Scientists have long thought that animals in Antarctica were isolated from harmful human-linked bacteria, but a new study provides evidence that this is no longer the case.
Previous studies had found scattered instances of transmission of bacteria from humans to animals, known as reverse zoonosis, in the region, but research remained fragmented.
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Now, however, researchers have found widespread evidence of human-linked pathogens among Antarctic seabirds for the first time -- which they say could have devastating consequences for the continent's wildlife.
"This is the first time that such a wide-ranging study, in terms of geography and bird species, has been carried out in the Southern Ocean, which shows reasonably solid evidence of reverse zoonosis in the Antarctic," study author Jacob González-Solís, a researcher in the Department of Zoology and Biological Anthropology at the University of Barcelona, wrote in an email.
Birds including penguins, brown skuas, southern giant petrels and kelp gulls were found to have picked up bacteria such as campylobacter and salmonella, according to the study, published in the journal Science of the Total Environment.
The team took samples of feces from more than 600 adult seabirds in four locations -- Livingston Island, Marion Island, Gough Island and the Falkland Islands -- between 2008 and 2011, with three findings suggesting reverse zoonosis.
Samples showed campylobacter jejuni, a common cause of food poisoning in the United States and Europe, including genotypes that had rarely or never been found in wild birds before.
Others contained campylobacter lari, common in skuas and gulls. However, the team found that these strains were resistant to commonly used human and veterinary antibiotics ciprofloxacin and enrofloxacin, which suggests contamination from humans or domestic animals.
Researchers also found a strain of salmonella usually detected in scavenging birds that live in urban areas.
Although these bacteria are not associated with high death rates in animals, their presence shows that other, more dangerous pathogens could arrive on the continent, said study author Marta Cerdà-Cuéllar, a researcher at the Institute of Agrifood Research and Technology in Barcelona.
"If these pathogens were able to arrive, so will others," Cerdà-Cuéllar wrote in an email. "We can say that any [Antarctic] bird or mammal could be affected by a zoonotic agent."
And that could have devastating consequences.
"This means that sooner or later human activity will introduce pathogens to Antarctic fauna that could cause mass deaths and even local extinctions," González-Solís said.
Although the study says evidence of reverse zoonosis is fairly solid, the authors are not sure how the birds came into contact with the bacteria.
"There are various possibilities, the most likely is contact between Antarctic and sub-Antarctic fauna with domestic birds in sub-Antarctic communities such as the Falklands, but they could also be the legacy of old whaling missions, Antarctic research stations and the growth in Antarctic tourism," González-Solís said.
With increasing numbers of tourists potentially to blame, the authors recommend tighter controls on visitors.
"To prevent the arrival of pathogens, stricter biosecurity measures will be necessary in order to limit the impact of humans in Antarctica," Cerdà-Cuéllar said.
According to Jonas Bonnedahl, lecturer in clinical sciences in the Department of Clinical and Experimental Medicine at Sweden's Linköping University, humans have spread pathogens in the Antarctic that have, on rare occasions, spread to wildlife.
"I also think it is fair to say that it is the permanent research bases that have to be blamed here, rather than the tourism industry," Bonnedahl, who has researched human-linked bacteria in the Antarctic and was not involved in the study, wrote in an email. "Much has happened in the last years and I think that most nations now put more effort in biosecurity concerning sewage from research bases."
However, he agrees that the rise in tourist numbers is a concern.
The number of visitors to Antarctica has been steadily creeping up in recent years, with 44,367 tourists traveling there during the 2016-17 season.
These revelations about the potential impact of tourism in Antarctica add to growing concern over visitor numbers in other parts of the world.
The Galápagos Islands off the coast of Ecuador are arguably the most famous natural habitat in the world, and visitors are limited to specific sites and marked trails only, and a guide is required at all times.
The Himalayan kingdom of Bhutan is another destination that attempts to control visitor numbers, with a "high value, low impact" tourism policy that sees visitors charged $200 or $250 per day, depending on the time of year.
Locals have cited concerns about the environmental impact on its fragile ecosystem, as well as an over-reliance on foreign visitors.
And South Africa's famous Otter Trail coastal hiking route has such a limited number of places available that hikers have to book up to a year in advance.