THE MERCK VETERINARY MANUAL
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Small Strongyles in Horses

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More than 40 species of small strongyles in several genera have been found in the cecum and colon of domestic equids, each with its own site of preference. They have been referred to as trichonemes, cyathostomes, and currently cyathostomins. They belong to the subfamily Cyathostominae of the family Strongylidae, and ~10 species are particularly prevalent. Most are appreciably smaller than the “large strongyles,” but Triodontophorus spp (sometimes classified as nonmigratory large strongyles) are almost as long as Strongylus vulgaris.

Unlike the large strongyles, small strongyles do not migrate extraintestinally, because early development is confined to the wall of the intestine. Third-stage larvae may progress to the fourth stage without interruption, or they may undergo hypobiosis and resume development only after prolonged periods of dormancy or hypobiosis. When L4 emerge from the gut wall, they feed superficially on the mucosa and may rupture capillaries but are less pathogenic than the large strongyles. They molt to the adult stage. An exception is T tenuicollis, which is found in clusters and can produce severe ulcers in the wall of the colon. Generally, however, the resulting erosions of the mucosa are slight and hard to visualize. Consequently, it is common to recover thousands of adult worms from apparently healthy horses that have received limited anthelmintic treatment. In heavier infections, however, disruption may be extensive enough to disturb digestive and absorptive function, resulting in loss of condition and even a catarrhal enteritis of the large intestine.

An acute syndrome of sudden weight loss, often with severe diarrhea, is seen in temperate areas in late winter and spring, particularly in young ponies and horses (<5 yr old). This is associated with mass emergence of previously hypobiotic larvae from the intestinal wall as L4. Although of relatively low incidence, larval cyathostominosis is nevertheless of concern because response to treatment is variable, and prognosis must be guarded even with intensive therapy. It is reported more frequently in the UK and Europe than in the USA.

Horses with larval cyathostominosis generally have a neutrophilia and hypoalbuminemia. Hyperglobulinemia, particularly involving the β-globulin fraction described as characteristic in some reports, has been a less consistent finding. Eosinophilia is not a consistent finding. Often, strongyle eggs are not seen on fecal examination. However, gross observation of L4 or L5 larvae, which are often bright red, in the feces is helpful in making a diagnosis. Biopsy of large intestine via laparotomy also may assist in diagnosis; rectal biopsy is less reliable. Gross pathologic findings include typhlitis or colitis with mucosal hyperemia, hemorrhage, congestion, ulceration, or necrosis; in protracted cases, there may be only mucosal thickening. At necropsy, cyathostomin larvae can be seen as small, gray dots (1–2 mm) in the mucosa, giving it a gritty sensation on palpation. Transillumination of the mucosa from the serosal surface may help in visualizing the larvae.

Adult cyathostomins are easily removed from the gut lumen by a wide range of anthelmintics, provided that the worm population is susceptible to the chosen drug. Benzimidazole-resistant strains of small strongyles are common in some regions, and pyrantel resistance has been demonstrated in some locations. Resistance to the macrocyclic lactones has yet to be demonstrated, but concerns exist. Drug efficacy and the presence of anthelmintic resistance may be determined by comparing the worm egg count at the time of treatment and 10–14 days later. An effective drug should reduce the egg count to 0 or to very low levels. If resistance is present, a different anthelmintic class must be used, because side resistance occurs within chemical groups.

Small-strongyle larvae in the intestinal mucosa are much more difficult to effectively remove with anthelmintics. Ivermectin has been used with mixed results; lack of efficacy has been reported at and above label dosages. Treatment with large dosages of fenbendazole (10 mg/kg for 5 consecutive days) or with moxidectin has been reported as effective and can be used during the winter to reduce the risk of larval cyathostominosis. Horses already suffering from this disease may not respond to treatment if submucosal inflammation is too severe. Consequently, treatment must be augmented by corticosteroids and other appropriate supportive therapy.

Routine or interval treatments are traditional and are intended to minimize the level of pasture contamination, thereby reducing the risks associated with accumulation of mucosal larvae and adult worms. Alternatively, infection may be prevented by daily administration of pyrantel tartrate. The interval between routine treatments depends on the duration a particular drug keeps the feces free of eggs and varies from 4–13 wk. The frequency of treatment is also influenced by the value of the horses and the perceived level of risk, which varies with access to pasture, stocking density, and management practices. In most cases, these procedures are ineffective in the face of widespread drug resistance to most compounds except ivermectin and moxidectin. Control measures should be designed to minimize the risk of resistance developing in the worm population. This includes preserving the refugia population of the worms, ie, worms not exposed or affected by the anthelmintic, and thus reducing the drug selection pressure. These populations are the arrested L3 found in the mucosa and L3 larvae on the pasture. Fewer treatments may be effective if given strategically according to local epidemiologic and climatic considerations. Most adult horses >3–4 yr old have developed some immunity to reinfection; thus, only a small portion of the herd harbors the adult worm populations and is responsible for contamination of the pasture with eggs. Selective treatment of only these infected horses will also reduce the exposure of the worm population to anthelmintics and selection for resistance. Removal of feces from paddocks and pastures aids in control and may also reduce the number of anthelmintic treatments required.

Generally in parasite control programs, all horses on a farm should be treated at some time, and those commingled on the same pasture or paddock should be treated at the same time. Boarded horses or horses returning after having been off the premises for an extended time should be quarantined and dewormed before being admitted to the herd. In administering the anthelmintic, all horses should receive the proper dose, as determined by body weight or an accurate estimation. Rotating different classes of anthelmintics in a fast rotation scheme (eg, every few months) or a slow rotation scheme (annually) is widely practiced to prevent development of resistant parasite strains, but there is little evidence to support the utility of this procedure. Whatever program is used, fecal samples should be examined periodically to monitor the effectiveness of the program and the presence of drug resistance. Treatment can be restricted to those horses in a group that routinely have positive egg counts >100–200 eggs per gram of feces.

Last full review/revision October 2014 by Thomas R. Klei, PhD

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