Lung Fluke Infections in Dogs

What are lung flukes?

Lung flukes are parasitic worms (trematodes). The most common lung fluke to affect dogs in North America is called 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 they are rarely found in North America.

What is the life cycle of a lung fluke?

The life cycle of a lung fluke is 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 larvae enter the 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, 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. 
• Digestive enzymes within the dog’s intestinal tract cause the cysts to break down, releasing 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.

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. The most common way a dog is infected is by eating infected crayfish, but dogs can also be infected by eating animals that prey on crayfish, such as rodents.

Where are lung flukes found?

Lung flukes can be found anywhere in North America but are more commonly around the Great Lakes region and the midwestern and southwestern parts of the United States.

Dogs living near streams are more likely to become infected. They are also found in other areas of the world, particularly in China and Southeast Asia.

What are the symptoms of a lung fluke infection?

The usual symptoms of a lung fluke infection include intermittent coughing or difficulty breathing. If infected with a large number of flukes, the 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 either feces or mucus that an infected dog coughs up. It can also be detected by analyzing a sample of material from the trachea and bronchi, through procedures called a “transtracheal wash” or “bronchoalveolar lavage”.

To detect eggs from the parasite, the sample must undergo special processing. Most veterinarians will submit suspect samples to a veterinary laboratory for a definitive diagnosis.

"X-rays of the lungs of an infected dog usually reveal white spots in the lungs, 
which are consistent with lung fluke cysts."

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

Could the symptoms be caused by something else?

Dogs can develop a cough or difficulty breathing from many other causes. Treatment depends 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?

No commercial products are specifically labeled for the treatment of lung flukes in dogs, but 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 clear up with the appropriate treatment.

Can I catch lung flukes from my dog?

No, the only way that people can be infected with lung flukes is to eat raw crayfish contaminated with lung fluke cysts.