When we talk about “worms” in dogs or cats, we’re usually talking about parasites that can infect pets or (less commonly) that harbour other pathogens. However, there are also certain worms that can cause other problems for our furry friends. For example, the hammerhead flatworm (Bipalium adventitium) produces a very potent paralytic neurotoxin, tetrodotoxin, which is the toxin famously associated with human deaths from improperly prepared pufferfish.

(Disclaimer: I’m neither an entomologist nor a toxicologist, so I’m drifting out of my lane here.)

Headlines about this worm can be pretty sensational…

…but the buzz may be greater than the actual risk.

There’s been another round of new reports recently following identification of the hammerhead worm in southwestern Ontario this spring, but it’s not actually a new problem. The first report of these worms in Canada actually dates back to 2018 when they were found in Montreal. Some interesting crowd-sourced tracking of hammerhead worms shows that they’ve likely been present in Ontario for at least a couple of years (including just down the road from me).

What’s the actual risk from hammerhead worms in the environment?

It’s hard to say. These worms are small and the amount of tetrodotoxin toxin in them is limited. I haven’t found good data on how much worm exposure would actually pose a health risk to an animal. Hammerhead worms are present in other parts of the world, yet I can’t find any reports of disease in humans or animals linked to them, despite lots of media reports saying “they’re toxic to kids and pets.”

As they say, “absence of evidence” isn’t “evidence of absence” but a lack of reports of something as dramatic as acute paralysis suggests that the risk from exposure to these worms is limited. I’d still avoid eating hammerhead worms of course, and I wouldn’t dismiss the potential that ingestion of one or more worms by a small animal (and we’ll include kids in the small animal definition here) could cause a problem.

What should pet owners do?

  • Relax (as is often the first step with topics like this).
  • Ultimately there’s not a lot that can be done specific to these worms. The main prevention measures are awareness and avoidance. We’re concerned about the potential impact of ingesting or touching hammerhead worms, so try to avoid any direct contact with them; unfortunately that may be easier said than done in some cases, especially with dogs like mine (Labradors) that consider anything (organic or non-organic) to be a viable food source.
  • The good news is that these worms are pretty obvious if you find one (since they have a very unique head). The bad news is, as with most wildlife, if you see one, you can be pretty sure there are lots more in the area that you don’t see. If you come across a single hammerhead worm, they are probably already well established in your area.
  • If you know that this worm is in a particular area, avoid the area if you can, or at least prevent uncontrolled (e.g. off-least) access to it by your animals. Walking a dog through an area where hammerhead worms are present is low risk. Letting a dog root around in areas like that increases the risk of worm contact. Knowing the dog’s behaviour (and any tendencies to eat random things on the ground) also helps with the risk assessment and determining how strict to be about controlling animal access.

An advantage we have in Canada is our (historically) tough winters (yes, there is a bright side to really cold weather), because cold weather kills a lot of parasites. However, we’re losing some of the protective effects of winter with climate change. We’re seeing the potential for expanding ranges of various critters (large and small) and a greater ability of those critters to survive Canadian winters. Given the number of reports of hammerhead worms over the past few years, and the massive underestimation of how common any particular worm is based on the number of reports, we have to assume that hammerhead worms are well established in various parts of Ontario and Quebec (and maybe beyond), and that they’re probably here to stay, at least in some areas.  Common sense would dictate that we should raise awareness and take some basic measures to avoid contact with these worms. We should probably also add Bipalium-associated tetrodotoxin exposure to the differential diagnosis list in the very rare situation when we see unexplained acute paralysis (or weakness) in an animal (or child) with potential exposure to worms. My guess is that this is a minor- or non-issue around here, but more information would be nice.

Image from: https://en.wikipedia.org/wiki/Bipalium_adventitium – by Sanjay Acharya – Own work, CC BY-SA 4.0