Capillaria in Dogs and Cats

Capillaria can affect a pet’s bladder and cause troubling discomfort.

Infection with the bladder form of capillaria is usually mild. By: odonata98
Infection with the bladder form of capillaria is usually mild. By: odonata98

Capillaria belong to a group of nematode worms known as threadworms. Within the threadworms are many different species that infect some animals, but, curiously, not others.

The name “threadworm” may seem familiar because humans can harbor threadworms, or pinworms, but this is a human-only parasite and is not caught from your dog or cat (and indeed, pets can’t catch it from people!).

The 2 species most likely to infect cats and dogs are Capillaria plica and Capillaria aerophilia, which live in the bladder and lungs, respectively.

For the trivia lovers out there, capillaria are the second most common parasite of the blackbird. It also infects wild animals such as mink, brown bears, raccoons and skunks.


Infection with the bladder form of capillaria is usually mild and doesn’t cause problems for dogs. Often the parasite is picked up as an incidental finding during routine urine analysis.

If the worm burden is heavy, the dog may show signs such as:

  • Discomfort when urinating
  • An urgency to empty his bladder
  • Blood in the urine

Although the lung form of capillaria is called “lungworm,” it is not the same parasite that is also known as lungworm, angiostrongylus. Whereas capillaria causes only mild, flu-like symptoms, such as coughing, sneezing or a runny nose, angiostrongylus is more serious and can cause death.

Capillaria infection is less common in cats than dogs, but the symptoms are largely the same.


Dogs are infected by eating earthworms (the bladder form) or wild animal feces (the respiratory form). The eggs are passed from urine, embryonate on the ground and are picked up by earthworms as they tunnel through soil. The larvae mature inside the earthworm, and, when eaten by a dog, migrate to the bladder.

The respiratory form has a similar life cycle but involves wild animals, and the parasite migrates to the lungs.


This condition is diagnosed by examining the patient’s urine (if Capillaria plica is suspected) or feces (Capillaria aerophilia) under the microscopic.

The clinician may see typical lemon-shaped eggs, with a plug at either pole. However, these eggs are shed intermittently and so infection can be missed if only 1 sample is checked.

Capillaria infection is less common in cats than dogs. By: aturkus
Capillaria infection is less common in cats than dogs. By: aturkus


As long as the dog does not reinfect himself, the infection is mild and self-limiting.

However, if threadworms are identified, it is easily treated with a course of fenbendazole. The animal is given 3 doses on consecutive days.

It is a good idea to bathe the animal at the start and end of treatment to remove any capillaria eggs clinging to his fur — otherwise, he may ingest these when he grooms.

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Dogs will be dogs, and eating feces — or rolling in it — is one of their peculiar joys in life. Therefore, it’s a good idea to worm with fenbendazole 3 or 4 times a year.

Another precaution for kenneled dogs living outside is to replace soil substrate in the yard with sand, gravel or concrete to minimize the chances of eating earthworms.


  • Capillaria plica infection in a dog with chronic pollakiuria: Challenges in diagnosis and treatment. Basso, Spanhauer, Arnold & Deplazes. Parasitol Int. 2014, Feb: 63(1): 140–2.
  • Intestinal and lung parasites in owned dogs and cats from central Italy. Riggio, Mannella, Ariti & Perrucci. Parsitol. 2013, Mar 31: 193(1­–3): 78–84.

Dr. Pippa Elliott, BVMS, MRCVS

View posts by Dr. Pippa Elliott, BVMS, MRCVS
Dr. Pippa Elliott, BVMS, MRCVS, is a veterinarian with nearly 30 years of experience in companion animal practice. Dr. Elliott earned her Bachelor of Veterinary Medicine and Surgery from the University of Glasgow. She was also designated a Member of the Royal College of Veterinary Surgeons. Married with 2 grown-up kids, Dr. Elliott has a naughty puggle called Poggle, 3 cats and a bearded dragon.

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