Internal Parasites of the Horse

Internal Parasites of the Horse

By Randall Holman

Internal parasites are a threat to the health of horses and may cause irreparable damage. Parasites are small organisms living a portion of their life cycle within the internal organs, body cavities and tissues inside the horse. There are more than 150 internal parasites; however, only a few commonly cause significant health problems.

The common class of internal parasites causing health problems for horses is nematodes such as large and small strongyles, ascarids, and tapeworms. Other less harmful parasites like pinworms and botfly larvae are also typically considered when applying parasite control.

Antiparasitic compounds have reduced the popularity of large strongyles causing the most damage to horses which leaves the the small strongyle as the most common parasite within horses. Infected horses may exhibit signs such as diarrhea and colic. Small strongyles have been known to cause stunted growth, anemia and weight loss.

Adult strongyles, large or small, live in the large intestine and produce ova that are passed out into the feces. The eggs then develop into larvae that may be consumed by the horse eating contaminated grass or drinking water, which in turn, infects the horse. The larvae can survive freezing weather; however, a hot and dry environment will kill them. Strongyle larvae may survive up to 31 weeks at winter temperatures, compared to up to seven weeks at summer temperatures.

Parasite control and prevention can be divided into management and chemical treatment. Management programs interrupting the life cycle of the parasite before infestation occurs is key to successful control. This can be accomplished by keeping stalls cleaned and managing manure appropriately – placed in compost pile or spread on cropland or pastures not being grazed by horses. Heat buildup in composted manure will destroy larvae. Manure that is spread thin on cropland will dry and heat from the sun causing larvae to be destroyed.

There are also chemicals known as antiparasitics or anthelmintics used to eliminate parasites by killing them or stopping the life cycle. These deworming chemicals work by paralyzing the parasite, preventing nutrient consumption by the parasite, or limit the parasite reproductive ability. Anitparasitics or dewormers are available in different forms such as paste, feed additives, and gel, and all are effective when the proper dose is administered.

A rotational treatment program of alternating between classes and brands of dewormers is commonly used to avoid resistance to an anitparasitic class. Common strategies include:

60-day rotation of two or more dewormers.
Annual rotation – deworming the same number of times and rotations per year, but concentrating treatments during infective periods.
Daily treatment via feed additives.
Targeted treatment, and strategic treatments.

It is best to consult a veterinarian to establish an effective parasite control program that will be effective for your horse. Some antiparasitics are toxic to young foals and the labels and package inserts should be read carefully. In most cases, though, a horse will need to be dewormed several times a year.

Randall Holman, site owner of Front Range Frenzy and horse enthusiast, is the author of the above article. You will find other easy and practical basic horse care information on his website:

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