Many infectious diseases—such as Ebola—can be transmitted between humans and animals. Researchers have long assumed that the spread of these so-called zoonotic diseases usually stops where land and water meet, however. Now, in one of the first studies to establish the land-to-sea transmission of a human pathogen, scientists have detected high levels of a human bacterial strain in gray seals. The study also suggests these bacteria may make some seals sick.

Terrestrial pathogens have made their way into marine waters before. Toxoplasma gondii, a parasite found in cat feces, for example, can infect sea otters when kitty litter gets washed out to sea. Few studies have found a similar route of transmission for human diseases to marine animals, however. Now, by employing genome sequencing and statistical models used to track the source of pathogens, researchers have found the strongest evidence to date that bacteria detected in a population of gray seals originated from a human source.

In the new study, a team led by Johanna Baily, a veterinary pathologist with the Moredun Research Institute in Penicuik, U.K., set out to collect baseline health data on gray seals breeding on Scotland’s Isle of May—an uninhabited island and nature reserve that sits just 8 kilometers off the mainland. During the fall of 2011, the team collected microbiological samples from nearly 100 live and 50 dead seal pups on the island. The researchers screened the samples for several pathogens including Campylobacter—a bacterium commonly found in wildlife and domestic livestock that triggers foodborne illness in humans. Symptoms of infection in people include cramps, diarrhea, and abdominal pain, all of which stem from inflammation of the gut.

To their surprise, not only did the researchers detect Campylobacter in the seals, but they also found an exceptionally high prevalence of it within the colony. The pathogen was detected in nearly 50% of the seals tested. Dead seals harboring Campylobacter had signs of intestinal inflammation, suggesting that the pathogen could affect the animals’ health.

“We suspected it was there, but we expected to find a very low prevalence,” Baily says. “Campylobacter has been previously detected in seals at very, very low levels. The prevalence we found in gray seal pups was absolutely shocking.”

To determine the source of the infection, the team sequenced the genomes of the Campylobacter strains isolated from seals. Next, they compared bacterial sequences from seals to those from potential source populations including poultry, livestock, wild birds, and humans using statistical models. The pathogens infecting seals were most similar to those found in sick humans, the team reports this month in Molecular Ecology.

But, it’s not clear yet whether humans are actually responsible for spreading the bacteria to seals, says Erin Lipp, a microbial ecologist at the University of Georgia, Athens, who was not involved in the study.

The genomic sequences of the bacteria isolated from seals were also highly similar to those found in poultry. Because humans typically contract Campylobacter infections through the consumption of infected poultry, both humans and seals could be acquiring the bacteria through contact with the domestic birds. Agricultural runoff from poultry farms could serve as one potential pathway for the birds to spread the bacteria to seals, without direct contact between the two.

For now, it is unknown how seals are contracting the pathogen. “Are the seals swimming in areas impacted by sewage or wastewater? Or is it that a carrier, such as a wild bird, is bringing the infection back to the island?” adds Dale Griffin, a public health microbiologist at the U.S. Geological Survey in St. Petersburg, Florida.

The most likely source of the bacteria is urban or agricultural runoff, but the researchers need to start screening environmental samples before they can say for sure, Baily says. “We’ve got a lot of gaps to fill in.”

The findings are ringing alarm bells; we shouldn’t be finding human pathogens in these seals, Baily says. The study raises important questions about how pervasive human pathogens are in the marine environment and what their impacts are on wildlife.