Dogs get flatworms by swallowing infected fleas, eating raw or undercooked fish, hunting small prey like rodents and rabbits, or wading in contaminated freshwater. The specific route depends on the type of flatworm, but in every case, the parasite needs to pass through another animal or life stage before it can infect your dog. There is no direct dog-to-dog transmission for most flatworm species.
Swallowing Infected Fleas
The most common flatworm in dogs is the tapeworm Dipylidium caninum, and the transmission route is surprisingly simple. Flea larvae on the ground eat tapeworm eggs from the environment. As the flea matures, the tapeworm larva develops inside it. When your dog chews or licks at itchy skin and accidentally swallows one of these infected fleas, the tapeworm larva is released in the intestine, where it attaches and grows into an adult worm. This often happens during normal grooming, so dogs with even a mild flea problem are at risk.
This is why flea control is so central to tapeworm prevention. A dog that keeps getting reinfested with fleas will keep picking up new tapeworms, even after deworming treatment.
Hunting or Eating Rodents and Rabbits
Several other tapeworm species use small mammals as their middleman. Rodents, voles, squirrels, and rabbits can carry larval tapeworms encysted in their tissues. When a dog catches and eats one of these animals, the larvae are released in the dog’s gut and develop into adult worms.
This route is particularly relevant for Taenia species and for Echinococcus multilocularis, a tapeworm that cycles between wild canids (foxes, coyotes, wolves) and rodents. Dogs that roam freely, live in rural areas, or are used for hunting have a significantly higher risk of picking up these parasites. Research on Echinococcus in Calgary found that free-roaming dogs and those frequently fed raw livestock offal were more likely to be infected. While this parasite historically stayed in rural areas, it can establish itself in urban settings once domestic dogs enter the cycle.
Eating Raw or Undercooked Fish
In the Pacific Northwest of the United States and parts of Canada, dogs can pick up a dangerous fluke called Nanophyetus salmincola by eating raw freshwater fish, particularly salmon and trout. The fluke itself causes intestinal irritation, but the real threat is a bacteria the fluke carries that causes salmon poisoning disease, a condition that can be fatal in dogs without treatment. Symptoms typically appear 5 to 7 days after a dog eats the parasitized fish.
Another fish-borne flatworm, Diphyllobothrium, is a broad tapeworm dogs can get from eating raw freshwater fish in northern regions. Cooking or freezing fish thoroughly before feeding it to your dog eliminates these parasites.
Swimming in Contaminated Water
One flatworm doesn’t need to be swallowed at all. Heterobilharzia americana, the parasite behind canine schistosomiasis, infects dogs through the skin. Snails living in freshwater marshes, ponds, mudflats, and canals release free-swimming larvae into the water. When a dog wades or swims in these areas, the larvae penetrate the skin directly, transform into a juvenile stage, and migrate through the body to the blood vessels around the intestines. Infected snails have been found along the Colorado River in California and in waterways across the southeastern United States.
What These Infections Look Like
Many flatworm infections cause no obvious symptoms at first. The most recognizable sign of a tapeworm is small, white, rice-like segments (called proglottids) in your dog’s stool or stuck to the fur around the tail. These segments are actually packets of eggs shed by the adult worm. You might also notice your dog scooting on the ground due to irritation around the anus.
Heavier infections or more aggressive species can cause weight loss, diarrhea, a dull coat, and general lethargy. Fluke infections tend to be more serious. Salmon poisoning from Nanophyetus causes fever, vomiting, bloody diarrhea, and swollen lymph nodes. Schistosomiasis from Heterobilharzia can lead to chronic intestinal inflammation, bloody stool, and liver or kidney damage over time. Lung flukes like Paragonimus kellicotti, which dogs pick up from eating raw crayfish, cause coughing and breathing difficulty.
Treatment and Prevention
A single drug, praziquantel, is the standard treatment for most flatworm infections in dogs. The World Health Organization lists it as one of 100 essential medications. It works effectively against both tapeworms and most flukes, though it does not kill every flatworm species equally well. Your vet may combine deworming with additional treatment if a fluke is carrying a secondary infection, as with salmon poisoning disease.
Prevention comes down to breaking the transmission cycle at its weakest points:
- Flea control: Treat all pets in the household with routine flea preventatives, whether collars, topical treatments, or oral chews. Treat the home and yard as well. This alone prevents the most common tapeworm.
- Limit hunting and scavenging: Keep your dog from catching and eating rodents, rabbits, or other small wildlife. Dogs that roam unsupervised are at the highest risk for Taenia and Echinococcus infections.
- Avoid raw fish: Never feed your dog raw or undercooked freshwater fish, especially in the Pacific Northwest.
- Be cautious around freshwater: In areas where canine schistosomiasis is known to occur, limit your dog’s access to slow-moving freshwater where snails thrive.
- Routine deworming: Cornell University’s veterinary college recommends using a dewormer that specifically covers tapeworms on a regular schedule, since standard dewormers for roundworms do not treat flatworms.
Can Humans Catch Flatworms From Dogs?
The risk is low but not zero. Humans cannot get tapeworms by touching an infected dog. However, the same flea route that infects dogs can technically infect people. If a person accidentally swallows an infected flea, the Dipylidium tapeworm can develop in the human intestine. Fewer than 350 human cases have been documented worldwide, and over two-thirds involved small children, likely because young kids are more prone to close face-to-face contact with pets and more likely to accidentally ingest a flea.
Echinococcus poses a more serious zoonotic concern. Dogs shed microscopic eggs in their stool, and if a person ingests these eggs (from contaminated soil, water, or direct contact with a dog’s fur), the larvae can form cysts in the liver or lungs. This is rare in most of North America but is a significant public health issue in parts of the world where dogs have close contact with livestock and wildlife.