Flatworm Parasite (Heterobilharzia) in Dogs

Joel DeJaynes, DVM
By Joel DeJaynes, DVM on Nov. 23, 2021

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What Is Heterobilharzia, or the Flatworm Parasite in Dogs?

Heterobilharzia americana is a flatworm (or fluke) parasite that swims or wades in freshwater, including ponds, lakes, swamps, canals, etc.). 

This parasite is more common in southern states such as Texas, Louisiana, Florida, Mississippi, Alabama, Georgia, and South Carolina. However, cases have been found as far north as Kansas. 

The parasite uses a type of freshwater snail as its intermediate host, meaning that it first develops in the snail before it infects other animals. As a result, infections are limited to where these snails thrive. When the flatworm reaches a certain stage, it’s shed from the snail into the water, where it swims around looking for a host such as a dog (or other mammal) to infect.

Flatworms infect a dog by penetrating the skin and migrating to the lungs, where they can cause symptoms such as coughing. From the lungs, they travel through the bloodstream to the liver, where the flatworms develop into mature adults.

Within the liver, male and female flatworms reproduce, and the eggs can pierce the intestinal wall, to be shed into an animal’s feces. The lifecycle repeats if the eggs hatch in water and find another freshwater snail to infect.

Symptoms of Flatworms in Dogs

The symptoms caused by the flatworm in dogs are general and often mistaken for other diseases. When the flatworm penetrates the dog’s skin, they can cause a rash-type of reaction. You may notice this if your dog has short hair or if the parasite penetrates the skin on the stomach, where there is little or no hair. If you’re swimming or wading with your dog, you might also develop a rash, known as swimmer’s itch. 

Other symptoms will develop as the flatworm migrates to other areas. If the flatworm migrates to the lungs, symptoms may include:

  • Cough

  • Loss of appetite

  • Lethargy

Eggs produced by adult flatworms will invade the intestinal tract and cause diarrhea (which may be bloody or contain mucous) as well as fever and loss of appetite. Other symptoms may include:

  • Drooling

  • Dehydration

  • Increased need to drink

  • Collapse in some cases

If not treated, Heterobilharzia americana can be fatal. The severity of symptoms depends on how many mature flatworms have infected a dog, where worms have penetrated, and where inflammation is occurring. Some studies have found this process may take as long as 84 days.

Causes of Flatworms in Dogs

Flatworm infection in dogs is caused by the parasite Heterobilharzia americana. This is a flatworm parasite that uses freshwater snails as its intermediate host until it can infect a warm-blooded mammal.  

A dog cannot be infected by swallowing the eggs from an infected dog’s stool. Flatworms rely on the intermediate host (the snail) for the eggs to develop further. The eggs themselves are not harmful to dogs. Humans cannot become infected by coming into contact with an infected dog’s stool.

How Do Veterinarians Diagnose Flatworms?

Unfortunately, flatworm infection often goes undiagnosed because the symptoms mimic other diseases, which is why chronic symptoms often lead to death or euthanasia due to a poor quality of life.

When your veterinarian suspects flatworm infection, they will use a direct saline preparation and your dog’s fresh stool to confirm the diagnosis. This is because flatworm eggs don’t float (as most other parasites would), which is why it is not readily found on routine stool checks.

An abdominal ultrasound may be done to identify changes in the liver, spleen, abdominal lymph nodes, and walls of the intestines. These changes may not lead to a direct diagnosis but can help identify the extent of damage.

Clinical tests, such as an ELISA and PCR assay, have been developed to specifically diagnose flatworm infections. The choice of the veterinarian to run either of these tests is often based upon the history you communicate to your vet. This would include clinical signs and recent exposure to freshwater in a region where flatworms live.

Treating Flatworms in Dogs

Your veterinarian will administer high doses of deworming medication to rid a dog of the parasite. Medication such as praziquantel or fenbendazole may be effective if the infection is caught in time. Your dog will probably be hospitalized during treatment.

If diagnosed in time, a dog may be successfully treated. However, many cases lead to irreversible damage and euthanasia is recommended to spare a dog suffering. 

Recovering from Flatworms

If infection is diagnosed early and treatment is successful, dogs can recover quickly with no longstanding negative effects. If infection is caught later, but treatment is still successful, a dog’s life can be saved, but chronic gastrointestinal issues, among others, may be permanent.

Early identification is the key to successful treatment. Make sure to give your veterinarian a very thorough history and include the area and whether your dog was exposed to freshwater.

Flatworm Parasite (Heterobilharzia) in Dogs FAQs

Can flatworms kill dogs?

Yes, if not identified early in the course of the disease, infection is often fatal.

Are flatworms dangerous to dogs?

Yes, flatworms are dangerous to dogs. They can cause severe debilitating disease that often leads to death if not treated properly.

Can humans get flatworms from dogs?

No, humans cannot directly become infected by flatworms from their dogs.

How do dogs get flatworms?

Dogs get flatworms by swimming in freshwater sources where infected freshwater snails are living.

Featured Image: iStock.com/Lukasz Kochanek


1.    Companion Animal Parasite Council. Schistosomiasis.
2.    Johnson EM. Canine schistosomiasis In North America: an underdiagnosed disease with an expanding distribution. Compend Contin Educ Vet. 2010;32(3):E1-E4.
3.    Texas A&M Veterinary Medical Diagnostic Laboratory. Heterobilharzia amercana Infection in a Dog
4.    Texas A&M Veterinary Medicine & Biomedical Sciences. 2019 Canine Conference Proceedings
5.    The Merck Veterinary Manual. Whitehouse Station, NJ :Merck & Co., Inc. Hepatobiliary Fluke Infection in Small Animals.

Joel DeJaynes, DVM


Joel DeJaynes, DVM


Hailing from the Hawkeye State, Joel DeJaynes always wanted to be a Cyclone. His early childhood dream of becoming a veterinarian came true...

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