Lung Fluke Infections in Cats

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 to affect cats in North America is the 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 the lung fluke is very complex, requiring several hosts to be able to mature from egg through to adult. The following is a simplified version of the life cycle of a lung fluke:

  • Cats that are infected with adult Paragonimus kellicotti shed eggs in their feces.
  • The eggs hatch within a couple of 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 cats, can also become infected when they eat raw crayfish. Digestive enzymes within the intestinal tract of the cat will cause the cysts to break down, releasing the immature flukes. These flukes 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. Although the most common way for a cat to become infected is by eating an infected crayfish, cats 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 commonly located around the Great Lakes and the Mississippi River drainage areas. 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?

This infection is diagnosed by detecting eggs in a sample of feces or mucus coughed up from the lungs of an infected cat or by analyzing a sample of material from the trachea and bronchi through procedures called a ‘transtracheal wash’ or ‘bronchoalveolar lavage’.

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 can also usually reveal white spots in the lung fields which are consistent with fluke cysts. X-rays are useful to determine how many cysts are present and where they are located. On occasion, 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 other causes. Treatment will depend 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?

Although no commercial products are specifically labeled for treatment of lung flukes in cats, there are several antiparasitic drugs that are effective for treating this infection. The preferred treatment for a lung flukes in cats is the antiparasitic drug praziquantel or alternatively, fenbendazole (Panacur™),

What is the success rate with treatment?

Most infections will be cleared with the appropriate treatment.

Can I catch lung flukes from my cat?

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

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