Seals and sea lions may be behind human cases of tuberculosis, one of the most persistent and deadliest infectious diseases in the world, according to new research.
"We found that the tuberculosis strains were most closely related to strains in pinnipeds, which are seals and sea lions," researcher Anne Stone said in a university news release.
Tuberculosis kills two million Americans each year, but how did it first spread to the United States? Researchers from Arizona State University speculate that it was transferred from humans in Africa to seals and sea lions in South America, and then to Europeans once they colonized the United States.
"What we found was really surprising. The ancient strains are distinct from any known human-adapted tuberculosis strain," Stone added.
Modern strains of tuberculosis currently circulating are most closely related to those found in Europe, but previous research shows that older strains were around in South America - Peru, to be exact - 1,000 years ago before the Europeans even arrived, begging the question, "What types of tuberculosis strains were present before contact?"
In the study, researchers collected genetic samples from throughout the world and tested those for tuberculosis DNA. Out of the 76 DNA samples from New World pre- and post-contact sites, three from Peru around 750 to 1350 AD had tuberculosis DNA that could be used. After comparing these to animal strains, the results showed a clear relationship to seals and sea lions.
"Our results show unequivocal evidence of human infection caused by pinnipeds (sea lions and seals) in pre-Columbian South America. Within the past 2,500 years, the marine animals likely contracted the disease from an African host species and carried it across the ocean to coastal people in South America," Stone explained.
Scientists had previously believed that human migration events caused the rise of tuberculosis, but this study's findings suggest that the disease is much more recent than that.
Stone and her team note that their research can help scientists better understand how tuberculosis is transmitted and how it evolves, important questions to be answered given the fact that it is a disease that is on the rise again worldwide.
The study findings were published in the journal Nature.