Small Strongyles (non-migrating) Cyathostomins

Small strongyles have several names. One is scientific Cyathostomins. Another is non-migrating due to the fact unlike migrating or large strongyles that the larvae migrate through several abdominal organs. Small strongyles only burrow into the mucosal lining of the large intestine and become encysted there normally for 2-3 weeks, but may remain encysted and viable for 2.5 years. They are now by far the most common parasite of horses. Dr Nielsen and Dr Reinemeyer state in their book Handbook of Equine Parasite Control there are over 50 species of small strongyles. They have similar life cycles and common modes of pathogenicity (ways of causing disease). The eggs of all strongyle species are very similar and cannot be used to identify which species are present. The main problem caused by small strongyles is the inflammation that the larvae cause in the large intestinal wall when they excyst. The adult small strongyles do not cause the horses much problem even when in high numbers. They do not attach to the lining and suck blood as large strongyles do. Sometimes large numbers of encysted larvae excyst at one time and rarely cause a syndrome termed Larval Cyathostominosis. Due to the substances released being somewhat toxic and the inflammation caused by the excystation of the large number of larvae the horse can develop this syndrome characterized by profuse diarrhea, weight loss, anemia, hypoproteinemia, ventral edema, and dehydration (Dr Nielsen and Dr Reinemeyer). This syndrome is difficult to diagnose. Larval Cyathostominosis most commonly occurs in young horses between 1-4 years of age. This syndrome may appear somewhat seasonal. With in the northern climates occurring in winter or early spring, in warm climates occurring in the summer. It may occasionally occur after deworming. What causes the larvae to remain in the cysts is unknown but due to when these large excystations occur it may have to due with the number of adults in the intestine. As the number of adults present in the intestine drop it seems a signal is sent to the larvae to leave the cysts. Just because this syndrome is discussed here is not intended to cause owners to worry. Horsemen’s Laboratory has examined over 70,000 samples since 1992. Between 1/1/14and 1/1/16 we examined 29,976 samples with 19,000 negative (63%) (No worm eggs found on counting chamber) and 10,976 (37%) samples were positive. 1563 (14%) of the positive samples were over 1000-strongyle eggs/g of stool. We never have had a report of a horse developing Larval Cyathostominosis. This may be because most our patients are mostly older horses. What I am trying to illustrate is that Larval Cyathostominosis is not a common problem just because horses have a fairly high number of small strongyles.