Strongyles (Redworms) in Horses: Causes, Symptoms and Treatment

Strongyles, commonly known as redworms, are among the most prevalent internal parasites in horses, infecting 80 to 99% of equids worldwide. Strongyles are also among the most damaging parasites to horses. While horses can carry small amounts without any cause for concern, heavy infestations can lead to outcomes ranging in severity, from weight loss and poor performance to life-threatening colic and intestinal damage. Because strongyles are widespread in pastures and barns, owners need to understand how these parasites spread, the warning signs and why veterinarian-guided deworming is the most effective prevention.

Key Takeaways

• Strongyles are one of the most common internal parasites in horses and can cause digestive issues, weight loss, colic and life-threatening complications if left unmanaged.

• Horses become infected by grazing on contaminated pasture, making proper management a key component of prevention.

• Fecal egg counts and fecal egg count reduction tests help veterinarians diagnose infections and identify possible dewormer resistance.

• Modern deworming programs focus on test-based treatment plans rather than outdated fixed schedules, making it imperative to rely on your veterinarian’s assistance instead of administering treatment on your own. 

• Small strongyles can remain encysted in the intestinal lining for months or years, then emerge in large numbers and cause severe inflammation, while large strongyles migrate into blood vessels and can trigger colic, anemia and intestinal damage.

• Pasture management, including manure removal, rotational grazing and quarantining new horses, works alongside deworming to reduce reinfection and slow the development of dewormer resistance.

What Are Strongyles in Horses?

Strongyles, or redworms, are parasitic nematodes (roundworms that derive nutrients at the host's expense), and they live in a horse’s digestive tract. They are a diverse group of roundworms in the Nematoda phylum and among the most important equine parasites to be aware of. This is due to their widespread nature, environmental resilience and potential to cause both subtle and severe health issues over time. 

While horses at any life stage can become infected, certain groups are more vulnerable to strongyle burdens. Young horses (under 3 years old) are at higher risk because their immune systems are still developing and have not yet built resistance. Senior horses with underlying health issues face similar vulnerability. Horses kept in boarding facilities, training barns or communal pastures are also more likely to encounter infective larvae due to increased manure contamination. Horses with inconsistent or poorly managed deworming programs face a significantly higher risk, demonstrating the need for thorough preventive practices under a veterinarian's guidance.

Life Cycle of Strongyles

Adult strongyle worms live in the horse’s large intestine, where they produce eggs that are passed out in the manure. The eggs hatch into first-stage larvae (L1), which feed on organic matter in the manure. They then molt into second-stage larvae (L2), which also feed and grow before molting again. From there, the larvae develop into third-stage larvae (L3), also known as the infective stage (the specific phase in a parasite's life cycle at which it is fully developed and capable of invading a host). They migrate from the manure onto blades of grass or vegetation, where they wait to be ingested, infecting horses. 

Once inside the digestive tract, the larvae penetrate the intestinal wall and develop into fourth-stage larvae (L4). The larvae later mature into fifth-stage larvae (L5), or young adults, and return to the large intestine where they develop into fully mature adult worms, restarting the cycle. 

Types of Strongyles

Once ingested, L3 larvae either remain in the digestive tract or migrate through tissues and blood vessels. The severity of the infection and the specific symptoms it causes depend on the type of strongyles, which are generally divided into two major categories: small strongyles and large strongyles. While they share a similar route of transmission, their behaviors within the horse can be quite different.

Small Strongyles (Cyathostomin spp.)

Small strongyles, also called cyathostomins, are the most common type found in horses, with most adult populations carrying at least some level of infection without showing signs. More than 50 Cyathostomin species (spp.) exist, with around 40 capable of infecting horses, and a single horse may harbor several species simultaneously.

After being ingested, larvae of small strongyles embed themselves in the lining of the large intestine, where they can remain encysted (dormant) for extended periods, even months to years. When they emerge in large numbers, they can create significant inflammation and damage to the intestinal lining, causing larval cyathostominosis, which is a severe and potentially life-threatening condition. Because cyathostomins are so widespread and increasingly resistant to some dewormers, they are a major focus of modern parasite control programs.

