Lung fluke infections in cats

What are lung flukes?

Lung flukes are parasitic organisms called trematodes. The most common lung fluke to affect cats in North America is called 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 are rarely found in North America.

What is the life cycle of the lung fluke?

The life cycle of the lung fluke is very complex, requiring several hosts in order to mature from egg through to adult. What follows is a simplified version of this life cycle:

• Cats that are infected with adult P. 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 become infected when they eat raw crayfish. Digestive enzymes within the intestinal tract of the cat will cause the cysts to break down, liberating the immature flukes. These flukes migrate through the cat’s body, reaching the lungs in about 2 weeks. Once in the lungs, the flukes will 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, 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 these lung flukes found?

Paragonimus kellicotti is found around the Great Lakes and the Mississippi River drainage areas of North America. 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 there are a lot of flukes in the infection, 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 leads to thickening, widening and scarring). Severely affected cats may become weak and lethargic. With mild infections, there may be no symptoms at all.

How is an infection with Paragonimus kellicotti diagnosed?

This infection is diagnosed by detecting eggs in 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 a procedure called a ‘transtracheal wash’.

In order to detect eggs from this parasite, the sample must undergo special processing. Most veterinarians will submit suspect samples to a veterinary laboratory for definitive diagnosis via microscopic examination after the fecal sample has been processed by a  fecal sedimentation test.

X-rays of the lungs of an affected cat will usually reveal white spots in the lung fields, 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 with this infection when x-rays are taken for other reasons.

Could the symptoms be caused by something else?

Cats can develop a cough or difficult breathing from many causes, and treatment will depend upon the cause. Diagnostic testing is necessary in order to differentiate the cause of the cat’s symptoms and 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 Paragonimus kellicotti infection in cats is the antiparasitic drug praziquantel, with daily administration of this drug for 3 days. An alternative treatment is fenbendazole, another antiparasitic drug, with daily treatment required for 10-14 days.

What is the success rate for treating?

Most infections will be cleared after the appropriate treatment.

Can I catch lung flukes from my cat?

No. Although this infection is zoonotic, meaning that people can develop this disease, the only way that people can get infected with Paragonimus kellicotti is to eat raw crayfish infected with the lung fluke cysts.