Strongyles are considered the most significant internal parasite that affects horses. It is estimated that these parasites affect between 80 – 99% of equids worldwide. [1][2][3]

There are two main types of strongyles: large strongyles and small strongyles. Both types can cause significant damage to the horse’s digestive tract, resulting in various clinical signs ranging from mild to severe. [4]

Proper parasite control measures, regular fecal egg count (FEC) monitoring, and good pasture management are essential to minimize strongyle infection.

Anthelmintic agents (dewormers) are an important part of parasite control and treatment. However, anthelmintic resistance is a growing concern due to overreliance on deworming medications.

If you suspect your horse may have a strongyle infection, consult with your veterinarian to receive a diagnosis and develop a treatment plan. Early detection and timely treatment are essential for managing strongyle infections in horses.

Strongyle Infestation in Horses

Strongyles, also known as strongylids or redworms, are parasitic nematodes that commonly infect horses around the world. They are a significant concern in equine health, potentially causing serious complications.

Parasitic nematodes, often referred to simply as “worms,” are a diverse group of roundworms in the Nematoda phylum. These internal parasites infect various hosts and can infest different regions within the host’s body.

Strongyles are intestinal nematodes, meaning these worms primarily inhabit the digestive tract of the host. They can cause gastrointestinal issues, such as diarrhea, weight loss, and colic, but also impact other bodily systems.

Strongyles encompass two main types: large strongyles (Strongylus spp.) and small strongyles (Cyathostomin spp.). Infections with either type of strongyle are collectively referred to as “strongylosis”.

Strongyle infections occur when horses ingest larvae while grazing on pastures contaminated with these parasites. The larvae then develop into adult worms within the horse’s gastrointestinal tract, primarily affecting the cecum and colon.

Large strongyles (Strongylus vulgaris, Strongylus edentatus, and Strongylus equinus) can cause damage to the blood vessels that supply the gastrointestinal system, potentially leading to life-threatening complications.

Small strongyles (Cyathostomins spp.) are the most prevalent internal parasites found in horses. They remain encysted in the intestinal wall as larvae, and their emergence can lead to inflammation, ulceration, and protein loss, causing a condition known as “larval cyathostominosis”.

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Risk Factors for Strongylosis

Risk factors for strongyle infection include factors that increase exposure to these parasites as well as impaired immune function.

  • Young Horses: Foals and yearlings exhibit the highest susceptibility to strongyle infections due to their still-developing immune systems. In contrast, horses over five years old tend to have minimal to moderate worm burdens. [3]
  • Contaminated pasture: Horses that graze on pastures contaminated with infective strongyle larvae are at an increased risk of infection. [5]
  • Overstocking: Overcrowding pastures with horses in a confined area can lead to rapid pasture degradation. Overstocking also increases the concentration of parasite eggs and larvae in the environment, heightening the risk of exposure and infection. Poor pasture management, including inadequate manure removal, can also contribute to higher parasite burdens. [6]
  • Lack of pasture rotation: Without proper pasture rotation strategies, strongyle larvae and other parasites can accumulate in the pasture. Horses repeatedly grazing on the same field increase their exposure to infective larvae, resulting in greater parasite burdens and an elevated risk of infections. [6]
  • Inadequate deworming protocols: Horses that are not placed on regular and appropriate deworming schedules are more susceptible to strongyle infestations. Failure to use effective anthelmintic drugs or improper deworming practices can lead to incomplete parasite elimination and the development of anthelmintic resistance.
  • Climate and season: Weather conditions can influence the survival and development of strongyle larvae in the environment. Warm and moist climates are favorable for larval development, increasing the risk of exposure during certain seasons.
  • Immune status: Horses with compromised immune systems or underlying health conditions may be more susceptible to severe strongyle infestations.

Small Strongyles (Cyathostomin spp.)

Cyathostomins, commonly known as small strongyles, have a global presence and are widespread in equine populations regardless of climate or management practices. It is estimated that small strongyles can be found in 70 – 100% of horses from all continents. [5]

These small worms are short in length and can appear red or white. [5] Cyathostomin parasites are incredibly resilient and can survive inside the horse’s body or on pasture for long periods of time.

There are over fifty species of Cyathostomin parasites, forty of which can infest horses. Multiple species can infest a single horse at the same time, and a healthy horse may contain millions of worms without showing any symptoms. [7]

Clinical Signs of Small Strongyles

Horses affected by small strongyles may either show no apparent clinical signs or exhibit symptoms indicative of a parasitic infestation. In horses with significant burdens of adult Cyathostomins, the following symptoms may be observed: [5][8]

In horses with small strongyle infection, there may also be evidence of neutrophilia (an elevated white blood cell count), hypoalbuminemia (reduced albumin levels), and hyperglobulinemia (an excess of globulins in the blood).

