Strongyles in horses

Last Updated on March 18, 2022 by Allison Price

Internal parasites Small organisms that spend a part of their lives in host animals. They live in the body’s tissues, organs and cavities. They also feed on the host animal for their nutrition. Many different parasites can affect horses. Parasites can cause varying degrees of damage.

Parasite infestation can cause loss of nutrients and blood from the host. This can lead to serious medical problems. Horses with parasites are likely to have poor health. They will experience decreased growth and performance as well as reduced fertility and reproductive capabilities.

Although there are many internal parasites that can infect horses, only a handful of them cause serious health problems. It is essential to understand the life cycle of parasites before you can establish a parasite control program. Effective prevention and control programs work because they disrupt the parasite’s life cycle. The primary parasites that cause horse health problems in some parts of the country are nematodes. These include largesmall strongyles and ascarids. When designing a parasite control program, other parasites may be less important, such as pinworms or botfly larvae.


The two main types of strongyles that infect horses are large and small. Three primary types of large strongyles infecting horses are strongylus vulgaris and strongylus endentatus.

All strongyles, large and small, are adults. They live in the large intestinal tract. The feces of adult strongyles contain eggs which are then released into the horse’s environment. These eggs become infective larvae and can be found in pasture vegetation or in horse stalls. Horses that ingest infected larvae are infected if they eat grass, feed, and water contaminated by them. Because of the protective sheath, these larvae are extremely resistant to extreme environmental conditions. Strongyles can survive in freezing conditions, but they will die in hot and dry environments. Infective larvae can survive for up to 31 weeks in winter temperatures. This is compared to just seven weeks in summer.

Large strongyle larvae can migrate to different parts of the body. The bloodworm Strongylus vulgaris will burrow into the walls of the arteries which supply blood to the small and large intestinal intestines. Blood clots can form from this migration, which can cause disruption to blood flow and scar tissue formation in the affected arteries. The larvae reach the lumen of their large intestine after 120 days. This is where they mature. These parasites can lay thousands of eggs per day as adults. The entire life cycle can take six to seven months.

Strongyles in horses

The life cycle of the Strongylus Endentatus as well as Strongylus Equinus are similar, however their larval migration occurs primarily through liver. The migration causes damage to the organ, but not as much as the S. vulgaris migration through intestinal blood supply. S. endentatus larvae and S. equinus larvae return to the large intestinales where they become adults. They live for approximately 8 to 11 months.

Effective antiparasitic (anthelmintic) compounds have reduced the number of large strongyles that caused the most harm to horses in the past. Horses’ most common internal parasite is the small strongyle. Small strongyles can infect horses and cause no symptoms. Horses with severe infections may show clinical signs such as colic and sudden onsets of diarrhea. In order to decrease feed efficiency, gain rate, and performance, small strongyles have been linked.

Cyathostomes, a small strongyle, has a life cycle that is similar to the large one. However, the larvae don’t migrate beyond their intestines. The larvae can burrow into or encyst within the wall of the large colon. Horses suffering from heavy infestations may experience diarrhea or a decrease in growth performance. These signs are usually caused by large numbers of encysted cystomes emerging from the gut wall. This causes inflammation. The severity of clinical symptoms is dependent on the extent of damage to the intestinal walls, which can vary depending on the severity of the infection. It is not known why the cyathostomes appear at once. The possibility that larval cyathostomosis is caused by:

  1. Seasonal factors (winter/spring northern areas, spring/summer southern areas)
  2. Antiparasitic Treatment in Two Weeks
  3. Young horses (less that six years old).
Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!