Cats, dogs, and sticky worms: invasion by land flatworms (Geoplanidae) is facilitated by household pets.

Abstract

BACKGROUND: It is well known that the main vector of invasion by land flatworms has been the export of potted plants from their countries of origin to the invaded country. Within the invaded country, transport to garden centres where the plants are sold, and transport to the buyer's garden, are also carried out by humans. However, it is less clear how flatworms can then invade neighbouring gardens, given their slow movement rates. METHODS: We re-examined citizen science reports in metropolitan France received over more than 12 years (2013-2025), searching the 6500 original emails for keywords suggesting transport by pets. RESULTS: We found 15 citizen science observations of cats (13) or dogs (2) with flatworms stuck to their fur. Surprisingly, all reports concerned the species, the two-tone planarian, even though this species is not the most abundant in gardens in France. Over the period 2020-2024, observations ofon dogs and cats represented 7.3% (10/137) of reports. DISCUSSION: We suspect that transport by domestic animals is a significant factor favouring invasion by, which possesses a particularly sticky mucus adapted to arthropod predation. This is compounded by the fact that the species reproduces asexually in Europe, and therefore the transport of a single individual may be sufficient to facilitate an invasion. We calculated a conservative estimate of the distances travelled outdoors by all the dogs and cats in France, which was 18 billion km/year; if only a tiny proportion of these journeys involve pets carrying flatworms, this transport as a dispersal factor becomes entirely plausible. CONCLUSIONS: We believe that animal transport is a significant factor favouring land flatworm invasion, but that this does not apply to all species. A citizen science initiative could provide a better understanding of the extent and importance of animal transport as a factor for land flatworm invasions in other countries.