Rounding up roundworms: Smart parasitology control for foals and young horses


Equine veterinarians can educate horse owners and farm managers about the risk of using a same-old, same-old approach to dewormingand offer a custom approach for each farm.

A foal is most susceptible to the dangers of roundworm infection before 18 months of age, which is when immunity generally develops. the first year of life, Parascaris equorum is one of the most pathogenic parasites a foal will encounter. This roundworm can cause respiratory disease, ill thrift, weight loss, small intestine impaction, peritonitis and death, says equine parasitology expert Wendy Vaala, VMD, DACVIM (equine).1

Immunity usually develops by the time a horse is 8 to 18 months of age, says Dr. Vaala, who is associate director of life cycle management with Merck Animal Health. But in the meantime P. equorum can wreak havoc. “When it comes to young horses, especially foals and weanlings, control of ascarids should be the primary focus due their pathogenicity,” Dr. Vaala says.

Complicating the problem is the increasing prevalence of drug-resistant ascarid populations due to overuse of deworming medications. “Signs that a parasite control program is failing on a breeding farm can include both overt respiratory illness as well as intestinal signs in young foals with uncontrolled P. equorum infection,” Dr. Vaala says. A strategic approach-deworming only those horses that need it and monitoring drug efficacy through fecal egg counts (FEC) and fecal egg count reduction testing (FECRT)-helps protect horses' health.

The other major parasite in horses is the small strongyle, Dr. Vaala says-thankfully, since they're not as deadly as roundworms, veterinarians don't often see much clinical disease associated with cyathostomin infection in well-cared-for horses on good nutrition plans.

The roundworm life cycle

Here's a summary of the P. equorum life cycle and its effect on young horses:

Foals ingest P. equorum larvated eggs from contaminated pasture, paddocks, dry lots or stalls.

After the eggs are swallowed, larvae emerge in the small intestinal lumen, penetrate the intestinal mucosa, enter the lymphatics and are transported to the liver.

After about a week, larvae molt in the liver and the L3 larval stage passes into the lungs through the posterior vena cava. Larvae then erupt from pulmonary capillaries to enter the alveoli.

After two to three weeks of pulmonary migration, larvae are coughed up, swallowed and pass back into the small intestinal lumen where they develop into mature, egg-laying adult ascarids over the next two to three months.

Eggs are passed back into the pasture or stall, where they become infective within two weeks under favorable environmental conditions. Due to their thick capsule, ascarid eggs are environmentally hardy, can withstand temperature extremes and can persist on pasture for five to 10 years.

The prepatent period in an infected horse is approximately 10 to 15 weeks. Clinical signs associated with pulmonary migration include purulent nasal discharge, cough and mild fever. Heavy burdens of late-stage larvae and mature worms may be accompanied by these same clinical signs. Large burdens of ascarid larvae and adults in the small intestine have been associated with poor growth or weight loss, unthrifty appearance, inappetance and diarrhea.

Dr. Vaala recommends the following parasitology control program in foals and young horses during their first 12 months of life:

Perform prepartum deworming of the mare or deworm 24 to 48 hours after foaling with a macrocylic lactone anthelmintic. Use an anthelmintic with demonstrated efficacy against P. equorum.  If the mare is dewormed pre-foaling, ensure that the effective egg reappearance period (ERP) for the drug used extends past the early postpartum period.

Do not treat the neonatal foal unless there are parasitic-related signs of disease such as diarrhea due to Strongyloides westeri (confirmed by a high FEC)-use ivermectin or oxibendazole-or respiratory disease suspected secondary to P. equorum larval migration-use ivermectin (only if proven effective on that farm by FECRT) or larvicidal fenbendazole. Note that young foals with prepatent ascarid larval infection will not have a positive FEC.

For a 2- to 3-month-old foal, deworm with a drug effective against P. equorum, such as fenbendazole, oxibendazole, pyrantel or ivermectin (if proven effective). Collect fecals 10 to 14 days after treatment from foals with patent ascarid infection and positive FEC and perform FECRT to test each drug class. 

