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Worms in Horses 101: Pictures + Explanations

  • Annual 4X Horse Dewormer Pack

    Annual 4X Horse Dewormer Pack

    Valley Vet Supply

  • Annual Foal Dewormer Pack

    Annual Foal Dewormer Pack

    Valley Vet Supply


Did you know? Some 90% of worms in horses come from the environment they are exposed to every day. This includes small strongyles, tapeworms, pinworms, roundworms, bloodworms and threadworms. Equine parasites can present a number of health risks to horses, including:

  • Poor nutrient absorption
  • Increased risk of colic and pneumonia
  • Lung damage
  • Decreased performance
  • Stunted growth
  • Weight loss
  • Poor coat condition

Help control your horse's risk for parasites; ensure they are on an effective equine deworming program. For young horses and horses deemed high parasite shedders, consider this annual deworming pack, available only at veterinarian-founded Valley Vet Supply. If you are deworming foals, look to this specific pack. Both annual deworming packs were developed by the Valley Vet Supply Technical Service Veterinarian team, with guidance from the AAEP Parasite Control Guidelines.

For a better understanding of worms in horses, review the descriptions and pictures below.

Horse Worm Facts

  • Pinworms


    Pinworm eggs are picked up by horses from contaminated feed, water, bedding, and may also be present on tail wraps, grooming materials, and even fence posts and stalls. The female pinworm deposits eggs around the anus, secreting a substance which can cause intense itching. This may lead to tail rubbing and even injury to the tail and rump. Washing the perianal region may help relieve the itching, but all materials used should be discarded or washed in hot water with soap. Pinworms may be difficult to diagnose in manure samples, but the eggs can usually be picked up on scotch tape pressed to the skin near the anus.

  • Ascarids (large Roundworms)

    Ascarids (large Roundworms)

    The adult stages of the large roundworm is found in the small intestine, where the female passes large numbers of eggs into the manure. In about two weeks, these eggs become infective and the horse picks them up while grazing. The larvae migrate into the blood vessels and are carried to the liver and lungs. The immature worms are coughed up and swallowed, maturing in the small intestine to complete the life cycle.

  • Bots


    Bots are the immature maggot stages in the life cycle of the bot fly, the adult of which resembles the honeybee in general appearance. The females lay their eggs by attaching them to the hairs of the front legs, throat, and under line. As the horse licks itself, the larvae attach themselves to the lips and tongue and burrow into these tissues. After about three weeks they attach themselves to the lining of the stomach, where they may remain for several months, causing additional damage.

  • Large Strongyles (Blood Worms)

    Large Strongyles (Blood Worms)

    Adult strongyles are found firmly attached to the walls of the large intestine, where the females pass large numbers of eggs into the manure. These eggs hatch and the larvae climb blades of grass and are swallowed. The larvae then migrate to the large arteries which supply the intestines. As the artery walls are damaged, blood clots form and break away, causing colic.

    Small Strongyles (Cyathostomins): Ubiquitous parasites, infecting nearly all grazing horses. Relatively mild pathogen, except when present in large numbers.

  • Tapeworms


    Difficult to diagnose, it is estimated that 40% of the horses in the U.S. are affected. Tapeworms are contracted during grazing, when the horse ingests the intermediate host, a mite found on plants. Once infected, tapeworms contribute to digestive problems, colic and malnourishment. Tapeworms are often undetected using normal fecal flotation methods.

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