- The major intestinal parasites in adult horses are small strongyles (or cyathostomes), tapeworms and large strongyles.
Small strongyles account for 80% of the total worm burden and are capable of encysting (or encasing) in the intestinal wall. They are capable of causing a clinical disease called larval cyathostomosis.
Tapeworms tend to congregate around the ileocecal valve in the intestine and are thought to cause certain types of colic. To help eliminate tapeworms, horses should be treated with either praziquantel or a double dose of pyrantel pamoate.
Although large strongyles are capable of causing severe damage to the vascular system and other organs through their migration, this parasite has declined dramatically in the last 20 years due to the highly effective dewormer ivermectin.
Stomach bots (Gastrophilus) are parasites that attach to the stomach and small intestine. The adult parasite is a large fly resembling a honeybee, which deposits the parasite eggs on the forelegs, lips and shoulders. Stomach bots are usually harmless unless the worm burden is exceedingly heavy. This parasite can be treated by removing the small yellow eggs from the hair coat and by administering either ivermectin or moxidectin after fly activity stops (winter) and again after peak egg laying (late spring).
Pinworms are a common parasite that is not associated with major disease. This parasite does not migrate through tissues and therefore has a very efficient life cycle. While pinworms do not damage to the intestines of the horse, their eggs create irritation around the rectum of the horse resulting in tailhead scratching. Pinworms are effectively treated by most anthelmentics (dewormers).
The intestinal parasites of major concern in weanling foals are roundworms (also known as ascarids). Roundworms are capable of causing an intestinal impaction as well as pneumonia. Prevention of roundworm infection is difficult because the eggs are very durable and can live in the soil for up to 10 years. To reduce the infection, foals should be dewormed at 6 weeks of age and then every 6-8 weeks until they are 6 months old. Dewormers that cause rapid killing of roundworms, such as piperzine or organophosphates, should be avoided since they can lead to impaction in heavily parasitized foals. Dewormers with slower kill, such as fenbendazole, oxibendazole, pyrantel and ivermectin, are safer options.
A fecal flotation provides valuable information on the parasite burden of both individual horses and the herd. This test may be used examine the effectiveness of an athelmentic program by determining the fecal egg count prior to and after the administration of a dewormer. In targeted treatment programs, fecal egg counts can be used to determine which animals harbor the highest parasite burden. One exception is the equine tapeworm, which can be missed on a routine fecal flotation.
Parasite resistance is a continual concern and may have arisen with the inappropriate or excessive use of anthelmentics. To reduce the emergence of resistance, dewormers must be used appropriately and combined with strategies to reduce the parasite contamination in the environment.
Some pasture management tools that may help reduce parasites include:
Reduce the stocking density to one horse per two acres of pasture.
Cross-fence and then rotate the horses biweekly between pastures.
Pick up the manure piles from the pasture twice weekly.
Separate the horses based on age groups (mare/foal, weanlings, yearlings, adults).
In the northern United States, horses become infected with intestinal parasites during spring through summer. In the southeastern United States, the situation is reversed, and horses become parasitized during the cooler season (fall through late spring). Therefore, deworming programs in the South target parasites from November to late June.
Underdosing a dewormer may increase the risk of the development of parasite resistance. Overdosing can also be harmful to the horse. For more accurate dosing, use a girth tape measure to estimate the body weight of horse prior to administering a dewormer.
Parasite control programs should be tailored to individual herds. A veterinarian can help decide the best deworming schedule based on the stocking density, age of the horse and use. A cookbook approach may result in either ineffective parasite control or anthelmintic overuse.