Your Dog or Cat May Be Spreading Invasive Worms Without You Knowing, Study Shows
Your pet could be carrying an invasive flatworm — and you might not know.
If your pet spends time outdoors, it could be helping an invasive species spread—and you might not realize it until a slimy hitchhiker shows up in your home.
Researchers have found that invasive flatworms can travel between gardens and neighborhoods by clinging to the fur of dogs and cats. A study published in PeerJ documented 15 pet-related incidents in France, involving 13 cats and two dogs whose owners discovered the worms after their animals came back inside.
A sticky problem
Parasitologist Jean-Lou Justine at the French National Museum of Natural History identified the pattern after reviewing reports collected across France. Owners described finding worms tangled in their pets’ fur following time outdoors.
Among roughly ten introduced flatworm species recorded in France, only the Australian species Caenoplana variegata was consistently found on pets. The reason appears to come down to its biology: the flatworm produces a sticky mucus coating used for movement and adhesion, which helps it cling to surfaces — including animal fur.
One owner in southwestern France reported, “They cling to the hair of my Persian cats,” while another stated, “It was very slimy, stuck in the fur,” describing removal of the worm with tweezers.
Not a parasite, but still a concern
The good news for pet owners is that these flatworms do not feed on or live within animals. Researchers categorize the interaction as phoresy, in which one organism simply uses another for transportation. That distinguishes it from a true parasitic relationship.
Still, researchers emphasize that while the worms do not infect pets, they may cause irritation and concern for owners who encounter them unexpectedly. Veterinary response focuses on removal rather than treatment for infection.
Why it matters for your garden
The real worry is ecological. Caenoplana variegata preys on small soil-dwelling organisms such as woodlice, insects and spiders—creatures that play a role in maintaining healthy garden ecosystems. Repeated introductions into new environments could affect local food webs over time.
The species can also reproduce asexually, meaning a single individual transported to a suitable environment can establish an entirely new population.
How flatworms spread—and why pets make it worse
Flatworms are often introduced to new regions through potted plants, which serve as the initial pathway for international spread. But once they arrive, their limited mobility makes it difficult for them to expand beyond the original point of introduction without help.
Dogs and outdoor cats provide that mobility by carrying worms across yards, sidewalks and fences, effectively enabling movement between nearby locations. This mechanism helps explain how a species that moves only short distances on its own can still appear across a wide geographic area.
Between 2020 and 2024, 10 of 137 recorded sightings of this species in France involved pets, representing 7.3% of cases. Over a broader dataset of 447 sightings, the species was found distributed across the country rather than concentrated in a single region.
Because many instances likely go unnoticed—when worms fall off outdoors or remain undetected—researchers note that current records likely underestimate the true frequency of pet-assisted spread.
A growing global concern
Similar cases of flatworms found on pets have been reported in countries including Australia, the United States, New Zealand and Brazil, suggesting that pet-assisted transport may not be limited to France.
In July 2025, the European Union listed three land flatworm species as invasive species of Union concern, reflecting growing awareness of their ecological impact.
Citizen-submitted observations have played a key role in identifying these patterns. Photographs and reports provided researchers with detailed evidence of occurrences that might otherwise go unrecorded.
For pet owners, the takeaway is straightforward: if your dog or cat comes inside with something slimy stuck in its fur, take a closer look. You may be witnessing an invasion in progress.
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