veterinarian to time treatment and avoid perpetuating drug-resistant parasites.

Deworming foals and young horses can be a bit of a Catch-22: They are more vulnerable to internal parasite infection than adult horses; however, aggressive deworming increases the risk of developing drug-resistant parasite populations. To reach a happy medium, owners should consider less-intensive deworming of this age group.

"Traditionally, we started deworming early, often less than a month of age, and repeated the treatments as often as monthly or bimonthly during the first year of life," explains Craig Reinemeyer, DVM, PhD, Dipl. ACVM, president of East Tennessee Clinical Research Inc., in Rockwood. "In hindsight, this was a bad idea."

In this article we will address good strategies for deworming youngsters, from foals to weanlings and beyond.

Foals' Foes: Threadworms and Roundworms

Many owners treat young foals for threadworms (Strongyloides westeri) they might acquire from the mare's milk. "There's debate whether to treat mares before foaling (and when/with what dewormer) or if it's better to treat the foal within the first days of life, or even to treat for threadworms at all," says Martin Nielsen, DVM, PhD, Dipl. EVPC, assistant professor at the University of Kentucky's Maxwell H. Gluck Equine Research Center, in Lexington.

If a mare has been on an appropriate deworming program and was treated within the two months prior to delivery, Nielsen says she probably doesn't need dewormer again before labor. "If you do treat the mare," he adds, "use a drug with efficacy against migrating stages of threadworms." These hide in the mare's tissues, waiting for foaling and lactation hormones to call them into action. At that point they migrate to the mammary glands and into the milk. Neither Nielsen nor Reinemeyer typically recommend treating foals for threadworms, however, as this parasite is only ¬occasionally pathogenic (disease-causing).

"You won't lose a foal from this parasite," Reinemeyer explains. "It's also very easy to treat if we diagnose a foal with diarrhea and lethargy associated with Strongyloides infection. (However), even if fecal samples tell you the worm is present (it) doesn't mean we have to treat. Foals develop complete immunity to threadworms by the time they are 5 to 6 months old."

He adds that such early deworming merely exposes other worms, especially ascarids, to suboptimal doses of the dewormer being used for threadworms. As a result, the nontargeted parasites can develop resistance to these drugs. "Anytime you treat a foal with ivermectin or moxidectin, for instance, you are treating nearly every worm inside that horse," says Reinemeyer. "Current deworming practices rapidly select for (in other words, they aid in development and proliferation of) resistant worms that are no longer susceptible to ivermectin and moxidectin in young horses."

The first real parasitic threat to foals is the large roundworm, or ascarid, known as Parascaris equorum. A foal ingests the worm's infective eggs while sampling pasture or exploring his environment, and these eggs release larvae in the small intestine. The larvae then migrate through the liver and lungs and eventually return to the gut, where they mature and reproduce. Heavy ascarid infection can trigger weight loss, poor growth, a rough hair coat, a pot-bellied appearance, lethargy, and/or colic.

"The best time to treat (for this parasite) is no sooner than 2 to 3 months of age," says Nielsen. "The reason for waiting this long is that efficacy of any anthelmintic treatment tends to be higher in later stages of the parasite (life cycle). If you treat too early, when the foal has only larval stages, efficacy is poor."

Gene Lyons, PhD, and colleagues at the Gluck Center, for instance, conducted a study in which they found that using oxibendazole at the standard label dose was only about 40% effective in killing 28-day-old ascarid populations. However, the same drug at the same dose was more than 95% effective in killing adult worms.

Thus, veterinarians should recommend owners never deworm foals younger than 60 days. If you wait much longer than two to three months, however, the worms mature enough to start shedding eggs, which contaminate the environment and remain infective for years. The worms will also grow larger within the gastrointestinal tract, posing a risk for small intestine impaction after deworming, Nielsen warns.

The choice of deworming drug depends on whether the foal is at risk for impaction--whether you want to kill the worms slowly or quickly. "Benzimidazoles tend to result in a slow kill (for a lesser chance of impaction)," says Nielsen. "Since Parascaris doesn't seem to be resistant to these drugs, they are a safe choice for deworming foals.

"Four dewormings during the first year of life should be adequate, unless stocking rate (number of horses per unit of land) is very high, or we've had clinical problems in those foals or have identified a certain parasite that warrants extra effort in deworming," he continues.

