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What is Internal Parasites ?

Most internal parasites are not life-threatening, but any of them can be debilitating without treatment. The most dangerous is the small strongyle because it can become resistant and do major damage to the intestines which may be fatal. Large Strongyles are also important because they can block the arteries and damage the liver or other organs. This can be fatal in some cases as well, but they are not resistant like small Strongyles can be. Lungworms are also dangerous because of their ability to cause respiratory damage and distress.

Internal parasites come in many different types, but they usually stick to the digestive tract, respiratory system, and sometimes the muscles. Although every horse has a small amount of parasites living in their body naturally, sometimes one of them can get out of control and overwhelm their system. The most common parasites include stomach bots (Gasterophilus spp.), roundworms (Ascarids-Parascaris Equorum), threadworms (Strongyloides Westeri), neck threadworm (Onchocerca spp.), bloodworms (Large and Small Strongyle), tapeworms (Anoplocephala perfoliata), pinworms (Oxyuris equi), and lungworms Dictyocaulus arnfeldi).

Symptoms of Internal Parasites in Horses

The symptoms of internal parasites depend on the type of parasite such as:

Stomach bots (Gasterophilus spp.)

  • Loss of weight
  • Abdominal pain
  • Poor hair coat
  • Lethargy
  • Weight loss
  • Slow healing wounds
  • Breathing trouble

Roundworms (Ascarids-Parascaris Equorum)

  • Dermatitis
  • Excessive tail rubbing
  • Colic
  • Pot belly
  • Poor wound healing
  • Respiratory Issues
  • Weight loss

Threadworms (Strongyloides Westeri)

  • High body temperature
  • Diarrhea
  • Abdominal pain
  • Anemia (weakness, listlessness)
  • Decreased weight
  • Lack of appetite

Bloodworms (Large and Small Strongyle)

  • Slow growth
  • Weight loss
  • Poor coat condition
  • Lethargy
  • Abdominal pain

Tapeworms (Anoplocephala perfoliata)

  • Severe spasmodic colic
  • Diarrhea
  • Loss of weight
  • Increased appetite
  • Unhealthy body condition

Pinworms (Oxyuris equi)

  • Severe itching of the anus
  • Biting on tail and rear end
  • Visible pinworm eggs around anus
  • Aggravation

Neck Threadworm (Onchocerca spp.)

  • Itching in the neck area
  • Hair loss
  • Inflammation
  • Crusty lesions
  • Self-trauma

Lungworm (Dictyocaulus arnfeldi)

  • Trouble breathing
  • Wheezing
  • Coughing
  • Appetite loss
  • Weight loss
  • Respiratory distress


  • Stomach bots (Gasterophilus spp.)
  • Roundworms (Ascarids-Parascaris Equorum)
  • Threadworms (Strongyloides Westeri)
  • Neck Threadworm (Onchocerca spp.)
  • Bloodworms (Large and Small Strongyle)
  • Tapeworms (Anoplocephala perfoliata)
  • Pinworms (Oxyuris equi)
  • Lungworms Dictyocaulus arnfeldi)


Causes of Internal Parasites in Horses

  • Stomach bots (Gasterophilus spp.) lay their eggs on your horse’s legs and are ingested so they can attach themselves to the intestinal walls; once they mature, they are excreted in the manure where they will hatch into adult flies to start the next cycle
  • Roundworms (Ascarids-Parascaris Equorum) are the largest and are most common in foals under 18 months old
  • Threadworms (Strongyloides Westeri) are parasites that affect foals up to six months old and cause damage to the intestinal lining
  • Neck threadworms (Onchocerca spp.) are transmitted to horses by gnats where they live in the ligaments and tendons and cause itching and severe dermatitis
  • Bloodworms (Large and Small Strongyle) are the most common worms in horses over two years old; they attach to the lining of the digestive system and they can stay there for several years before laying their eggs (they are the hardest to get rid of because of their long hibernation inside the lining of the intestines)
  • Tapeworms (Anoplocephala perfoliata) are serious parasites that can grow up to four feet long and cause severe colic
  • Pinworms (Oxyuris equi) are not common but can be found in some horses where they cause extreme itching around the anus
  • Lungworms Dictyocaulus arnfeldi) cause a major deterioration in your horse’s health by getting into the airway through the bloodstream


Diagnosis of Internal Parasites in Horses

If you believe your horse may have internal parasites, you need to consult an equine veterinary professional as soon as possible. The longer you wait, the more damage they can do and the harder they will be to treat. Bring a stool sample for testing so the veterinarian can use it to diagnose your horse. There should be eggs, live worms, or dead worms in the manure that can give a definitive diagnosis. Blood tests such as a CBC and chemical analysis are also used in some cases. In addition, the veterinarian may want to get some x-rays or ultrasound to look for obstructions or other abnormalities.



Treatment of Internal Parasites in Horses

Roundworms (Ascarids-Parascaris Equorum)

These worms have become resistant to many dewormers on the market so veterinarians may have to switch drugs several times to get rid of them.

Stomach bots (Gasterophilus spp.)

Bots can be treated with a broad-spectrum dewormer such as ivermectin.

Bloodworms (Large and Small Strongyle)

Large strongyles can be treated with macrocyclic lactones and dewormers. Moxidectin and fenbendazole are used to treat small strongyles.

Tapeworms (Anoplocephala perfoliata)

Pyrantel and Praziquantel can help rid your horse’s body of tapeworms.

Threadworms (Strongyloides Westeri), Neck Threadworms (Onchocerca spp.)

Broad-spectrum dewormers such as pyrantel or ivermectin are good choices to treat threadworms.

Lungworms (Dictyocaulus arnfeldi)

Ivermectin is the drug of choice for treating lungworms.

Pinworms (Oxyuris equi)

These can be treated with ivermectin, pyrantel, fenbendazole, or moxidectin.



Recovery of Internal Parasites in Horses

In almost all cases, your horse’s prognosis is excellent with treatment. To prevent another occurrence or a reinfection you have to be sure to thoroughly clean and disinfect your horse’s living area and it is also a good idea to treat any other horses that has been in contact with your horse.



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Internal Parasites Questions and Advice from Veterinary Professionals

Need pet health advice? Ask a vet


Ask a Vet



Tennessee Walker


12 yo


Unknown severity


0 found helpful


Unknown severity

Has Symptoms

Thickness Below The Mane

The people I bought her from insist she is foundering. With the exception of being overweight she only has one symptom that is not described on this website or any other for that matter. I really don't think she has founder. Can you please tell me something about this thickness below her mane?? To me it looks like a muscle but I'm no veterinarian.

July 23, 2020



Dr. Michele K. DVM


0 Recommendations

Thank you for your question. Unfortunately, without being able to see your horse, there is no way that I can comment on the thickness below her Mane and whether that may be normal or not. It would be best to have her seen by your veterinarian, as they can give her a full examination, see what might be going on, and let you know if she is having problems or not. I hope that all goes well for her.

July 23, 2020

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