Affected horses may also experience anemia due to the blood and nutrient loss caused by these parasites while feeding. [5]

Life Cycle of Small Strongyles

Adult female Cyathostomins residing in the horse’s large intestine lay eggs that are eliminated from the horse’s body through the manure. These eggs are microscopic and not visible to the naked eye. [1]

These eggs hatch into first stage larvae (L1) in the manure. After emerging, the larvae grow and molt into second stage larvae (L2), eventually reaching third stage larvae (L3) where they develop a protective membrane around their body. [1][5]

The L3 stage, commonly known as the infective stage, is when the larvae transition from the manure and migrate onto the ground or pasture. [1][5] When a horse grazes on this pasture, L3 larvae are ingested and passed to the small intestine.

After shedding their protective sheath, the larvae burrow into the cecal and colonic endothelium (or lining) and become encysted. In this state, encysted small strongyles can remain dormant and shielded from the horse’s immune system and some deworming treatments for extended periods, lasting from a few months to up to two years. [5]

While encysted, these larvae can cause severe damage to the mucosal lining of the intestines. They undergo a simultaneous emergence from the lining as fourth stage larvae (L4), a process commonly referred to as “mass emergence” or “cyathostominosis”.

Larval Cyathostominosis

Larval cyathostominosis, is a severe and potentially life-threatening condition caused by the invasion or emergence of encysted small strongyle larvae from the intestinal lining. [8]

Type I cyathostominosis typically occurs in the late fall or early winter and is triggered by the invasion of L3 cyathostomin larvae into the intestinal wall. This larval invasion can lead to weight loss, soft feces, and colic. [8]

Type II cyathostominosis takes place in the late winter or early spring when L3 larvae emerge simultaneously from the intestinal wall as fourth stage larvae (L4) and enter the gut lumen.

This emergence can lead to substantial damage to the intestinal wall, resulting in severe inflammation, digestive upset, diarrhea, colic, and even death. [8]

This phenomenon of mass emergence is relatively uncommon and usually observed in young horses carrying a high burden of small strongyles. Nevertheless, any horse can be affected regardless of the season. The mortality rate for this condition can reach up to 50%. [5]

Large Strongyles (Strongylus spp.)

Large strongyles are considered one of the most pathogenic parasites that can affect equines. Fortunately, large strongyle infestations are relatively rare in domesticated horses today.

Horses can be affected by three primary species of large strongyles, namely: [3][9][10]

  • Strongylus edentatus (toothless strongyle)
  • Strongylus vulgaris (double tooth strongyle)
  • Strongylus equinus (triple tooth strongyle)

S. vulgaris, also known as the bloodworm, is the most significant and detrimental large strongyle. However, this species is not as prevalent as it used to be due to the widespread use of anti-parasitic medication. [4]

Large strongyles reside in the horse’s cecum and colon where they feed on blood and tissues. Without intervention, they can inflict significant organ damage. [2]

Clinical Signs of Large Strongyles

The clinical signs of large strongyle infestation in horses can vary depending on the severity of the infection and the specific species involved. The following signs may indicate a large strongyle infestation: [4][10]

  • Poor body condition
  • Weight loss
  • Distended abdomen (pot-belly)
  • Poor appetite
  • Poor performance
  • Delayed shedding of coat
  • Diarrhea
  • Colic

Large strongyle infection can present significant challenges, as it can result in persistent gastrointestinal symptoms that hinder digestive function. This impairment in nutrient absorption can lead to weight loss and reduced performance in affected horses. [1]

Adult large strongyles travel to blood vessels and burrow through the walls of arteries, which can cause inflammation and impair blood flow to the intestines. In situations where there is substantial damage, horses may develop secondary anemia or die from complications of severe colic. [3]

Life Cycle of Large Strongyles

The life cycle of the large strongyle commences with adult females in the gastrointestinal tract producing eggs. These eggs are expelled from the horse through the manure, and the larvae develop in the manure, maturing into third stage larvae (L3) after approximately two weeks. [3][4]

Once the larvae reach the infective L3 stage, horses ingest them while grazing on contaminated feed or pasture. After ingestion, the larval development process starts to differ based on the species of large strongyle.

The life cycles of the three main species of large strongyle are described below:

Strongylus vulgaris (double tooth strongyle)

Once ingested, S. vulgaris larvae penetrate the mucosal lining of the horse’s gastrointestinal tract, typically in the cecum or colon. They molt into fourth stage larvae (L4) in the submucosa, migrating into the small arteries. [3][4]

Subsequently, these larvae migrate into the cranial mesenteric artery and its branches, which are responsible for delivering oxygenated blood and nutrients to the intestines. In rare cases, the larvae can enter other vessels and spread throughout the horse’s body. [11]

Several months later, these larvae molt into fifth stage larvae (L5) and return to the gut wall where nodules form around the larvae. These nodules rupture releasing young adult parasites into the intestinal lumen.

