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.