For a 4- to 6-month-old foal (weaning age), perform a second treatment with a drug effective against P. equorum, such as benzimidazole or pyrantel. Perform FECRT for drugs not tested after the first treatment. Avoid drugs that kill via spastic or flaccid paralysis (such as pyrantel and ivermectin) in foals you suspect of having heavy ascarid burdens to reduce the risk of impaction colic. Consider a larvicidal dose of fenbendazole on farms with multidrug-resistant P. equorum.

For 7- to 9-month-old weanlings, deworm with a drug effective against small strongyles: ivermectin, pyrantel or benzimidazole. Perform FECRT in select weanling groups with patent strongyle infections to determine drug efficacy. Incorporate cestocide treatment if tapeworms are a concern: praziquantel or a double dose of pyrantel.

For 10- to 12-month-old weanlings and yearlings, treat with a drug effective against encysted small strongyles: moxidectin or a larvicidal five-day course of fenbendazole.

Conversation: Crucial for control

The potential lethality of a heavy P. equorum infection makes it a health threat that's impossible to ignore. And frequent use of dewormers unaccompanied by fecal monitoring is a recognized risk factor for development of drug-resistant ascarids and foals with heavy parasite burdens-which means conversations between farm owners and their veterinarians are essential.

AAEP on roundworms

According to the American Association of Equine Practitioners Parasite Control Guidelines:

P. equorum is the most important parasite infecting foals and weanlings.

Anthelmintic resistance is highly prevalent in P. equorum, which should be factored into treatment decisions.

Horses less than 3 years of age require special attention because they're more susceptible to parasite infection and at higher risk for developing disease.

The best method for performing FECs is the modified Wisconsin technique; it's more sensitive than the modified McMaster FEC procedure.

“Here's the message to farm managers: Work with your veterinarians; spend a little more money doing properly timed fecals to identify which dewormers are effective against which parasites and attempt to decrease the frequency of treatments rather than going out and buying the most inexpensive tube of dewormer (which is often an ivermectin product) without first evaluating efficacy,” Dr. Vaala says.  

She emphasizes that much of the pathology from Parascaris species can occur during the prepatent period, before any eggs are seen in the feces. “You have to get ahead of the curve and anticipate the potential problem rather than wait to see the aftermath,” she says. “Ascarid impactions are difficult to treat even with surgery. Postsurgical recovery can be complicated by peritonitis, ileus and systemic toxemia. Therefore, if the first indication of heavy intestinal ascarid burdens is a surgical roundworm impaction, we're late to the game. Other foals on the farm are likely harboring similar parasite burdens and the pastures are contaminated with long-lived, resistant ascarid eggs waiting to infect future foal crops.”

Ascarids in the small intestine of a horse. Photo courtesy of Dr. Wendy VaalaFarm owners may be reluctant to change their deworming strategies and begin using fecal monitoring for various reasons. Perhaps they haven't yet lost a foal due to ascarid impaction or a weanling with respiratory disease associated with prepatent ascarid larval infection, or maybe they're not willing to give up frequent treatment with inexpensive dewormers in exchange for paying for diagnostics such as FECs and FECRTs. Veterinarians can have a hard time battling long-held traditions on these farms.

One thing that can help veterinarians convince these reluctant owners is to keep diagnostics affordable and targeted-to determine which horses or groups of horses are candidates for fecals. Here are valid reasons to perform fecals:

To identify patent parasite infections

To evaluate drug efficacy

To screen new arrivals on the farm prior to turning them out on home pastures

To identify which mature horses (older than 3 years) are naturally low egg shedders and require less frequent drug treatments (usually more than 80% of the herd) and which horses are high strongyle egg shedders (often less than 10 to 20% of the herd) in need of more frequent deworming.


