A surprising new study reveals that dogs and cats may be helping an invasive flatworm spread.
A new study published today (February 10) in the journal PeerJ has uncovered an unexpected way an invasive flatworm is spreading across France. The research was led by a scientist from the Institute of Systematics, Evolution and Biodiversity (ISYEB) at the French National Museum of Natural History, working with a colleague from James Cook University in Australia. Their findings point to household pets as unintentional carriers of the species.
How invasive flatworms usually spread
Terrestrial flatworms (Platyhelminthes) are known invasive organisms that typically expand their range through the movement of plants. This process is mostly driven by human activity, such as gardening and plant trade. Yet this explanation left a key mystery unresolved. These worms move extremely slowly, so it was unclear how they were able to appear in nearby gardens that had not received new plants.
Citizen science reveals an unexpected carrier
To investigate, researchers examined more than twelve years of observations gathered through citizen science programs across France. This long-term dataset revealed something previously overlooked. Several reports showed flatworms clinging to the fur of dogs and cats, suggesting that domestic animals can transport the worms between locations.
One species stands out
Of the roughly ten flatworm species that have invaded France, only one was linked to this form of transport. That species is Caenoplana variegata. It produces an especially sticky mucus, likely connected to its diet as a predator of arthropods. The worm can also reproduce without a partner, a trait that further increases its ability to establish itself in new areas.
Because pets travel significant distances each year, researchers say this hidden method of movement could play an important role in spreading certain invasive flatworm species worldwide.
Reference: “Cats, dogs, and sticky worms: invasion by land flatworms (Geoplanidae) is facilitated by household pets” by Jean-Lou Justine and Leigh Winsor, 10 February 2026, PeerJ.
DOI: 10.7717/peerj.20713
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