Sunshine Equine

Performance Horses


D. Craig Barnett, DVM

Today, equine parasitologists consider cyathostomes (small strongyles) to be the #1 nematode problem in horses. In fact, small strongyles can account for up to 90% of the horse’s total worm burden. Unfortunately, most horse owners don’t know about encysted small strongyles and the problems they can cause.

Life cycle

The adult small strongyles are very small white worms that reside primarily in the large colon of the horse. Small strongyle populations may range from several thousand to more than 1 million per horse. Fortunately, the adult small strongyles do not cause the horse much problem. However, the larval stage of the parasite burrows into the lining of the horse’s colon and can cause significant inflammation of the colon and clinical problems such as weight loss, diarrhea, and even death in severe cases. I generally get at least one report per year of encysted small strongyles causing death in a horse. To better understand this problem it is helpful to know a little bit about the small strongyle life cycle. The adult small strongyles lay eggs that are passed in the feces. In the environment these eggs mature into first-stage larvae (L1), second-stage larvae (L2), and eventually into third-stage larvae (L3). Infective third-stage larvae (L3) are picked up by horses grazing contaminated pastures. The L3 migrate into the lining (mucosa or submucosa) of the horse’s colon. Here a cyst forms around the third-stage larvae (L3) in the intestinal wall (thus the term encysted small strongyles). The encysted third-stage larvae (L3) have been further divided into "early third-stage larvae" (EL3) and "late third-stage larvae" (LL3). Studies show that up to 75% of encysted small strongyles are early third-stage larvae (EL3) and that these larvae may remain in the lining of the intestinal tract for up to 3 years prior to resuming development to the late third-stage larvae (LL3) and fourth-stage larvae (L4). The fourth-stage larvae (L4) emerges from the cyst and enters the lumen of the large colon where it develops into the fifth-stage larvae (L5) and into adult small strongyles to start the cycle again. For the horse, the primary problem is with the encysted small strongyles (L3 and L4 stages) when they emerge from the lining of the colon.

Clinical Disease (Larval cyathostomosis)

Larval cyathostomosis is the name given to the disease caused by these small strongyles. Encysted small strongyles can cause severe clinical signs and even death in extreme cases. The synchronous emergence of the fourth-stage larvae (L4) from the intestinal wall creates the clinical signs associated with this disease. As might be expected, severe damage to the intestinal mucosa may result when potentially thousands to millions of larvae emerge from the mucosal cyst into the lumen of the intestine.

Typical acute clinical signs consist of diarrhea, weight loss, weakness, listlessness, anorexia, recurring colic, edema of the legs and ventral abdomen, and potentially death. Less severe subclinical signs such as decreased performance, poor feed utilization, dull hair coat, unthriftiness, or "ain’t doin’ right" may also be present. This clinical disease seems to be primarily reported in winter to early spring. It is worth noting that affected horses usually have been dewormed regularly.

Treatment & Control


If horses are dewormed regularly, why are they still getting encysted small strongyles? The reason is that there are only two anthelmintics (dewormers) that are effective against the encysted stage (L3 and L4) of this parasite. It is important to recognize that a given horse will probably have a mixed population of arrested EL3, developing L3 (LL3), and L4 in the intestinal wall as well as larval and adult small strongyles in the lumen of the intestine. Therefore, to prevent the onset of larval cyathostomosis (the clinical disease associated with the emergence of the encysted small strongyles) treatment must be effective against all stages that may be present in the horse. If a particular larval stage is not removed, those larvae will eventually continue development and may produce clinical disease. The two products that are effective against encysted small strongyles are Quest® and Panacur.

Unfortunately, Quest is not very effective against the encysted early third-stage larvae (EL3). Remember that the EL3 can account for up to 75% of the encysted small strongyles. This is why we have elected to use Panacur as it has a high degree of efficacy against all stages of small strongyles. However, it is only effective against encysted small stongyles if given at double the normal dose for five consecutive days.

Hopefully this will help explain why we feel it is important to implement an encysted small strongyle treatment into your deworming program.


Study: Small Strongyles Developing Resistance to Ivermectin