TB has human, not animal, origins - says study

The origins of human tuberculosis have
been traced back to hunter-gatherer
groups in Africa 70,000 years ago, an
international team of scientists say.
The research goes against common belief that
TB originated in animals only 10,000 years ago
and spread to humans.
The work, published in Nature Genetics,
outlines the strong relationship between the
evolutionary history of both humans and TB.
The disease causes more than one million
deaths every year.
Previous research has indicated that human TB
originated about 10,000 years ago in Africa
during the Neolithic Demographic Transition
(NDT), when the human population was
expanding and agriculture was becoming
prominent.
Researchers combined geographic and genetic
data from 259 strains of TB to reconstruct its
evolutionary history and compare it to the
origins of humans in Africa.
Prof Sebastian Gagneux, from the Swiss
Tropical and Public Health Institute, said: "We
found that the most basal - the earliest -
lineages of TB and humans originated in the
same place, in Africa, 60,000 years earlier
than what people previously thought.
"What we have done is provide a strong
hypothesis to reinforce the idea that TB
originally started in humans, and migrated to
animals during NDT."
Killer
The question the scientists are now trying to
answer how TB managed to survive 60,000
years among small groups of people.
A striking feature of TB, which is not common
in other diseases, is that people can be
infected with it for years before showing any
symptoms. The disease is able to reactivate
itself after a certain time period.
This latency is what the researchers suggest
kept TB alive during early years.
Prof Gagneux said: "If there are only a few
people to infect, it makes no sense to kill
them, as you would risk killing itself too. It's
an evolutionary dead-end."
Once the human population started expanding
during the NDT, TB became more active and
was able to spread farther.
So, as the number of hosts increased during
and after the NDT, so did TB's drive for
increased virulence.
"The next step in this research would be to use
genetic information to understand this
activation and deactivation mechanism of TB,"
said Dr Inaka Comas, lead author of the
research.
TB remains a global threat, causing 1.4 million
deaths in 2011, according to the World Health
Organisation. If scientists can understand how
TB and humans co-developed, it may help find
a way to reduce its prominence.
Dr Ruth McNerney, a lecturer in pathogen
biology and diagnostics at the London School
of Hygiene and Tropical Medicine, said the
study could lead to further developments in
understanding the movements of TB.
"There are now thousands of TB genomes
being sequenced in big databases so that in the
next five years we'll know more about TB than
we ever have, which is exciting.
"This is the first step in that area of research."