Parasite Control for Young Horses

Parasitologists paint a realistic picture of what roundworm-related diseases in foals look like and dispel common myths.
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two foals in field
The AAEP recommends deworming foals four times during the first year of life, with the first two targeting roundworms. | Photo: iStock

Parasitologists paint a realistic picture of what roundworm-related diseases in foals look like

Many of us have heard a horror story or two about the dangers of roundworms (Parascaris spp, ascarids) in young horses. Some of us might have even seen the pictures of an anesthetized foal’s intestine cut open during surgery to reveal the spaghettilike worms spilling out onto a blue surgical drape. But if you trust statistics, odds are you haven’t witnessed problems related to Parascaris firsthand. Even if you think those small, potbellied foals with dull coats and runny noses must be victims of roundworms, parasitologists will disagree.

“In general, parasitic disease is a rare and exceptional event in well-managed horses and foals,” says Martin K. Nielsen, DVM, DVSc, PhD, Dipl. ACVM, EVPC, a professor of parasitology and the Schlaikjer professor in equine infectious disease at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, in Lexington.

Nielsen makes this observation from not only years of his own experience but also research conducted around the globe. In one of his many recently published studies, he reported that foals dewormed at only 2 and 5 months of age had no measurable negative health consequences (e.g., no difference in average daily weight gain, no incidents of colic) compared to foals dewormed more aggressively.

“By more aggressive we mean deworming every two months and rotation between dewormers,” Nielsen ­clarifies.

While Nielsen assures us roundworm impactions are uncommon, he does not endorse declaring parasitism in foals a nonissue.

“Even though roundworm-related disease is rare, it can happen. And when it happens, it can be serious and life-threatening,” he explains. “Any impaction in the small intestine is a more serious event than our usual large intestinal impaction. Small intestinal impactions are much more painful and difficult to resolve medically. Surgery is often required.”

In this article Nielsen relays the life cycle of the equine roundworm, dispels myths associated with Parascaris-related diseases, and describes effective control strategies.

The Parascaris Life Cycle in Horses

Parascaris infections occur almost exclusively in juvenile horses 18 months of age and younger. Adult female pencil-sized roundworms residing in the small intestine of young horses lay eggs. Those eggs pass in infected foals’ feces. On the ground the eggs develop into microscopic “larvated eggs” within weeks to months, depending on the environmental conditions (heat, humidity, etc.). These eggs measure about 100 micrometers, which is about the diameter of a strand of hair.

“Larvated eggs are the infectious stage for roundworms as they contain infective larvae,” says Nielsen. “After being ingested by foals, the larvae hatch from the infective eggs in the foal’s small intestine.”

But those larvae do not simply stay in the small intestine and grow into adult worms as one might think. They first take a circuitous journey.

To begin, the larvae penetrate the wall of the small intestine and invade the lymphatic system. The lymphatic ducts take the larvae to their first stop: the liver. The larvae migrate throughout this important internal organ for about a week. They then enter the bloodstream and travel to the foal’s lungs. They break free of the bloodstream and enter the airways (bronchioles).

“After two to three weeks, the larvae migrate up the respiratory tree to the bronchi and trachea where they are ultimately coughed up into the foal’s pharynx (the back of the oral cavity),” Nielsen explains. “The foal then swallows the larvae, and they return to the small intestine and, finally, develop into adult worms.”

Common Myths and Misconceptions

With all its gallivanting through the foal’s liver and lungs, it’s easy to see why this parasite is accused of causing disease. But, as Nielsen emphasizes, clinical disease rarely develops in well-managed foals and juvenile horses. Here are some common myths and misconceptions associated with Parascaris in foals:

Myth: Foals can have hundreds of worms in their small intestine. So many, in fact, the worms could fill a 1-liter bucket.

“Yes, they can, but most of the time they don’t,” says Nielsen. “We often struggle finding more than a handful of worms in the foals that we use in our research.”

Myth: Young horses with Parascaris have trouble gaining weight, presumably because the mass of adult worms in the small intestines competes with the host for digested nutrients, especially amino acids, and/or because the foals are eating less.

“I don’t think there are any data ­supporting these statements,” he says. “We have performed two studies evaluating weight gain and Parascaris in foals and found no association between the two.”

 Myth: Foals with internal parasites such as roundworms have poor/dull hair coats.

“Most foals have roundworms,” says Nielsen. “So, if a few of those foals have dull hair coats, is it because of the worms? This is an example of association versus ­causation.”

Myth: Heavy burdens of Parascaris are always the cause of a pot-bellied appearance.

“Foals can have a pot-bellied appearance just like the one observed in heavily parasitized children,” says Nielsen.

He warns, however, that foals and young horses can also appear pot-bellied simply due to eating roughage, which fills the large intestine.

“In this case, the belly appearing large has nothing to do with parasites,” he adds.

 Myth: Parasitized foals are ­lethargic.

“There is, again, no evidence to support this misnomer,” Nielsen says. “All horses are parasitized. So when we come across a lethargic one, is it because of the parasites?”

In sum, the only specific disease syndrome equine ascarids cause is small intestine impactions, which are well-described. Most roundworm infections are completely asymptomatic, he says.

“Further, roundworms are usually eliminated between six and eight months of age,” says Nielsen. An age-dependent immunity occurs regardless of whether the animal has been infected.

