About 5,000 years ago, a bacterium that was primarily transmitted via ticks made a switch to louse. That may have been a strategic evolutionary opportunity, since the change from tick to louse as a vector coincided with human adaptation of wool textiles, more densely populated cities, and a more sedentary lifestyle, according to a paper in the journal Science.
“We and our pathogens are very intrinsically linked,” says Pooja Swali, a research fellow at University College London and an author of the paper. “What we are doing as humans makes a difference to the transmission dynamics of the pathogens we carry. B. recurrentis might be a very good example of where changes in our behavior around 5,000 years created an environment which allowed B. recurrentis and its louse vector to flourish.”
Ancient Ticks and Lice
To this day, the louse route remains unusual. Ticks still spread the majority of relapsing fever bacteria that infect humans. Therefore, the researchers were interested in what made B. recurrentis one of the few exceptions.
To better understand this adaptation, they sequenced B. recurrentis dating back from about 2,300 years to 600 years ago. Samples came from throughout England, including Wetwang Slack (2,250 years to 2,050 years ago); Fishmongers Swallet (2,300 years to 2,100 years ago); Poulton (733 years to 633 years ago); and Canterbury (736 years to 563 years ago).
When they compared the sequences to other organisms, they found that the closest relative was B. duttonii, from about 4,700 years to 5,600 years ago. They saw an overall loss of genes — especially of ones that would normally help the human immune system detect them. In essence, B. recurrentis became better at hiding in humans over time.
“We show that much of this reduction in genome size, which presumably started around 5,000 years ago, had already occurred by our oldest sampled infection,” Swali says.
Read More: What Were Stone Age Bacteria Like? We Now Have the First Clues
Bacteria and Human Hosts
Over time, B. recurrentis got so good at specializing in using the human body louse as a vector and the human as a host, it has so far not been detected in any other animal. Animal hosts have been found for two other bacterium that also switched from tick- to louse-borne: Bartonella quintana (trench fever) and Rickettsia prowazekii (endemic typhus).
“Interestingly, unlike the other louse-borne pathogens, Borrelia recurrentis has specialized to a human body louse and has only been identified in human hosts,” Swali says. “To date, it has not been found in animals, which brings to question when this transition occurred, and what human behavior may have resulted in this change."
Identifying and Controlling Infections
While discovering the human social and cultural forces behind B. recurrentis evolution and transmission change is historically interesting, it could also be medically useful. For example, understanding disease transmission routes can help identify and control infections — potentially even preventing a pandemic.
“Any change in transmission dynamics is interesting but also super important when it comes to infection control,” Swali says. “Knowing what causes bacteria to adapt to different mechanisms of spread in an ever-changing climate means we can try and be better prepared.”
Read More: The "First Predators" Ruled a World Full of Bacteria
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
