Liver Flukes: What Every Dog Owner Needs to Know

By Jane Robertson DVM DACVIM

You may have seen the headlines: ‘YOUR PETS ARE IN PERIL’: DOG-KILLING WORM FOUND IN CALIFORNIA FOR THE FIRST TIME¹; A DOG-KILLING WORM IS ON THE LOOSE IN CALIFORNIA²; or one of my favorites: DOG-KILLING “LIVER FLUKE” PARASITE SPREADING FAST ACROSS US³. 

Here at dogpacking.com our objective is to keep you and your dog active and enjoying outdoor activities together. To this end, we want to keep you informed of health risks your pets may be exposed to and how to mitigate those risks. We will not give you sensationalized headlines as click bait, which are what these headlines are.

What is this liver parasite?

This liver parasitic flatworm is called Heterobilharzia americana and in dogs it can cause an infection called schistosomiasis, after a dog has been swimming or wading in water in which the parasite is found.

This is NOT a new parasite, but one that has historically been found in Florida, Texas, and other gulf states; with random cases having been reported in other states including Kansas, North Carolina, Indiana, and Utah. Finding this parasite for the first time in Southern California is newsworthy but should not be sensationalized. And it is not “spreading fast” across the United States. Case in point, there have been no reported cases in Northern California at this time.

How are dogs infected?

For its life cycle this parasite needs snails, freshwater, and a mammal to host it. Mammals such as racoons, horses, and dogs are considered the definitive hosts and snails are called the intermediary hosts.

The lifecycle starts when a definitive host sheds parasite eggs in its stool. When these eggs hatch, free-swimming forms of the parasite are released into the water, then penetrate snails, wherein they develop into flatworms.

A second free-swimming form is released back into the water where it can penetrate directly through the skin of a definitive host, such as a dog, and migrate initially to the lungs and liver and then once matured move to the blood vessels of the intestines. Here they lay eggs that penetrate the intestinal lining to be released into the intestines to be excreted in the feces. And then the cycle starts again.

How many dogs are known to have been infected in California?

Los Angeles County of Public Health issued an advisory in 2023 confirming 11 cases of canine schistosomiasis (the disease caused by this liver fluke) from 2018 to 2023.⁴ These dogs were from 5 households that all swam in a pond that was fed by a tributary of the Colorado river. One dog did unfortunately die from its infection. A follow up study of 2000 snails were collected from the Colorado river where it borders Arizona and some were found to harbor the parasite. 

Can people be infected?

Exposure to Heterobilharzia americana may cause a mild skin rash or swimmer’s itch in people, but nothing more serious. The infection cannot be spread from dog to dog or from dogs to humans.

Can I still take my dog swimming in lakes, rivers, and ponds?

Of course you can! Common sense still needs to prevail. Other hazards, infections and intoxications are still more likely. Not allowing your pet to swim in a strong current, drink from stagnant water or water suspected to be contaminated with blue-green algae, swim in water that contains run off from a cattle farm are best to be avoided. And I probably would not take my dog to Mill Pond in Southern California, but I am not sure I would avoid the entire Colorado River!

What are the signs that my dog could have been infected?

Schistosomiasis is not an acute disease, so your dog will not get sick overnight. These dogs typically have a gradual onset of chronic gastrointestinal signs. A study of 60 dogs over a 10-year period from Texas found the most common clinical signs were diarrhea, vomiting and weight loss with or without a loss of appetite. Less common signs include increased thirst and urination.⁵

How is infection with this liver fluke diagnosed?

The key to diagnosing schistosomiasis is first having an index of suspicion that infection with this liver fluke is possible.

There could be changes on routine blood testing that may increase the level of suspicion including increases in liver enzymes, changes to blood proteins (high globulin, low albumin), mild anemia, increases in kidney parameters and increased calcium levels. However, routine laboratory results can be normal and when changes are present, they are often mild and nonspecific.

So, again, to make the diagnosis, you and your veterinarian must specifically consider infection with Heterobilharzia americana, the causative agent of canine schistosomiasis, and then perform a test to find it.

Remember, part of the life cycle of this parasite results in eggs being shed into the dog’s intestinal tract and out into the feces. And fortunately, there is now an accurate test to detect this parasite in your dog’s stool. This is a PCR (polymerase chain reaction) test that is performed at Texas A&M gastrointestinal laboratory. Your veterinarian can send a test directly to this lab or through their commercial lab who will forward it along.

It is possible to find these eggs on other fecal exams, especially a fecal sedimentation exam (not performed routinely), but these methods of detection are far less reliable.

Do monthly preventative parasite medications kill liver flukes?

Unfortunately, monthly preventatives do not work against this liver fluke.

Is there a treatment for schistosomiasis?

Yes! These dogs are usually treated with two different drugs to kill this parasite (praziquantel and fenbendazole). Treatment is typically well tolerated. Some dogs may require hospitalization with supportive care depending on how sick they are. The key is to make the diagnosis early before your dog is too sick.

What can I do to keep my dog safe?

As always, just remain vigilant. If there are known cases of schistosomiasis from dogs swimming in a specific location, then of course, I would avoid that body of water. However, this information is not likely to be available. So, the most important thing is to just be aware of this disease and what clinical signs to look for in your dog.

Always provide your veterinarian with your dog’s travel history and where he or she has been swimming. And if your dog develops chronic gastrointestinal signs and other more common causes have been ruled-out, then request that your veterinarian perform the PCR test forHeterobilharzia americana.

With the above knowledge and guidelines, you and your furry friend can enjoy water-based adventures.

About the author

Jane Robertson, DVM, DACVIM is a veterinarian board-certified by the American College of Veterinary Internal Medicine. She grew up near Toronto obtaining her veterinary degree from the Ontario Veterinary College in Guelph. She moved to Davis, California for her small animal internal medicine residency where she continues to reside. Dr. Robertson has worked in private referral hospitals and for a large veterinary diagnostic company where she was Director of Medical Affairs. In her free time she enjoys cycling, rock climbing and going on outdoor adventures with her Goldendoodle, Max.

References

¹Bartlett, A. ‘Your pets are in peril’: Dog-killing worm found in California for the first time. SF Gate. Last updated March 18, 2024. Retrieved March 20, 2024, from https://www.msn.com/en-us/news/us/your-pets-are-in-peril-dog-killing-worm-found-in-california-for-the-first-time/ar-BB1k3BTx#image=BB1k3nWm|1.

²Cara, E. A Dog-Killing Worm Is on the Loose in California. Gizmodo. March 14, 2024. Retrieved March 20, 2024, from https://gizmodo.com/dog-killing-parasite-california-liver-fluke-water-1851335680.

³Chaturvedi, A. Dog-Killing “Liver Fluke” Parasite Spreading Fast Across US. NDTV World. Last updated March 18, 2024. Retrieved March 20, 2024, from https://www.ndtv.com/world-news/dog-killing-liver-fluke-parasite-spreading-fast-across-us-5261448. 

⁴Animal Health Advisory: Heterobilharzia americana (canine schistosomiasis) confirmed in 11 dogs in three Southern California Counties (LA, Orange, and Riverside) April 19, 2023. Issued by LA County of Public Health. Retrieved on March 20, 2024, fromhttp://publichealth.lacounty.gov/vet/docs/AHAN/AHAN_Heterobilharzia11dogs_04192023.pdf.

⁵Graham AM, Davenport A, Moshnikova VS, et al. Heterobilharzia americana infection in dogs: A retrospective study of 60 cases (2010-2019). J Vet Intern Med. 2021;35:1361–1367. https://doi.org/10.1111/jvim.16127