Seals, sea lions likely spread tuberculosis globally

Tuberculosis - one of the most persistent and deadliest infectious diseases in the world - may have spread globally via seals and sea lions, a new study suggests.
Scientists have long debated the origins of tuberculosis, a disease that kills one to two million people each year.
New research shows that tuberculosis likely spread from humans in Africa to seals and sea lions that brought the disease to South America and transmitted it to Native people there before Europeans landed on the continent.
"We found that the tuberculosis strains were most closely related to strains in pinnipeds, which are seals and sea lions," said Anne Stone, from Arizona State University School of Human Evolution and Social Change.

"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, and there was a complete replacement of the older strains when European disease reached the Americas during the age of exploration.

Researchers found that genomes from humans in Peru dating from about 1,000 year ago provide unequivocal evidence that a member of the tuberculosis strain caused disease in South America before Europeans arrived.
"The connection to seals and sea lions is important to explain how a mammalian-adapted pathogen that evolved in Africa around 6,000 years ago could have reached Peru 5,000 years later," said Johannes Krause of the University of Tubingen in Germany, co-principal investigator on the project.

Researchers collected genetic samples from throughout the world and tested those for tuberculosis DNA. Of 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.
They then focused on these three samples and used array-based capture to obtain and map the complete genome.
These were compared against a larger dataset of modern genomes and animal strains. Research results showed the clear relationship to animal lineages, specifically 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 said.

Africa has the most diversity among tuberculosis strains, implying that the pathogen likely originated from the continent and spread, researchers said.
After tuberculosis was established in South America, it may have moved north and infected people in North America before European settlers brought new strains in.
The research was published in the journal Nature.