‘Weird Looking’ Otter Poo Reveals an Unexpected Role in Parasite Control

Receiving an unsolicited photo of worm-infested animal poop would make anyone cringe, unless you’re a parasite expert like Katrina Lohan. When a colleague sent her a snapshot of a watery pile of feces with a fire-engine red worm inside, she was instantly intrigued.

“She sent this to me and was like, ‘I think this is a parasite, are you interested in studying river otters?’ And I was like, ‘Ooh, I think that’s a parasite too. And yes, I am,’” Lohan, a parasite ecologist who leads the Coastal Disease Ecology Laboratory at the Smithsonian Environmental Research Center (SERC), told Gizmodo.

Her colleague discovered the otter scat on a dock at the SERC campus on the Chesapeake Bay. Scientists know surprisingly little about river otters that live in tidally influenced coastal areas, but these predators play a vital role in ecosystems. Studying the parasites in their poop would offer valuable insight into their dietary habits and the health of their habitat. So, Lohan teamed up with Calli Wise, a biological research technician at SERC and first author of the new study published in the journal Frontiers in Mammal Science on Thursday, August 14.

Wise was particularly well-suited for this study, as she’d already conducted research on river otter latrines. Yes, you read that right, but wipe that image of a tiny, adorable outhouse from your mind. Otter latrines are essentially on-land congregation areas, Wise told Gizmodo. River otters don’t just relieve themselves at these hubs but also eat, play, and lounge there. These animals are nocturnal and semi-aquatic, which makes it difficult to observe them in their natural habitat, but researchers can learn a lot from the stuff they leave behind at latrines.

Wise and her colleagues hunted for latrines along roughly 7.5 miles (12 kilometers) of the shoreline of the Rhode River, a tidally influenced subestuary of the Chesapeake Bay. “You’re looking for areas of disturbance where it looks like an animal could have gone from the water to land,” Wise explained. “Actually, beyond just looking, my best sense was my nose. You can kind of smell out a latrine,” she said. Go figure.

The researchers collected scat from 18 active latrines and brought it back to the lab. In addition to observing samples under the microscope, they performed DNA analysis using metabarcoding. This DNA sequencing technique involves extracting small snippets of DNA, amplifying them, then comparing them to a database of snippets from known species. This allows researchers to identify species present in complex biological samples.

“This method has never been used on scat left at latrines in general, for diet or parasites,” Wise said. She and her colleagues found DNA from a wide variety of parasites spanning six taxonomic classes in the scat samples. Most of the species they identified infect river otters’ prey, but there were also some that infect otters themselves.

“The fact that this method provided such a level of resolution that we could not only figure out what it was they were eating, but also see the parasites in their prey, was really cool,” Lohan said. What’s more, the findings suggest that parasites play a key role in river otters’ diets. When they infect prey species, their negative impact on the host’s health may make it easier for river otters to hunt them, Lohan explained. Thus, these predators may be culling infected individuals from populations of their prey, but confirming this will require further research, she said.