Small Strongyles

The term 'small strongyles' refers to approximately 40 species of parasitic worms (belonging to the subfamily Cyathostominae of the family Strongylidae) found in horses, of which about 10 different species are commonly found. Although there are a number of different species, they are similar in terms of their life cycle, symptoms, treatment and prevention (see below). 

One also hears of 'large strongyles', which have a different effect on horses. Most of the species of 'small strongyles' are physically smaller than the large strongyles, but not all, so the difference between the two groups is primarily in terms of the effect they have on the horse rather than just physical size. See large strongyles for further details.

Life Cycle

The adult form lives in the large intestine of horses, where it feeds and lays eggs. The eggs are passed out with the manure, after which they hatch into larvae, which crawl into the grass. Then the horse eats the grass, they ingest the larvae. The ingested larvae burrow into the intestinal wall, where they mature. The matured worms then emerge from the intestinal wall, to enter the large intestine, where they feed and lay eggs, completing the life cycle.


The main problem with this parasite is damage to the intestinal wall. This damage occurs at two times in the life cycle:

  • When the matured worms emerge from the intestinal wall, their emergence damages the wall of the intestine. In temperate climates, this occurs in late winter and early spring. If the horse is heavily infected, the emergence of large numbers of parasites over a short period of time can do substantial damage to the intestine.
  • As the mature parasites feed on the mucosa on the intestinal wall, they can also damage the intestine, although such damage is normally minor unless the worms are present in large numbers.

If the damage to the intestine is minor, there is very little in the way of symptoms, except for the presence of eggs in the manure. More extensive damage can lead to weight loss and diarrhea, with an increased risk of colic. 


The adult worm is relatively easily treated with most most types of wormers. However, this leaves the larvae stage untreated, which can damage the intestinal wall when they emerge. If it is suspected that the horse may be heavily infected with larvae, one should consult with a veterinarian about treating the larvae, to prevent this damage. Since most types of worming medication are ineffective against the larval stage, one will need to use a type which is. For details on the various types of wormers and which ones are effective, see worming medications.


In addition to regular deworming with an appropriate wormer, proper herd and field maintenance will help control the parasite population and reduce the frequency with which treatment is required.