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Also Known As

Worms, Flies, Gnats, Lice, Mites, Mosquitos, Ticks


Parasites pose a major threat to the health and comfort of most horses both internally and externally. Any veterinarian will tell you that parasitism is one of the most costly and damaging conditions that affect the lives of horses and their owners.

Because all parasites feed on the horse, they contribute to many health issues by causing internal damage, robbing the horse of needed nutrients, causing debilitating irritation, lowering performance, as well as serving as carriers of disease. If not kept in check, infestations of parasites can lead to disability and the death of the horse.

Being omnipresent, internal parasites are especially damaging to young horses or those with compromised health from other diseases and conditions. The major internal horse parasites can be divided into four types: large and small strongyles, ascarids, pinworms, and bots.

External parasites include flies, mites, ticks, lice, gnats, and mosquitoes, all of which can carry diseases, as well as making the horse miserable and requiring extra time, effort and money on the part of the horse owner.


  • Loss of condition
  • Anemia
  • Dry, patchy coat
  • Diarrhea
  • Itching
  • Increased temperature and heart rate
  • Colic
  • Intermittent constipation or diarrhea
  • General debility
  • Loss of exercise capability


Because of the ubiquity of parasites and the ease of transmission from horse to horse or from environment to horse, most horses will be afflicted with parasites from the time they are foals until after their deaths.

To date, no program developed through science or any other method has come close to diminishing the number and/or kinds of parasites that plague horses. In fact, they are as much a part of the horses' world as the air they breath.


Given the conditions of the equine world, eradication of parasite infestations is currently impossible. However, much can be done to minimize both the population of parasites and the effects they have. The American Association of Equine Practitioners has developed a comprehensive list of suggestions to get rid of parasites before they attack the horse:

  1. Pick up and dispose of manure droppings at least twice weekly.
  2. Mow and harrow pastures regularly to break up manure piles and expose parasite eggs and larvae to the elements.
  3. Rotate pastures by allowing other livestock such as sheep or cattle to graze them, thereby disrupting the life cycles of parasites.
  4. Group horses by age to reduce exposure to certain parasites and maximize the deworming program geared to that group.
  5. Keep the number of horses per acre to a minimum to prevent overgrazing and reduce fecal contamination.
  6. Use a feeder for hay and grain rather than feeding on the ground.
  7. Remove bot eggs quickly and regularly from the horse's coat to prevent ingestion.
  8. Rotate worming agents, not just brand names, to prevent chemical resistance.
  9. Consult a veterinarian to set up an effective and regular deworming schedule.

To rid stalls, barns, pastures, and fields of external insect parasites, such as flies, mites, ticks, lice, gnats, and mosquitoes, attack them on the premises by applying or administering appropriate repellents and insecticides. Pyrethrins, organophosphates, insect growth regulators, synergists, and repellents offer protection from flies and mosquitoes.

Light coverings for horses can protect the body from flies and other biting insects. Stabling horses before sunset and eliminating all water sources that might be used by mosquitoes as breeding grounds will cut down on exposure to mosquitoes.

Topical insecticides applied to the horse will kill lice, chiggers, and ticks. Examining the horse for ticks after trail rides or being out in brushy areas will prevent infections and diseases from tick bites. Keeping tall grass, weeds, and brush cut back will help control the presence of biting insects.


Treatment for internal parasites should be prescribed by a veterinarian who can establish a routine, effective deworming program. All horses on the premises should be dewormed at the same time and at regular intervals.

Dewormers should be highly effective for the species in question and used in the correct dosage. Expectant mares should be treated to prevent foals from becoming burdened with parasites shortly after birth.

All labels and directions should be read and thoroughly understood before use. Extra precautions should be taken to make sure horses are not overdosed because of toxicity of the drugs.

Treatment for external parasites will vary. Bites that cause swelling or itching can be treated with topical ointments. If infestation is severe, a veterinarian can determine if the horse is suffering from anemia as a result. Infection and diseases transmitted through bites will require treatment related to the specific condition.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.