Lung Fluke Infections in Cats

What are lung flukes?

Lung flukes are parasitic worms (trematodes). The most common lung fluke to affect cats in North America is Paragonimus kellicotti, also known as the North American lung fluke. Other species of lung flukes can infect cats 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:

• Cats infected with adult Paragonimus kellicotti shed eggs in their feces.
• The eggs hatch within a couple of 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 cat’s intestinal tract cause the cysts to break down, releasing immature flukes, which migrate through the cat’s body, reaching the lungs in about 2 weeks.
• Once in the lungs, the flukes pair up and form cysts within the cat’s lungs. Adult flukes begin to produce eggs within 5-7 weeks. Fertilized eggs are released into 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 cats, can also become infected when they eat raw crayfish. The most common way a cat is infected is by eating infected crayfish, but cats 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 located around the Great Lakes region and the midwestern and southwestern parts of the United States.

Cats 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 are intermittent coughing or difficulty breathing. If a cat is infected with a large number of flukes, the cat may cough up bloody mucus or may develop pneumonia, pneumothorax (lung collapse due to air in the chest cavity outside the lungs), or bronchiectasis (damage to the bronchioles of the lungs that lead to thickening, widening, and scarring). Severely infected cats 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 cat 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”.

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

To detect eggs from this 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 cat usually reveal white spots in the lungs, which are consistent with fluke cysts. X-rays are useful to determine how many cysts are present and where they are located. Sometimes an asymptomatic cat will be diagnosed when X-rays are taken for other reasons.

Could the symptoms be caused by something else?

Cats can develop a cough or difficulty breathing from many causes. Treatment depends on the cause. Diagnostic testing is necessary to differentiate the cause of a cat’s symptoms and to determine the appropriate treatment.

What is the treatment?

No commercial products are specifically labeled for treatment of lung flukes in cats, but there are several antiparasitic drugs that are effective for treating this infection. The preferred treatment for lung flukes in cats is the antiparasitic drug 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 cat?

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