Large Strongyles (Strongylus spp.)

Large strongyles are considered one of the most pathogenic parasites that can affect horses, but they are less common due to improved deworming practices. However, they are much more dangerous than small strongyles. Their larvae migrate out of the intestines and into the main blood vessels (arteries) that supply the gut, where they reside in the horse’s cecum (a large, comma-shaped muscular sac located in the right side of the abdomen that acts as a fermentation vat to digest fiber) and colon and feed on blood and tissues. This can lead to blood vessel damage, clotting and reduced blood flow to the gut, which can cause life-threatening aneurysms, permanent organ damage, anemia, intestinal infarction (tissue death) in the intestines and severe colic (abdominal pain).

Three primary species of large strongyles affect horses: Strongylus edentatus (toothless strongyle), Strongylus vulgaris (double tooth strongyle) and Strongylus equinus (triple tooth strongyle). Strongylus vulgaris, also called the bloodworm, is the most significant and detrimental species. 

Expert tip: Pasture management is just as important as deworming for preventing parasitic infections. Regular manure removal, avoiding overstocking pastures and rotating grazing areas can dramatically reduce exposure to infective larvae. Consult your veterinarian about the most effective prevention strategies to pair with deworming.

What Are the Signs of Strongyles in Horses?

The clinical signs of strongyles are heavily influenced by the type causing infection. Small and large strongyles each affect a horse’s body in different ways, such as:

Signs of Small Strongyles

Because small strongyles live in and damage the lining of the large intestine, symptoms are often related to poor nutrient absorption and inflammation. Common signs include:

• Gradual weight loss or difficulty maintaining body condition 

• Dull, rough or unhealthy-looking coat 

• Reduced energy or overall performance

• Intermittent or chronic diarrhea 

• Mild, recurring colic or other signs of digestive discomfort  

In severe cases, large numbers of encysted larvae can emerge all at once, causing larval cyathostominosis. This can lead to sudden, severe diarrhea, rapid weight loss, fever and significant weakness, requiring urgent veterinary attention.

Signs of Large Strongyles

The signs of large strongyles tend to be much more concerning than those of small strongyles. This is because their larvae can penetrate the intestinal wall and invade major blood vessels and organs. Common signs include:

• Recurrent or unexplained colic episodes

• Anemia (pale gums or inner eyelids)

• Lethargy or reduced stamina

• Weight loss despite adequate nutrition

• Swelling or signs of internal inflammation

• Intestinal damage (extreme cases), which can become life-threatening 

Expert tip: Do not assume a horse is clear of parasitic infection solely based on their outwardly healthy appearance. Strongyle infections, especially small strongyles, can progress quietly for months. Routine monitoring and guidance from your veterinarian are far more reliable than waiting for visible symptoms to appear.

How Do Veterinarians Diagnose Strongyles?

Veterinarians diagnose strongyles infection through a combination of physical evaluation, medical history and laboratory testing. Veterinarians will assess a range of factors, including:

• Body condition and changes in weight 

• Coat quality 

• History of colic

• Deworming schedule and pasture management practices 

• Exposure risk 

Because horses with parasitic infections may not show obvious symptoms early on, diagnostic testing is a crucial component for assessing the severity of the infestation and determining the most effective treatment and prevention plan. 

Fecal Egg Count (FEC)

Fecal egg count (FEC) is the most common diagnostic method. This quantitative test, which is recommended 1 to 2 times per year for adult horses, measures the number of parasite eggs present in feces when examined under a microscope, helping vets estimate the horse’s parasite burden and identify whether strongyles are likely contributing to health problems, such as weight loss, diarrhea or poor performance. 

