Study Highlights Unexpected Ecological Benefits of Parasite-Ridden Diets

Edgewater, Md. — August 14, 2025 — North American river otters may not be known for their table manners, but their messy eating habits could make them valuable allies in tracking environmental health threats. A new study from the Smithsonian Environmental Research Center (SERC) reveals that the parasites found in the otters’ prey may serve as important indicators of ecosystem changes.

The study, published in Frontiers in Mammal Science, is the inaugural thorough examination of otter diets and latrine environments in the Chesapeake Bay area.

“River otters are impressive apex predators that play a vital role in ecosystems,” said Calli Wise, lead author of the study and a research technician at SERC. “The parasites consumed by river otters may also teach us about the health of the environment.”

Elusive Predators with a Hidden Role

River otters are among the Chesapeake’s most secretive residents. Nocturnal, semi-aquatic, and wary of humans, they are rarely spotted in the wild. Historically abundant across North America, their populations declined sharply due to the fur trade and habitat loss. A reintroduction program in Maryland during the mid-1990s helped restore their numbers, but population estimates for the Bay remain uncertain.

“It is shocking how little information there is about their biology and ecology,” said Katrina Lohan, co-author and head of SERC’s Coastal Disease Ecology Lab.

Studying Otters Through Their “Latrines”

Because direct observation is difficult, researchers study river otters through the feces they leave at “latrines”—land sites where otters gather to eat, socialize, and scent-mark.

In this study, scientists collected scat samples from 18 active latrines on the SERC campus in Edgewater, Maryland. While most sites were natural (beaches and riverbanks), some were on manmade structures such as docks and boardwalks.

The team analyzed the samples under a microscope and used DNA metabarcoding to identify prey species and parasites.

Findings: A Parasite-Rich Diet

DNA results showed that otters’ diets were dominated by finfish and crabs, making up 93% of all prey items. They also consumed amphibians, worms, birds, and even invasive species such as common carp and southern white river crayfish.

Parasites from six taxonomic classes were identified, with trematodes (parasitic flatworms) being the predominant type. Additional identified parasites were microscopic dinoflagellates and flatworms impacting fish gills, skin, or fins.

Most parasites likely came from infected prey and did not harm the otters. In fact, researchers believe the otters’ predation may benefit ecosystems by removing infected individuals from prey populations.

“While parasites have negative impacts on individuals, they are extremely important in food webs,” Lohan explained. “It is possible that river otters, like other top predators, wouldn’t be able to find enough food to eat without parasites.”

Potential Human Health Implications

Some parasites found—such as roundworms and single-celled apicomplexans—likely infected the otters themselves. None detected are known to infect humans directly, but some are closely related to parasites that can cause diseases in people, including cystoisosporiasis, a gastrointestinal illness.

As otters increasingly occupy urban waterways, they may encounter pollutants and pathogens of greater concern to humans.

“As river otters move into more urban waterways, they will be increasingly exposed to pollutants and parasites of concern to humans,” Wise said. “As mammals, river otters may be disease sentinels that we can study to learn more about environmental risks to humans.”

Key Findings

• Study area: 18 active otter latrines on SERC campus in Chesapeake Bay.
• Main diet: 93% finfish and crabs; also amphibians, worms, birds, and invasive species.
• Parasites detected: Six major types, predominantly trematodes.
• Ecological impact: Otters may help manage prey populations by targeting parasite-infected individuals.
• Human health link: No human-infecting parasites detected, but related pathogens found.
• Urban expansion risk: Otters could serve as sentinels for monitoring environmental health threats in urban waterways.

Original Publication
Journal: Frontiers
DOI: 10.3389/fmamm.2025.1620318
Method of Research: Observational study
Subject of Research: Animals
Article Title: North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay
Article Publication Date: 14-Aug-2025

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