SMALL STRONGYLES
#1 Parasite Problem in Horses
by D. Craig Barnett, DVM
As common intestinal parasites, such as Strongylus vulgaris (large strongyles), are being effectively controlled by the routine sue of anthelmintics, a new problem is emerging. Today, parasitologists consider cyathostomes (small strongyles) to be the #1 nematlde problem in horses. In fact, small strongyles can account for up to 90% of the horse's total worm burden.
Life Cycle
The adult small strongyles reside primarily in the large colon of the horse. Small strongyle populations may range from several thousand to more than 1 million per horse. The adult small strongyles lay eggs, which are passed in the feces. In the environment these eggs mature into first-state larvae (L1), second-stage larvae (L2), and eventually into third-stage larvae (L3). Infective 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 L3 larvae in the intestinal wall thus the term encysted small strongyles. At this point, the larvae may enter a state of arrested development during the early L3 stage (EL3). 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 L3 stage (LL3) and fourth-stage larvae (L4). The 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.
Clinical Features
Larvae cyathostomosis, the disease caused by these small strongyles, can cause severe clinical signs and even death in extreme cases. The synchronous emergence of the 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, and recurring colic, edema of the legs and ventral abdomen, and potentially death. Less severe sub clinical signs such as decreased performance, poor feed utilization, dull hair coat, unthriftiness, or "ain't doin' right" may also be present. It is worth noting that effected horses usually have been de-wormed regularly, and that the disease seems to be primarily reported in winter to early spring.
Treatment & Control
It is important to recognize that a given horse will probably have a missed 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 cyathostomoses ( 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.