“Once they reach a certain age, they no longer get ascarids,” he says. “A few adult horses can be found harboring ascarids, but that is relatively rare.”

 Myth: Lung invasion, which occurs two to four weeks after the foal consumes the infective eggs in the environment, can cause clinical signs of disease.

“Foals rarely develop a cough and infrequently produce a grayish-white purulent (pus-filled) nasal discharge,” says Nielsen. “If a foal were to undergo an endoscopic examination to directly visualize their airways, some discharge might be seen in the trachea.”

Even though larvae migrate around the foal’s liver for about one week after infection, they don’t cause clinical signs of liver disease.

Diagnosing Parasite Infections and Predicting Disease

Foals are not born with Parascaris infections and do not become infected via the mare’s milk. Instead, they can become infected soon after birth by ingesting infective eggs.

“While rumor also has it that pregnant broodmares can have Parascaris eggs stuck to their udder or coat, serving as a source of infection for neonatal foals, there is no evidence to support this,” says Nielsen. “The mares are usually not shedding ascarid eggs themselves, so they are unlikely to have many eggs on their skin or hair coat.”

The prepatent (incubation) period for Parascaris is about 90-110 days. This means it takes more than three months from the time the foal is infected with the larvae until the adult roundworms lay the eggs that can be measured in feces by a fecal egg count or fecal flotation.

“Thus, testing foals, even those currently infected with Parascaris, before three months of age will yield negative results, as the adult roundworms have not yet developed or reproduced,” says Nielsen.

“We don’t recommend egg counts until about five or six months of age, and that is more to monitor the decline of the ascarids and the takeover of the strongyles rather than simply diagnosing roundworms in foals,” he adds. “This is because we no longer have a single dewormer product that can be expected to effectively treat both parasite categories (i.e., ascarids and strongyles). So we need to know what we are treating so we can make the right choice of dewormer product.”

While egg counts do not provide information about the number of worms present in the foal, Nielsen says an available ultrasound technique can semiquantify the burdens.

Treating Foals to Avoid Parasite-Related Diseases

Despite the naturally occurring clearance of roundworm infections with age, young animals should still be dewormed to minimize the chances of small intestine impaction.

Chemical deworming

The American Association of Equine Practitioners (AAEP) Parasite Control Guidelines recommends deworming foals four times with a chemical dewormer during the first year of life, but only the first two target the roundworms. The first deworming should be performed when the foal is about 2 or 3 months old, preferably with a benzimidazole product (e.g., 10 mg/kg fenbendazole, oxibendazole). The AAEP recommends a second deworming with a benzimidazole just prior to weaning or when the foal is about 4 to 6 months old. Those medications target the adult roundworms in the small intestine; no products on the market can kill the migrating larvae.

The AAEP does not recommend targeted treatments (selective therapy) based on FEC (fecal egg count) in this age group. The No. 1 use of FECs in adult horses is for the fecal egg count reduction test (FECRT).

“There are large biological differences between strongyles that occur in adult horses and ascarids that occur in young horses,” explains Nielsen. “With strongyles we want to identify the (horses that are) consistent high shedders in a herd via an FEC and get those treated appropriately, because those horses will maintain high shedding levels year after year. With ascarids, about 50% of foals will be shedding eggs for a very short amount of time, so there is less value in targeting high shedders.”

Pro tip: When deworming, do not underdose! Use weight tapes or scales to determine body weight and deworm these animals appropriately. Be sure to administer the entire dose by mouth. The portion of the dewormer that hits the ground provides no benefit to the foal.

“The No. 1 cause of losing dewormer on the floor is if the horse has food (grain, hay, or grass) in its mouth when administered,” Nielsen says.

Pro tip: If a foal is suspected of having a heavy burden of Parascaris, then he should still be dewormed with a full dose of a benzimidazole (e.g., fenbendazole at 10 mg/kg). Some reports suggest performing a “slow kill” with a smaller dose in heavily parasitized foals to avoid causing a small intestine impaction (from killing all the adult roundworms at one time).

“There is no such thing as slow kill,” says Nielsen. “This is a big misconception. Treat with a registered dose with an anthelmintic that works. Period.”

He adds, “When people ask me for guidance on ‘slow kill,’ they often want to treat with half a dose of a product. I usually respond by saying that half a dose is not ‘slower’ than a full dose. It does not kill just 50% of the worms.”

Cleaning the environment

Disinfecting foaling stalls can help reduce the number of infective eggs neonates ingest. Lysol (a 50% solution of cresol in saponified vegetable oil), phenol, and chlorine bleach can help decrease the eggs, but none can fully eliminate them. And unlike with strongyles, pasture management might not play a major role in controlling ascarids.

“While reports on roundworms have historically suggested that ascarid eggs are capable of surviving in the environment for years, we now know they really aren’t that hardy,” says Nielsen. “They do, however, survive well in cold weather and can make it through a winter, but not for years. In contrast, they don’t do well in hot weather. Six days at 104º F kills eggs, and temperatures easily can reach those temperatures in a manure pile or on the surface of a pasture on a sunny day.”

Take-Home Message

Many of the classic “clinical signs” we attribute to wormy foals have little to no data supporting them, says Nielsen. The only truly documented ascarid-related disease is small intestine impaction, which occurs rarely in well-managed animals. Routine deworming for ascarids twice early in a foal’s life should be sufficient to avoid impaction until the normal age-dependent development of immunity naturally eliminates infection.


Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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