Meet — but don’t touch — the toxic invasive worm that experts say has been hiding in plain sight

By Mindy Weisberger, CNN

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They’re long and skinny. They’re boomerang-headed and coated in toxic mucus. And they’ve been hiding in plain sight in the US for a long time.

Invasive hammerhead flatworms have distinctive curved heads, striped bodies ranging in color from light yellow to dark brown, and they can secrete tetrodotoxin — a neurotoxin found in puffer fish and blue-ringed octopuses. Some species are no more than an inch (2.5 centimeters) long, but others measure up to 15 inches (38 centimeters) in length. If you chop them into bits, each part will regenerate into a complete worm.

The worms are known across the southeastern US, and with recent hammerhead sightings in Washington, DC, New Yorkers and others in the Northeast Corridor may be wondering how long it will be before the toxic invaders inch their way farther north.

Trouble is, the worms are already right at home in New York state (and likely more of the country than we suspect) and have been around for decades, said Peter Ducey, a professor of biological sciences at the State University of New York at Cortland.

“These animals are widespread in New York,” Ducey told CNN. “They’re widespread and abundant.”

Their numbers may be on the rise in the Northeast, as climate change brings warmer temperatures and increased rainfall, enabling southern populations to expand northward, “but we don’t yet have enough data to tell,” Ducey said.

One reason why people may be more aware of the worms right now is their prominence in the news and on social media, which tend to favor stories about “unusual or scary creatures,” Ducey said, and these worms — and their potential impact on surrounding ecosystems — “are certainly unusual.”

Look, but don’t touch

Hammerhead worms are planarians, a type of flatworm. Five species of invasive hammerhead worms — four in the genus Bipalium and one in Diversibipalium — are established in North America, said Bruce Snyder, an associate professor of biology at Georgia College and State University. The worms originated in Southeast Asia and are thought to have arrived in the US in 1891 in landscaping material, according to the US Department of Agriculture’s National Invasive Species Information Center.

Today, most hammerhead worms (also known as broadhead planarians) are concentrated in the Southeast, where they favor warm, damp habitats.

“They’re in forests quite a bit, but they’re also associated with human development,” Snyder told CNN.

Because the worms tend to end up wherever soil is transported, “they’re in a lot of gardens and yards and around houses,” he said. “Within those habitats, they’re in leaf litter. You often find them under rocks or logs or pieces of trash.”

To date, more than 3,000 sightings in southeastern states of just one invasive hammerhead species — Bipalium kewense — have been shared to the citizen scientist database iNaturalist. But the worms are also present in California and Oregon, and since 2022 there have been reports of sightings in southern and central Maine, according to the Maine Department of Agriculture, Conservation and Forestry.

Hammerhead tetrodotoxin, which disrupts neurons’ signaling to muscles, can sicken pets if they eat the worms, according to the New Jersey Department of Environmental Protection. Direct contact with the worms can cause skin irritation in humans, but the effects may be more severe if toxins enter the body through a cut, Ducey warned.

“In my lab, my students and I wear gloves when we handle the worms,” he said. “We try to touch them as little as possible in general.”

Hammerheads prey on earthworms, mollusks and insects, and may use their venom to subdue prey or deter predators, according to the New Jersey department.

Widely distributed

Ducey began researching the worms in the 1990s, but hammerheads arrived in New York decades earlier; he recalls seeing them while growing up, in his Long Island backyard.

The species Bipalium adventitium was first detected in the New York area in 1947 by zoologist Libbie Hyman, who reported the finding in 1954 in the journal American Museum Novitates. She discovered a population of the worms in a garden in Westchester County, about 35 miles north of New York City.

Hyman also identified B. adventitium worms in California in 1943, “which showed the worm was already widely distributed across the country,” Ducey said.

However, after Hyman’s discovery “nobody looked for them very much,” he added.

Around 25 years ago, Ducey and students in his lab went searching for the two species most widely distributed in the US — B. adventitium and B. kewense. The scientists found the worms nearly everywhere they looked, even where hammerheads hadn’t been reported before.

And once such invasive species are successfully reproducing in the wild, they’re not going anywhere, Ducey said.

“The time to get them is before they even arrive at a new place. And then the time to stop them is right at the very beginning of those invasions,” he said. “There are very few invasive species that once they become established can be removed completely.”

Worm vs. worm

Invasive animals can disrupt ecosystems by eating native species or outcompeting them. B. adventitium preys mostly on earthworms. However, earthworms in the northeastern US are also invasive species from Europe and Asia, so it’s unclear what the environmental cost of this invader-eats-invader predation might be, Snyder said.

“We also don’t have great data on exactly which species of earthworms the different flatworms will eat,” he added. “So it’s very challenging to try and figure out how much of an impact they’re having.”

Of greater concern are the dining habits of the hammerhead species B. vagum, which eats snails and slugs. Some snail species in the southeastern US are endangered, “so that particular invading planarian might be the most damaging, even though it’s not as widespread as the other two,” Ducey said. However, the extent of this predation is unknown, and more research would be required to know if B. vagum poses a threat to endangered snails, he added.

For those looking to rid their yards or gardens of hammerheads, slicing up the worms isn’t recommended, as that only leads to more worms. Pennsylvania State University’s College of Agricultural Sciences suggests placing the worm in a plastic bag so it doesn’t crawl away and applying salt or ethanol — but very carefully.

“When handling flatworms, gloves should be worn or the hands washed after,” according to a fact sheet from the college. “The risk from prolonged exposure to chemicals in land planaria mucus is likely low but unknown, so caution should be used regardless.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.

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