Study Find Human Pathogens in Gray Seals


By Julianne Glaser

Zoonotic, or cross-species, diseases have plagued humans for centuries—from Ebola to malaria to bubonic plague. Scientists had believed that transmission of infectious diseases stopped at the water’s edge, making spread of disease between land and sea animals impossible. A recent study to collect health data on Scotland’s gray seals, however, revealed elevated levels of a human pathogen, shattering the previous understanding of pathogenic transmission

Terrestrial and Marine Transmission

Johanna Baily, a veterinary pathologist with the Moredun Research Institute in Penicuik in the United Kingdom, led an innovative study involving collection of microbiological samples from live and deceased gray seal pups on the Isle of May, an uninhabited island off the coast of Scotland. Baily and her team analyzed the samples to determine the presence of several pathogens and were surprised to find Campylobacter bacterium in almost half of the tested seals.


Primarily found in domestic livestock and wilds, Campylobacter can cause foodborne illness and inflammation of the intestines in humans. Dead seals with this bacterium also showed evidence of intestinal inflammation, suggesting that the pathogen could affect animal health as well as human health. According to Baily, “Campylobacter has been previously detected in seals at very, very low levels. The prevalence we found in these gray seal pups was absolutely shocking.”

Researchers subsequently compared the pathogen genome sequence in seals to potential animal and human source populations. The team reported in Molecular Ecology that the pathogens infecting seals were most similar to those found in sick humans.

What's the Source of the Bacteria?

Microbial ecologist Erin Lipp from the University of Georgia cautions that further research is needed to determine if humans are in fact spreading the bacteria to seals. The pathogenic sequence isolated from seals was also highly similar to those found in poultry. Thus, both humans and seals could be contracting the pathogen from contact with contaminated chickens, or agricultural runoff.

Nonetheless, the presence of human pathogens in marine wildlife is cause forof concern. “We’ve got a lot of gaps to fill in” to determine the potential for further terrestrial-marine transmission and how human pathogens might be affecting wildlife on a global scale, Baily said.

Extension Questions

  • What other ways can agricultural runoff impact the environment?
  • What other marine animals could possibly be affected by human pathogens?