Unseen Hitchhikers: How Household Pets Are Fueling the Spread of Invasive Flatworms

PARIS — While gardeners have long known that invasive species can arrive in a shipment of potted plants, new research suggests a more personal “last mile” delivery system is at work in our own backyards. A study published on February 10, 2026, in the journal PeerJ reveals that domestic cats and dogs are unintentionally acting as local transport for Caenoplana variegata, an invasive land flatworm currently colonizing gardens across France and wider Europe.

By analyzing over a decade of citizen-science data, researchers from the French National Museum of Natural History and James Cook University found that these slow-moving invertebrates are hitching rides on the fur of pets to jump the gap between neighboring properties—a finding that adds a crucial piece to the puzzle of how invasive species saturate local environments.

The “Last Mile” Discovery

Terrestrial flatworms are notorious for their ability to hitchhike across continents via the global plant trade. Encapsulated in the soil of decorative shrubs or tucked beneath plastic nursery pots, they travel thousands of miles with ease. However, once they arrive in a neighborhood, their natural pace is glacial.

The new study, led by Professor Jean-Lou Justine, highlights a secondary, localized pathway. The team scrutinized more than 6,500 original emails and reports submitted by the public between 2013 and 2025. They identified 15 verified instances where flatworms were found stuck to the fur of domestic animals—13 on cats and two on dogs.

While 15 cases might seem small, the timing is significant: 10 of these records occurred between 2020 and 2024. During that four-year window, pet-related sightings accounted for approximately 7.3% of all C. variegata observations.

“This is what we call phoresy,” explains Dr. Elena Rossi, an independent ecologist not involved in the study. “It’s a non-parasitic relationship where one organism hitches a ride on another. In this case, the ‘commuter’ is a flatworm, and the ‘bus’ is your Golden Retriever.”

Why These Worms Stick

Not all flatworms are equal-opportunity hitchhikers. Interestingly, the study found that only one species—Caenoplana variegata, native to Australia—was consistently found on pets. Another common invasive flatworm, Obama nungara, was absent from the pet reports during the same period.

Researchers believe C. variegata possesses a “perfect storm” of biological traits that facilitate its travels:

• Sticky Mucus: The worm produces a particularly adhesive slime that allows it to cling to mammalian hair.

• Body Shape: Its long, slender form likely enables it to wrap around individual strands of fur more effectively than broader species.

• Asexual Reproduction: Perhaps most critically, these worms can reproduce through fragmentation. This means a single “hitchhiker” successfully dropped in a new garden has the potential to found an entire local population.

The sheer scale of pet movement makes even rare events statistically significant. The study authors estimate that in France alone, dogs and cats spend enough time outdoors to cover roughly 18 billion kilometers annually. Even if the probability of a worm sticking to a pet is one in a million, the sheer volume of “trips” makes dispersal inevitable.

Implications for Ecosystems and Pet Owners

From a public health perspective, there is no cause for immediate alarm. These flatworms are not parasites; they do not bite, sting, or infect mammals. They are predators of soil invertebrates, such as earthworms and snails.

“Veterinarians should reassure pet owners that these are not fleas or ticks,” says Dr. Marc Thorne, a veterinary consultant. “They aren’t feeding on the pet. However, the mucus can be an irritant, and some pets might show discomfort or attempt to groom the ‘sticky’ spot excessively.”

The real concern is ecological. Land flatworms can significantly alter soil biodiversity by preying on native species that are essential for healthy garden ecosystems. By moving these worms from “Garden A” to “Garden B,” pets are accelerating a process that would otherwise take years or decades.

Limitations of the Research

The researchers acknowledge that because the study relies on citizen science, the data likely represents just the tip of the iceberg. Many pet owners might simply brush off a “bit of mud” without realizing it is a living organism, leading to an undercounting of the actual frequency.

Conversely, the study is geographically limited to France. While the findings are highly suggestive for other regions where C. variegata is present—including parts of the United Kingdom and the United States—local environmental factors and pet behaviors vary.

A Call for “Pet Hygiene”

The study does not suggest that pets should be kept indoors, but it does advocate for increased awareness. Experts suggest a few simple steps for those living in areas known for invasive flatworms:

1. Post-Walk Inspections: Quickly check your pet’s paws and underbelly after they have been in damp, shaded garden areas.

2. Safe Removal: If you find a flatworm, do not touch it with bare hands, as some species can secrete irritating chemicals. Use a paper towel or tweezers.

3. Reporting: Use biodiversity apps or contact local wildlife authorities to report sightings. Documenting the location helps scientists track the invasion front.

“If the plant trade is the cargo ship of this invasion,” says Dr. Rossi, “then our pets are the bicycle couriers. They are finishing the delivery. Understanding this helps us better manage the health of our local environments.”

Medical Disclaimer: This article is for informational purposes only and should not be considered medical advice. Always consult with qualified healthcare professionals before making any health-related decisions or changes to your treatment plan. The information presented here is based on current research and expert opinions, which may evolve as new evidence emerges.

References

• https://www.earth.com/news/pets-are-spreading-invasive-flatworm-species-around-the-world/