Liver Fluke in Dogs: Schistosomiasis

What are liver flukes?

A liver fluke is a type of parasitic worm called a trematode. Heterobilharzia americana is the fluke species that affects dogs in North America. Other species of flukes can affect other animal species. Liver flukes cause a disease called schistosomiasis. Dogs infected with this disease can develop granulomas (inflammatory nodules) in the liver and other abdominal organs.

What is the lifecycle of a liver fluke?

The lifecycle is complex and involves several stages. 

1. Affected animals shed eggs in their feces. Raccoons are the natural host to these parasites, but dogs, wild canines, bobcats, and rabbits can also shed eggs.
2. These eggs hatch on contact with water, releasing the first larval stage, called miracidia. This larval stage infects freshwater snails (Galba cubensis, most commonly). Inside the snail, the larva transforms into the next stage, called cercariae.
3. The cercariae leave the snail and are free living in fresh water.
4. Dogs are infected when they swim or wade in the water and cercariae penetrate their skin, potentially leading to skin inflammation. The cercariae then migrate to the liver and lungs.
5. In the dog’s liver, the cercariae mature into adult flukes. They then migrate to intestinal blood vessels to mate. The flukes lay eggs that travel to the intestines to be released in the feces, and the cycle begins again.

Where are liver flukes found?

H. americana flukes are found mostly in the southern Atlantic and Gulf Coast states, including Florida, Texas, Louisiana, and North Carolina. This range corresponds to the range of the fluke’s favorite snail, Galba cubensis. H. americana has been sporadically reported in other states, but its exact prevalence is unknown. Young to middle-aged, large-breed dogs seem to be more commonly affected.

What are the symptoms of liver flukes?

Symptoms of liver flukes may range from mild to severe, with multiple organs involved. Migration of eggs and adult flukes can lead to granulomas forming in the liver, intestines, pancreas, and kidneys. Mild cases may have no clinical signs at all.

Non-specific symptoms include:

• weight loss
• vomiting
• diarrhea (possibly bloody)
• lethargy
• dehydration
• poor appetite

More severe signs include: 

• increased drinking and urination
• fluid in the abdomen (ascites)
• thickened bowel loops
• abdominal pain
• yellowing of the skin (jaundice)

How is the disease diagnosed?

Diagnosis is not always straightforward. Finding eggs in a fecal sample indicates the disease is present, but standard fecal testing can miss the eggs. Some laboratories have a PCR test that is more accurate.

If your dog has clinical signs consistent with this disease, your veterinarian will likely recommend additional testing, starting with bloodwork. A complete blood count may show anemia (low red blood cell count) due to blood loss, and increased eosinophils, which are white blood cells that are often elevated with parasitic disease. The biochemistry profile may show elevated liver values. Elevated calcium is also a common finding. 

Imaging with X-rays may show mineralization around the small intestines, as well as an enlarged liver and spleen. An ultrasound of the abdomen may show thickened bowel loops, enlarged lymph nodes, fluid in the abdomen, and changes in the liver.

Biopsies of the liver, small intestine and pancreas can reveal parasite eggs and occasionally adult flukes.

Diagnosis is often presumed based on a combination of clinical signs, supportive blood testing and imaging, and geographical location, as infection is more likely to occur in certain locations.

What is the treatment?

There are no commercial products specifically labelled for the treatment of liver flukes, but treatment with praziquantal and fenbendazole (anti-parasitic drugs) has been successful. Other treatments include nutritional and fluid support, pain medications, anti-nauseant medications and antibiotics for secondary infections.

What is the prognosis?

If clinical signs are mild and caught early, prognosis is good. Prognosis is more guarded if diagnosis and treatment are delayed, if there are extensive granulomas in multiple organs, or if the elevated calcium affects kidney function.

Can I catch this disease from my dog?

People cannot catch this infection from dogs, though swimming in areas where the cercariae are present can lead to a skin irritation called “swimmers itch”. Adult flukes do not develop in people.