Eventually, most young horses develop acquired immunity to ascarids just as they do to threadworms. On the whole, Reinemeyer recommends avoiding early deworming prior to 2 to 3 months of age and knowing which drugs are effective when treatment in necessary.

Weanling Regimens

Weanlings require a different and more complicated deworming approach because they might still have ascarids, but they might also have some strongyles. According to Andrew S. Peregrine, BVMS, PhD, DVM, Dipl. EVPC, ACVM, associate professor in the Department of Pathobiology at the University of Guelph's Ontario Veterinary College, small strongyles (cyathostomes--the main parasite targeted in adult horses) are becoming more prevalent in weaning-age foals.

"Toward the end of grazing season and in fall/winter, these worms can cause ¬serious problems in foals," he explains. "I dealt with a large number of 8-month-old foals that died in early December 2011 from damage due to these parasites.

"Older foals have always been infested with both roundworms and small strongyles, but we're now seeing many foals with massive numbers of (these) worms because their environment has become more heavily contaminated--mainly due to drug resistance," he continues. "Horse owners need to work with a veterinarian to determine the extent of the problem and how to treat it."

Fenbendazole is excellent for treating ascarids, but veterinarians note it might not be as effective against small strongyles as it used to be. When treating weanlings primarily for strongyles, Nielsen suggests using ivermectin; however, this drug is facing high levels of resistance in ascarids. "Weaning age is the transitional period where we see immunity kicking in against roundworms and see strongyles coming in and taking over--becoming the dominant worm burden," says Nielsen. This might happen later in some foals than in others.

Because you can't tell by looking at a foal which drug to use, he recommends taking a fecal sample to determine egg count and type. "The actual egg count is not important, but the type of egg you find is very important," Nielsen explains. "This can help you make the right decision on which parasite you should be treating."

The Science Behind Fecal Samples

"Today you can't assume the drugs you use are working unless you monitor foals by taking fecal samples," says Peregrine. "The horse owners I know of who lost foals because of massive worm burdens had never done a fecal sample. We surveyed more than 100 horse owners, and only 2% had ever done a fecal on a horse. The vast majority are just deworming and assuming it's working, even though today there are significant problems with drug resistance, both in roundworms and cyathostomes."

For both parasite types, problems typically occur in foals during late summer, fall, and winter. Peregrine recommends owners perform fecal monitoring in July/August to detect and treat heavy worm burdens before a clinical problem manifests.

To determine whether a drug is working, order a fecal egg count reduction test. "When deworming foals, do egg counts (for roundworms and/or small strongyles) on the day of treatment," says Peregrine. "Then take another fecal count 10 to 14 days later. With ivermectin we like to see an average reduction of at least 95% in egg count. If it doesn't drop that much, and you know you dewormed properly, this is strong evidence that the parasites are resistant and you should stop using that drug."

Importance of Acquired Immunity

"Though we want to prevent clinical disease in foals, we also want them to be exposed to a few parasites so they can start developing some resistance/immunity," says Nielsen. In one immunity study, researchers at Louisiana State University subjected horses to intensive deworming regimens for one year. The following year, the researchers found those horses were more susceptible to parasitic invasion.

"Immunity against parasites is never complete but it plays a role in reaching a balance in worm burden," Nielsen explains. "The ascarid migrates through the foal's liver and lungs, exposing itself to the immune system. We find a well-developed level of immunity against this parasite compared with tapeworms, for instance, that never leave the gut, or small strongyles that only make it into the intestinal wall and then go back into the gut."

"Young horses develop full immunity to ascarids by the time they reach 15 to 18 months of age," says Reinemeyer. "After that point, in most herds the major issue for yearlings and adults will be small strongyles."

Most herds don't have problems with large or small strongyles as long as those worms' numbers are kept low. But if you've kept parasite numbers so low that horses don't encounter enough worms to stimulate immunity, these horses are defenseless if they are exposed to worms while traveling; they could easily develop heavy worm burdens.

"These horses are suddenly dealing with parasites they've never encountered," explains Reinemeyer. "It's like when you send kids to first grade: They come home every week with a different kind of cold because they haven't encountered these viruses before."

Thus, most veterinarians and researchers agree it's better to give young horses some worm exposure and let them develop immunity, but keep worm numbers low enough so as not to be damaging. Then when they really need the immunity it will be there.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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