Pathogens like viruses, bacteria, and fungi seem to jump between species quite often. Ebola, West Nile, Lyme disease, dengue, salmonella, and malaria are just some of the zoonotic diseases that are transmitted between animals and humans. Researchers used to think that their spread stops at the land-to-sea interface, but now, surprisingly high levels of a human bacterial strain have been found in gray seal pups in Scotland. The study, published in Molecular Ecology, is one of the first to establish a direct land-sea transfer of a human pathogen to marine wildlife.
As we expand and urbanize, sewage and wastewater inevitably pollute coastal environments. One well-known case of a terrestrial pathogen that made its way out to sea is Toxoplasma gondii: Sea otters can become sick with toxoplasmosis when cat litter containing parasites is flushed out to sea.
To explore the potential for human pathogens to spread to marine wildlife, Moredun Research Institute’s Johanna Baily and colleagues isolated the common zoonotic bacterium Campylobacter from gray seals (Halichoerus grypus) in a breeding colony on the Isle of May in Scotland. These coastal “hook-nosed” pinnipeds are an important bioindicator (or sentinel species) for environmental pollution—they’re like the canary in a coal mine, except for monitoring coastal contamination. People who become ill with the foodborne disease campylobacteriosis might get abdominal pain, fever, and diarrhea. It’s occasionally life-threatening if it spreads to the bloodstream.
They found Campylobacter jejuni present in half of all the pups sampled: 24 out of 50 dead pups and 46 out of 90 live pups. The infected dead seals also showed signs of intestinal inflammation, similar to the human symptom. “We suspected it was there, but we expected to find a very low prevalence,” Baily tells Science. “Campylobacter has been previously detected in seals at very, very low levels. The prevalence we found in gray seal pups was absolutely shocking.”
All 19 of the returning yearling animals, on the other hand, were negative for C. jejuni. That suggests the seals can become cleared of their infection while away from the localized source in the colony.
So, to get at the source, the team sequenced the genomes of 90 strains isolated from seals and compared those to 192 published genomes from multiple sources, including Campylobacter isolated from marine mammals, wild birds, agriculture (like livestock), and human clinical samples. Their analyses grouped 76 percent of the seal samples with those from human and agricultural samples.
“The findings are consistent with either a common source or direct transmission of human and gray seal Campylobacter infection,” Baily says in a news release. However, the genomic sequences of the seals’ bacteria were also very similar to those from poultry. And since we typically contract Campylobacter from eating infected poultry, it’s quite possible that humans and seals are both getting the bacteria from domestic birds. If that’s the case, agricultural runoff—and not direct contact—could explain why a human pathogen is found in seals.
Images: shutterstock.com (top), Johanna Baily (middle)
