Lung Fluke Infections in Dogs

By Krista Williams, BSc, DVM, CCRP; Cheryl Yuill, DVM, MSc, CVH

What are lung flukes?

Lung flukes are parasitic organisms called trematodes. The most common lung fluke that affects dogs in North America is called the Paragonimus kellicotti, also known as the North American lung fluke. Other species of lung flukes can infect dogs in other areas of the world but are they are rarely found in North America.

What is the life cycle of a lung fluke?

The life cycle of a lung fluke is very complex, requiring several hosts to mature from egg to adult. The following is a simplified version of the life cycle of a lung fluke:

  • Dogs that are infected with adult Paragonimus kellicotti shed eggs in their feces. The eggs hatch within a couple weeks and the emerging larval form enters its first intermediate host, which is a snail.
  • In the snail, the larva develops into its second stage, then leaves the snail and infects its second intermediate host, a crayfish.
  • In the crayfish host, the fluke continues to develop, eventually forming a cyst within the tissues of the crayfish.
  • The final stage of development occurs when the crayfish is eaten by a predatory animal. The natural final host of this parasite appears to be the mink, but other animals, including dogs, can become infected when they eat raw crayfish.
  • Digestive enzymes within the intestinal tract of the dog cause the cysts to break down, releasing the immature flukes. The flukes migrate through the dog’s body, reaching the lungs in about 2 weeks.
  • Once in the lungs, the flukes pair up and form cysts in the bronchioles ( air passages inside the lungs) where they are coughed up, swallowed, and passed through the intestines into the feces, thus completing the life cycle.

Although the most common way for a dog to become infected is by eating an infected crayfish, dogs can also be infected by eating other animals, such as rodents, that prey on crayfish.

Where are lung flukes found?

Lung flukes can be found anywhere in North America but are more likely to be found near the Great Lakes and the Mississippi River drainage areas. This lung fluke is also found in other parts of the world, including China and Southeast Asia.

What are the symptoms?

The usual symptoms of a lung fluke infection include intermittent coughing or difficulty breathing. If infected with a large number of flukes , a dog may cough up bloody mucus, or may develop pneumonia, pneumothorax (lung collapse due to air in the chest cavity outside of the lungs), or bronchiectasis (damage to the bronchioles of the lungs that leads to thickening, widening and scarring). Severely affected dogs may become weak and lethargic. With mild infections, there may be no symptoms at all.

How is a lung fluke infection diagnosed?

A lung fluke infection is diagnosed by detecting eggs in a sample of feces or mucus coughed up from the lungs of an infected dog. A sample of material from the trachea and bronchi can also be analyzed through procedures called ‘transtracheal wash’ or ‘bronchoalveolar lavage’.

To detect eggs from the parasite, the sample must undergo special processing. If using a fecal sample, most veterinarians will submit suspect samples to a veterinary laboratory for a definitive diagnosis with a microscopic examination.

“X-rays are useful to determine how many cysts are present and where they are located.”

X-rays of the lungs from an infected dog will usually reveal white spots in the lung fields which are consistent with lung fluke cysts. X-rays are also useful to determine how many cysts are present and where they are located. On occasion, an asymptomatic dog will be diagnosed when X-rays are taken for other reasons.

Could the symptoms be caused by something else?      

A cough or difficulty breathing can be caused by many different ailments and treatment will depend on the cause. Diagnostic testing is necessary to differentiate the cause of a dog’s symptoms and to determine the appropriate treatment.

What is the treatment?

Although no commercial products are specifically labeled for the treatment of lung flukes in dogs, there are several antiparasitic drugs that are effective for treating this infection. Dogs can be treated with antiparasitic drugs such as praziquantel or Fenbendazole (Panacur™).

What is the success rate with treatment?

Most infections will be cleared after the appropriate treatment course is followed.

Can you catch lung flukes from your dog?

No. The only way that people can become infected is to eat raw crayfish that are contaminated with lung fluke cysts.


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