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Small strongyles



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Cyathostomum (syn. Trichonema), Cylicocydus, Cylicodontophorus, Cylicostephanus, Triodontophorus spp.

An extremely prevalent diverse group of over 40 species in eight genera, that are denoted arbitrarily for practical reasons as cyathostomes (Cyathostominae); more prevalent are those belonging to gene ra Cyathostomum (syn. Trichonema), Cylicocydus, Cylicodontophorus, Cylicostephanus, etc. (white to dark red worms, 1-2cm in length) and the non-migratory large strongyles Triodontophorus spp. (reddish, up to 2.5 cm in length).

Life cycle:

„Small strongyles“ are carried out through a histotropic phase confined to the mucosa or submucosa of the caecum and colon, where small, blackish-dark-red nodules are formed around the developing larvae (3rd moult); adult stage is reached by about 1.5-3 months; due to arrested larval development, hypobiotic, encysted larvae occur frequently in the intestinal mucosa, especially in foals and yearlings in temperate climates; in tropical regions, cyathostomes demonstrate no tendency for arrestment.


They are visible on inspection of the large intestinal mucosa or contents; buccal capsule exists but less developed (ringshaped), and mature small strongyles are usually not attached to the mucosa of the large intestine and caecum, and do not suck blood.

Small Strongyles (there are many species but most common are the

Cyanthosomes) are non-migrating parasites that live in the large intestine. The immature larval stages of this parasite develop in the

walls of the intestine where they are

protected until they emerge as early adult

parasites. While they are in the walls of the intestine there are several stages of

development when the larvae can encyst and undergo a period of inhibition or arrested

development that is called hypobiosis; first, as newly arrived third staged larvae and

second, as a late third staged larvae or early fourth staged larvae. When these larvae are

encysted, they go through a period of arrested development or hypobiosis that can last

from several months to as long as three years.

The physiological mechanism that causes hypobiosis to occur is unknown but hypobiosis

is seasonal and occurs only when parasite populations or parasite density in the gastrointestinal

tract is the greatest. It’s has become obvious, that the greater the parasite

population, the more changes that occur in the physiology of the gastro-intestinal tract

and the higher the percentage of incoming larvae that become encysted and undergo

hypobiosis. The small strongyles as a group are the primary parasites of horses that are

considered to the greatest “drug resistance” problems.

The problems for the horse come when these encysted cyanthostome larvae begin

development again and emerge, rupturing through the intestinal wall. Usually this is a

dynamic process with older worms dying off and new larvae replacing them. However,

due a parasitic purge by a dewormer where a large number of worms are removed at the

same time, mass emergence of encysted larvae often occurs causing severe

gastrointestinal problems called cyathostomiasis. Clinical cyathostomiasis is often

associated with the mass emergence of inhibited larvae from the mucosa of the cecum

and colon which is a prominent cause of diarrhea in horses. Clinically, it is characterized

by sudden onset of diarrhea, weight loss, subcutaneous edema and death. It is reported to

be seasonal in occurrence and yo


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