Based on the results of the FEC, horses are classified as: 

• Low shedder (less than 200 eggs per gram of feces)

• Moderate shedder (200 to 500 eggs per gram of feces)

• High shedder (greater than 500 eggs per gram of feces)

Horses with a high fecal egg count are considered significant contributors, creating high infection pressure for themselves and other horses grazing the same field. High shedders typically require 4 to 6 deworming treatments per year (every 8 to 12 weeks), whereas low shedders require only 1 or 2 treatments per year, according to the American Association of Equine Practitioners (AAEP) guidelines.

Fecal Egg Count Reduction Testing (FECRT)

In some cases, veterinarians may also perform a fecal egg count reduction test (FECRT). This test compares fecal egg counts before and after deworming to determine whether the dewormer effectively reduces the parasite burden. This is crucial for monitoring and managing anthelmintic (antiparasitic drug) resistance in strongyles.

Fecal samples are collected before deworming and 10 to 14 days after. The target reduction thresholds vary by drug class per 2024 AAEP guidelines: 

• Macrocyclic Lactones (Ivermectin and Moxidectin) should achieve 95% or greater reduction.

• Benzimidazoles (Fenbendazole and Oxibendazole) should achieve 90% or greater reduction.

• Pyrimidines (Pyrantel Pamoate) should achieve 85% or greater reduction.

Results below these thresholds suggest resistance within the parasite population and require a change in deworming strategy. FECRTs are generally recommended annually to assess dewormer efficacy on your farm. 

Additional Testing

For suspected severe strongyle cases, additional diagnostic procedures may be performed. Your vet may order bloodwork, such as an enzyme-linked immunosorbent assay (ELISA) or abdominal imaging (ultrasound), to evaluate inflammation, anemia or intestinal damage caused by migrating larvae. While neither of these tests are the primary tools for a routine strongyle diagnosis, they are useful for investigating complications.

Expert tip: Horses sharing a pasture are usually exposed to the same parasite population, and if a dewormer proves ineffective for one horse following a FECRT, it is likely to be ineffective for the entire herd. Speak to your veterinarian about adjusting your deworming strategy and selecting more effective parasite control options.

How Do Veterinarians Treat Strongyles?

Treating and preventing strongyles entails using targeted deworming medications combined with long-term parasite management strategies. Your vet will help determine the exact treatment and prevention plan, which will depend on the type of strongyles involved, the severity of the infestation, the horse’s age and health status and whether resistance is suspected.

Strongyles Treatment

Treatment often starts with an appropriate anthelmintic (deworming medication). Commonly used dewormers for strongyles may include Moxidectin (the treatment of choice), Ivermectin, Fenbendazole or Pyrantel Pamoate. The most common dewormers for strongyles include: 

• Quest® Plus Gel (moxidectin 2%/Praziquantel 12.5%): As confirmed by the 2024 AAEP Internal Parasite Control Guidelines, this is the AAEP-recommended treatment of choice for both adult and encysted small stronglyes (cyathostomins), as well as tapeworms. A single dose suppresses strongyle eggs for 84 days, and it is particularly recommended for late grazing season use.

• Quest® Gel (moxidectin 2%): This is recommended for early grazing season use and targets small strongyles and large strongyles without tapeworm coverage. 

• Zimecterin® Gold (ivermectin 1.55%/praziquantel 7.75%): This offers broad-spectrum coverage including tapeworms, and it is 99% effective against natural tapeworm infections. 

• Zimecterin® (ivermectin 1.87%): This offers broad-spectrum strongyle coverage without the tapeworm component of Zimecterin® Gold. 

• Safe-Guard® (fenbendazole): This product falls within the Benzimidazole class, but its efficacy against strongyles must be confirmed by FECRT on your farm due to widespread resistance. 

• Pyrantel Pamoate paste: This has shown widespread small strongyle resistance documented per 2024 AAEP guidelines, so efficacy must be confirmed by FECRT before use.

Your veterinarian will help you determine which medication and which form, including medicated feeds, oral pastes and gels, are most effective based on fecal testing results and the risks most prevalent in your area.

