Your Pets Could Be Spreading Invasive Flatworms Without You Realizing It, Study Finds
What if the biggest threat to your garden wasn’t hiding underground — but hitching a ride on your pet?
That’s the surprising finding from a study published in PeerJ, which documented how invasive flatworms are spreading across France by clinging to the fur of household dogs and cats. The slow-moving creatures can’t travel far on their own. But with a little help from an unsuspecting tabby, they don’t have to.
A Parasitologist Spots a Pattern
The discovery reads like a scientific detective story. Parasitologist Jean-Lou Justine at the French National Museum of Natural History (MNHN) was reviewing reports from pet owners across France when he noticed something recurring: people kept finding worms tangled in their animals’ fur after time outdoors.
Justine’s study documented 15 pet-related incidents, involving 13 cats and two dogs. Among roughly ten introduced flatworm species recorded in France, only the Australian species Caenoplana variegata was consistently found on pets. Its secret weapon? A sticky mucus coating used for movement and adhesion that 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 — Something Stranger
Here’s where it gets fascinating. These flatworms aren’t parasites. They don’t feed on pets or live inside them. What they’re doing has an entirely different name: phoresy.
Phoresy is a relationship in which one organism uses another purely for transportation. The flatworm climbs aboard, rides along and drops off somewhere new — no harm done to the host. Veterinary response focuses on removal rather than treatment for infection. Researchers emphasize that while the worms do not infect pets, they may still cause irritation and concern for owners who encounter them unexpectedly.
The distinction matters. A parasite exploits its host. A phoretic organism simply books a free ride. And this particular passenger is remarkably well-equipped for the journey, thanks to that sticky mucus.
Citizen Science Made This Discovery Possible
This is where everyday pet owners became accidental researchers. Citizen-submitted observations played a key role in identifying these patterns, as photographs and reports provided researchers with detailed evidence of occurrences that might otherwise go unrecorded.
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. Without those owner-submitted photos and descriptions, the scope of pet-assisted transport would have remained invisible.
Owners frequently noticed the worms only after their pets returned indoors. And 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 Global Hitchhiker
The phenomenon isn’t limited to France. Similar cases of flatworms found on pets have also been reported in countries including Australia, the United States, New Zealand and Brazil, suggesting pet-assisted transport may be a worldwide pattern.
Flatworms are often introduced to new regions through potted plants, which serve as an initial pathway for international spread. But once they arrive, dogs and outdoor cats provide local mobility — carrying worms across yards, sidewalks and fences, effectively enabling movement between nearby locations. The species can also reproduce asexually, meaning a single individual transported to a suitable environment can establish an entirely new population.
That matters ecologically. Caenoplana variegata preys on small soil-dwelling organisms such as woodlice, insects and spiders that contribute to soil health. In July 2025, the European Union listed three land flatworm species as invasive species of Union concern.
Next time your cat comes in from the garden, it might be worth a closer look.
This article was created by content specialists using various tools, including AI.