Strongylus edantatus (toothless strongyle)

Upon ingestion, the larvae of S. edantatus penetrate the intestinal mucosa and travel through the portal vein, taking several days to reach the liver. After approximately two weeks, they undergo molting and transition into L4 larvae, continuing their migration through the liver, which can lead to inflammation. [3][10]

Around six to eight weeks following the initial infestation, the L4 larvae enter the lining of the abdominal cavity (peritoneum). They continue their migration, usually travelling to the horse’s flanks and liver ligaments.

After four months, the larvae enter their fifth life stage (L5) and migrate through the lining of the abdominal cavity to the large intestine wall, where they become encysted. These L5 larvae develop into adults while encysted before being released into the lumen.

S. edentatus infestation puts horses at risk of developing hepatitis (inflammation of the liver) and peritonitis (inflammation of the peritoneum). [10]

Strongylus equinus (triple tooth strongyle)

After ingestion, the L3 larvae of S. equinus penetrate the wall of the cecum and colon and become encysted within the abdominal cavity within a week. Unlike other types of large strongyles, this species does not migrate as extensively. [3]

Within the cysts, the L3 larvae molt into L4 larvae before traversing through the lining of the abdominal cavity to reach the liver. They reside in the liver for at least six weeks before making their way back to the large intestine. [3]


Your veterinarian will diagnose strongyle infection in your horse by observing their general appearance and demeanor, as well as conducting diagnostic tests. Horses that harbor a significant number of strongyles may exhibit lethargy, weight loss, and poor coat condition.

Conducting a thorough health history on your horse is an important part of the diagnostic process and helps your veterinarian understand your horse’s risk of parasite exposure. Providing accurate information also enables your veterinarian to formulate an effective treatment plan and recommend management practices to mitigate the future risk of parasite infestations.

Fecal Egg Count (FEC)

A fecal egg count (FEC) is a quantitative diagnostic test that measures the number of parasite eggs present in a horse’s feces. This test is a critical tool for assessing the horse’s parasite burden, especially concerning internal parasites such as strongyles. [2][12]

FEC results can be classified into the following three categories:

  • High shedder: FEC greater than 500 eggs per gram (EPG) of feces
  • Moderate shedder: FEC between 200-500 eggs per gram (EPG) of feces
  • Low shedder: FEC less than 200 eggs per gram (EPG) of feces

Horses are categorized high, moderate, or low shedders (contaminators) based on the number of eggs they excrete in their manure. Horses with a high fecal egg count (FEC) are considered significant contributors to pasture contamination, posing a risk to other horses in the population.

Generally, horses identified as low, moderate, or high shedders tend to remain stable within their respective category. For instance, a healthy horse classified as a low shedder and kept under proper management practices will likely exhibit a consistently low fecal egg count (FEC).

Fecal Egg Count Reduction Testing (FECRT)

The fecal egg count reduction test (FECRT) is a diagnostic procedure used to evaluate the effectiveness of a dewormer in reducing the parasite burden in horses. This test is crucial for monitoring and managing anthelmintic resistance in strongyles. [2]

Fecal samples are collected prior to deworming and 10 – 14 days after deworming. After deworming, the FEC test should indicate a substantial reduction in fecal egg count, typically around 90 – 95%, if the treatment was effective.

However, if there is only a minimal decrease in the egg count, it suggests resistance within the parasite population, and an alternative treatment approach may be necessary. [13]

Keep in mind that all horses sharing an area or pasture will usually be impacted by the same population of parasites. Therefore, if a particular dewormer proves ineffective for one horse, it is likely to be ineffective for all of them. FECRT is usually employed as a herd test, rather than an individual test. [2]

While FECRT is valuable for assessing the efficacy of dewormers, many factors can affect the results, including individual variation and sample size.

It’s important to note that FEC and FECRT do not provide information about the total number of adult strongyles or encysted larvae present in the horse. These tests focus on assessing the reduction in fecal egg counts after deworming and are useful for monitoring the response to treatment, but they do not give a complete picture of all stages of the parasite’s lifecycle within the horse’s body.


The treatment of strongyle infestation in horses involves a comprehensive approach to effectively manage and control these parasites, while addressing secondary complications that may arise due to significant blood loss or severe damage to the intestinal lining.

It is crucial to consider both the immediate elimination of adult worms and the management of encysted larvae, as well as the implementation of preventive measures to reduce the risk of reinfestation and anthelmintic resistance.

Working closely with your veterinarian is important to develop a tailored treatment plan that incorporates strategic deworming, pasture management, and proper hygiene practices, ensuring the best possible outcomes for the horse’s health and long-term well-being.