For horses with heavy parasite burdens, treatment may need to be carefully managed. Rapid die-off of large numbers of parasites can increase the risk of inflammation or colic. This is mostly seen in horses with larval cyathostominosis, as deworming can sometimes trigger a lethal inflammatory reaction. In these cases, your veterinarian may recommend supportive care, including intravenous (IV) fluids, anti-inflammatory medications or dietary adjustments. 

Expert tip: Per the 2024 AAEP Internal Parasite Control Guidelines, there is widespread resistance to Pyrimidines (including Pyrantel Pamoate) and Benzimidazoles (including Fenbendazole) in small strongyle populations. These drug classes may still be effective on some farms, but their efficacy must be confirmed by fecal egg count reduction testing (FECRT) before relying on them. Macrocyclic Lactones (Ivermectin and Moxidectin) currently remain the most reliably effective options for strongyle control, with Moxidectin being the preferred choice for encysted small strongyles.

Strongyles Prevention: What Can Be Done?

The AAEP recommends a targeted, two-tiered parasite control program based on fecal egg counts, helping reduce pasture contamination and slow the development of dewormer resistance. Most adult horses benefit from a baseline deworming program once or twice a year (typically in spring and fall) to target strongyles and other parasites. An equine weight tape is an excellent tool for determining your horse's weight, which is essential for calculating the correct dewormer dose.

While low shedders usually only require 1 to 2 baseline treatments, moderate and high shedders need additional protection. This can include 1 to 2 targeted treatments in addition to the baseline treatments, especially during peak parasite transmission seasons.

Because young horses have egg counts that are often higher and more variable than those of adults, they should not be managed the same way. They benefit from a standardized treatment schedule rather than relying on FEC testing. The first deworming treatment in foals is recommended at 2 to 3 months of age and should be a Benzimidazole (specifically Fenbendazole) to target ascarids (roundworms), which are the primary concern in young foals. Macrocyclic Lactones, such as Ivermectin and Moxidectin, have widespread ascarid resistance and should not be used as the first-choice treatment in foals at this age. Subsequent treatments should continue at approximately 3-month intervals, transitioning focus toward strongyles as the foal matures. 

Successful prevention involves more than simply administering a dewormer – it also requires ongoing management to help reduce the likelihood of reinfection. In addition to deworming schedules, veterinarians recommend preventing exposure and improving overall parasite control through the following:

• Regular manure removal from paddocks and pastures, using appropriate equipment, such as wheelbarrows and manure forks

• Avoiding overgrazed or overcrowded fields

• Rotating grazing areas when possible 

• Performing routine fecal egg counts

• Quarantining and testing new horses before introducing them to the herd

Expert tip: Deworming horses on the same rigid schedule is no longer considered best practice, according to the AAEP. Work with your vet to create a targeted deworming program based on fecal egg counts, pasture conditions and each horse’s shedding level to help preserve dewormer effectiveness over time.

Frequently Asked Questions About Strongyles in Horses

Can strongyles in horses be deadly?

Yes, strongyle infestations can be fatal if left untreated, particularly those caused by large strongyles. Migrating redworms may damage blood vessels that supply the intestines, potentially leading to severe colic, intestinal injury, anemia or other serious complications. 

How do horses get strongyles? 

Horses become infected by grazing on pasture contaminated with parasite eggs passed in manure. Once ingested, the larvae develop inside the horse and continue the parasite life cycle. 

How often should horses be tested for strongyles?  

Horses should receive FEC testing at least once or twice yearly. Some horses may benefit from more frequent monitoring based on their environment and shedding level. Consult your vet to learn the most effective testing frequency for your horse.

Support Strongyles Treatment and Prevention with Tractor Supply’s Horse Pharmacy

Strongyles are common in horses, which is why proper testing, strategic deworming and good pasture management are essential for equine health and wellness. Staying proactive is a crucial component of Life Out Here with horses, and demands an effective parasite prevention plan, including deworming, coordinated by your veterinarian. Whether you manage a large herd or a single horse, Tractor Supply’s horse pharmacy carries trusted dewormers, prescriptions and other wellness products for comfort and performance throughout every season.