Smart monitoring, targeted control

When evaluating drug efficacy, Dr. Vaala collects fecals in horses that have not been dewormed recently-at least four to five weeks after administration of pyrantel or benzimidazole, eight to nine weeks after ivermectin, or 12 weeks after moxidectin. She administers a dewormer, then 14 days later collects fecals from individuals shedding significant numbers of eggs before treatment to see if the drug used was effective in reducing the egg count.

“By performing a FECRT on selected individuals I can determine what drugs work on each farm,” she says. “Never assume anything. Drug efficacy can vary between farms within the same state or region. A variety of factors contribute to development of drug-resistant parasites, including a farm's history of drug use, stocking density, whether it's a closed or open herd, and pasture management practices.”

Eighty percent or more of mature horses are genetically resistant to parasites and are classified as low egg shedders, Dr. Vaala says. So once a herd has been tested with a sensitive fecal assay, only the small percentage of high shedders (often just 10% to 20% of the herd) requires more frequent FECs.

“One fecal a year on every horse, whether they need it or not, does not make sense,” Dr. Vaala says. “Only the small subpopulation of high shedders needs to be retested on a regular basis to determine which drugs are effective.”

Young horses, due to their developing immune systems, are more susceptible to parasite infections and require a more regimented approach, with occasional FECs to confirm that the dewormers being used are still effective against ascarids and strongyles. “It's not unusual for more than one drug class to be needed to control both ascarids and strongyles,” Dr. Vaala says. “Using the same drug class repetitively on a breeding farm is not a sustainable or effective approach.”

An ascarid egg. Photo courtesy of Dr. Wendy Vaala.A common mistake among horse owners and farm managers is to blame respiratory signs in young horses on a bacterial or viral infection when in fact it's prepatent infection with P. equorum. “The veterinarian can explain the life cycle of P. equorum-the larvae are not mature ascarids shedding eggs-and inform the client that snotty-nosed, coughing foals with fevers may be harboring a parasite,” Dr. Vaala says. “Owners then understand that a negative FEC does not rule out disease due to prepatent infection, and if they wait until all their foals are shedding large numbers of ascarid eggs, they've waited too long and mature roundworms are shedding eggs and re-infecting pastures.”

Nonchemical strategies

In addition to using affordable and sensitive FECs in a strategic approach, a farm's parasite control program should include realistic nonchemical control measures. Veterinarians can start the conversation by asking farm managers what they're already doing in terms of pasture management. Important tactics include:

Removing manure twice weekly from mare-foal pastures. This is highly effective in reducing environmental exposure.

Implementing a composting system that attains high enough temperatures to render P. equorum eggs nonviable. “If composting is done properly, the internal temperature will get high enough to kill strongyle and even ascarid eggs,” Dr. Vaala says. “It's a matter of time, effort and some education. Farm owners can simply Google ‘horse manure composting' and find the details.”

Rotating other livestock, such as sheep and cattle, into horse fields to reduce pasture egg burdens. This might seem like old-school husbandry, but it works. “Sheep and cattle are great biological vacuums when it comes to equine parasite eggs,” Dr. Vaala says.  

Such a comprehensive approach may seem like a tough sell, but you might be surprised how receptive your clients are, Dr. Vaala says. “As practitioners, we often don't realize how much owners value veterinary advice tailored for their farm,” Dr. Vaala says. “If we let horse owners know that we're in the parasite control business for the long haul, and our goal is to design a comprehensive but practical program that includes FECs, anthelmintics and basic farm management practices, it might be an easier conversation than we think!

“I always tell horse owners, it's not sexy or glamorous, but if you go out and pick up manure twice a week where your mares and foals congregate in your paddocks or pastures, that will be one of the most effective (and least expensive) parasite control measures you can do-more effective than frequent purchases of dewormers that aren't working like they used to,” Dr. Vaala continues. “Nonchemical management practices are far better than giving poor-performing anthelmintic medications several times a year.”

Potential barriers to success

According to Dr. Vaala, the following risk factors may counter the success of a farm's parasite control program:

High-stocking pasture density that limits pasture management options available to control seasonal pasture contamination with parasite eggs and larvae.