Deworming Medications

Anthelmintics or dewormers are medications that kill parasitic worms in the horse. Drugs such as ivermectin, fenbendazole and pyrantel pamoate, and oxibendazole are used to control small and large strongyle infestations.

In the past, horses were routinely dewormed at regular intervals. This method of internal parasite treatment was considered convenient and believed to offer broad protection against various parasites.

The approach proved to be highly successful in controlling S. vulgaris infections, as the regular treatment prevented the worms from maturing and reestablishing their population. [2] Consequently, large strongyle infestation in horse populations has become relatively rare.

However, small strongyles have now emerged as the primary parasite, and their distinct life cycle means that regular deworming is less effective as a management strategy. Due to frequent overuse, certain types of dewormers are now subject to anthelmintic resistance from species of small strongyles. [3]

To address anthelmintic resistance, the equine industry has shifted towards a strategic deworming approach. [3] This approach involves targeted use of deworming medications under the supervision of a veterinarian.

Administering the dewormer at the recommended dose based on the horse’s weight is also imperative. Underdosing can contribute to the development of anthelmintic resistance since parasite populations exposed to sublethal doses have a higher likelihood of survival. [2]

Should all Horses with Strongyles be Dewormed?

Anthelmintics should not be administered to horses with low numbers of strongyles (under 200 eggs per gram), unless otherwise recommended by a veterinarian. Unnecessarily deworming horses with low fecal egg counts (FECs) can contribute to anthelmintic resistance. [8]

Although horses with low fecal egg counts may not need immediate deworming, regular FEC monitoring is essential to ensure that the horse’s parasite burden stays manageable.

Deworming for Small Strongyles

Small strongyles are the most common internal parasites in horses and can cause serious health issues. Anthelmintic resistance is now highly prevalent in small strongyles, so deworming treatments must be effective in killing resistant parasite populations. [2]

While encysted, small strongyles are only susceptible to a limited number of dewormers, making them particularly challenging to treat.

Macrocyclic lactones, such as ivermectin and moxidectin, are safe at low doses and possess a broad spectrum of activity. Although they can effectively eliminate immature adult strongyles, their effectiveness against encysted larvae may vary. [8][12]

Moxidectin is described as one of the last effective anthelmintics against small strongyles, but studies have observed resistance in certain horse populations. Horses can also develop resistance to combination treatments using multiple medications. [14]

Benzimidazoles and pyrantel can be used, but their effectiveness is limited to adult parasites, and widespread resistance has been observed. Fenbendazole, on the other hand, can be up to 95% effective against strongyle larvae and adult parasites. However, resistance to fenbendazole is becoming increasingly evident. [2][8]

It is typically recommended to rotate anthelmintics annually or more frequently. Horses may be treated with benzimidazoles one year, pyrantel the next year, and an avermectin (ivermectin or moxidectin) the following year. [8] This strategy minimizes resistance risks and maintains effectiveness in managing parasites.

Deworming for Large Strongyles

Large strongyles are generally susceptible to all dewormers. The standard treatment for horses with a significant large strongyle infestation involves deworming with ivermectin, pyrantel pamoate, or moxidectin. [2][10]


Effective pasture management plays a crucial role in preventing strongyle infestations in horses. By employing proper management practices, horse owners can reduce pasture contamination and limit the transmission of infective strongyle larvae.

Remove or regularly spread piles of manure and droppings throughout the pasture. This will helps promote uniform grazing across the area while also minimizing parasite populations. [6]

Strongyle larvae prefer dark, warm, and moist environments, often found in manure piles or taller grass. Dispersing manure piles by harrowing or raking the pasture on hot and dry days exposes the larvae to sunlight, diminishing their viability. [8][12]

To disrupt the strongyle life cycle and reduce the presence of larvae, pastures should be mowed regularly and kept at a shorter height (between 3-8 inches). Implementing a rotational grazing system can provide pasture resting periods and further aid in controlling strongyle infestations.

A rotational grazing system may be implemented to allow for pasture rest periods and to limit the accumulation of strongyle larvae.


The prognosis for strongylosis in horses is influenced by several factors, such as the severity of the infection, the horse’s overall health, the effectiveness of treatment, and the implementation of parasite control measures.

With prompt intervention, most mild to moderate small strongyle infestations are successfully resolved using appropriate deworming protocols and good pasture management practices.

Large strongyles infestations are more concerning because these parasites can cause significant tissue damage during their migratory phase. If left untreated, this can result in severe colic or other serious health issues.

In general, the prognosis for strongyle infestations in horses is favorable with effective management. Working with your veterinarian and adhering to a comprehensive parasite control program will help preserve the health and well-being of your horse while minimizing the impact of strongyle infestations.

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  5. Corning, S. Equine cyathostomins: a review of biology, clinical significance and therapy. Parasites & Vectors.2009. View Summary
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