An “open herd” with frequent introduction of new mares and foals harboring unknown prepatent parasite burdens with unknown drug sensitivity.

A long history of intensive deworming practices for foals and weanlings starting at an unusually early age and followed by administration of the same anthelmintic class at frequent intervals without any FEC monitoring to verify drug efficacy.

A reluctance by farm management to deviate from traditional deworming practices that have been followed for years if not decades.

Economic restraints that limit FEC-based surveillance in terms of labor and diagnostic expenses.

When these risk factors are present, it's even more important that veterinarians keep the program highly targeted and educate farm managers about the role of FECs and FECRTs in reducing resistance to anthelmintic medications.

Bucking tradition

It can be tough for equine practitioners to get through to farm managers regarding excessive use of anthelmintics, Dr. Vaala says. “It's hard to change the mindset of ‘that's the way I've always done it,'” she says. “The top goal for veterinarians working with these intense mare-foal operations is re-education to help them break from tradition and to demonstrate a measurable benefit.”

On the other hand, Dr. Vaala finds that smaller farms and owners just beginning to raise horses are more open to suggestions for proper parasite control. They also don't have huge numbers of foals, so they have room in their pastures to avoid overcrowding, which helps decrease worm transmission.

For larger commercial operations that have to pay attention to the bottom line, practitioners can help keep FECs affordable by training a technician to do the egg counts. A practice can refrigerate fecals and analyze them once a week with the same tech doing the test to ensure consistency. “Improved efficiency can help a practice can keep its overhead down, make a profit and offer an affordable diagnostic tool,” Dr. Vaala says.

Large farms also don't need to test every horse every time, Vaala continues. “Consider performing fecals on groups of foals prior to weaning to determine if the drugs used to control roundworms are working,” she says. “If you test your foals and the ascarid count is low or negligible, the next question is, can you get that same result with fewer treatments? If the FECs reveal high numbers of ascarid eggs, the question is, what drugs have you been using and which ones are no longer working?” Farm managers and veterinarians also need to monitor mares that come back pregnant from other farms-they're potentially returning with acquired parasites they'll pass on.

“Common sense, basic science, good animal husbandry and lots of education is going to be the best approach,'” Dr. Vaala says.

Unfortunately there's no magic bullet dewormer available today for equine parasites as in past decades, Dr. Vaala says. In fact, there's been no new deworming medication since the early 1990s. And deworming practices that make frequent use of the same drug class without fecal monitoring have contributed to the increasing prevalence of drug-resistant parasites. To help horse owners make better decisions, the veterinarian needs to re-enter the equation.

“We've got to do more with less until a new class of dewormer comes along,” Dr. Vaala says. “And in that effort, ascarids will be our litmus test.”


In summary, the true goal of equine parasite control is to limit infection so animals remain healthy and clinical illness does not develop.

“My message to fellow veterinarians is to value what we can offer a farm in terms of customized chemical and nonchemical parasite control,” Dr. Vaala says. “It's more than just selling anthelmintics or one fecal every year for every horse. Fecals should be used strategically-to calculate egg count reduction and determine drug efficacy, to screen newcomers on the farm for patent parasite infections, or to decide if the current deworming program is controlling ascarids and strongyles.”

Veterinarians can view this process as performing a “physical” on the farm itself: spending time walking around the farm and creating an all-around parasite control plan that includes sound husbandry practices, properly timed FECs on the horses that need them, and targeted use of drugs that still work. “It's a two-pronged attack-get clients to go to their veterinarian and get veterinarians to start offering practical, affordable services along with customized farm counseling,” Dr. Vaala says.


1. Vaala W. How to design a parasite control program for the first year of life: Focus on Parascaris equorum, in Proceedings. Am Assoc Equine Pract 2016;62:469.

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.

Related Videos
merck leptospirosis panel
merck leptospirosis panel
merck leptospirosis panel
dvm360 Live! with Dr. Adam Christman
Vet Perspective parasitology discussion
© 2024 MJH Life Sciences

All rights reserved.