Whooping cough was once one of the leading killers of babies around the world. Now that it's largely controlled with a vaccine, scientists have had a chance to figure out how the disease came into being in the first place.
That story is told in a study published online this week in the journal mBio. And it turns out that whooping cough arose quite late in human history.
Julian Parkhill, at the Wellcome Trust Sanger Institute in Cambridge, England, was one of more than a dozen scientists who pulled together a few hundred samples of the bacterium that causes whooping cough, Bordetella pertussis. The team's aim: to create a pertussis family tree.
It appears to have evolved from a single event, and that event was the adaptation to the human population.
"It's very much like doing a genealogical study and [being able to] say I've identified my great-great-grandfather, who lived between these dates," Parkhill tells NPR.
In this case, the scientists discovered that the ancestor of modern-day pertussis emerged suddenly in the 15th or 16th century.
"That isn't very long ago," Parkhill says. "And really, that was quite a surprise."
The genetic analysis is consistent with historical documents. The distinctive cough with its "whooping" sound was mentioned in a Korean medical text from the 15th century. The first documented epidemic arose in Paris in 1578. Before that, there was no sign of such a distinctive disease.
"It's not mentioned in any of the Ancient Greek or any of the Ancient Egyptian records, or anything like that," Parkhill says.
A close reading of the genes also provides clues about where the human disease came from.
"It appears to have evolved from a single event, and that event was the adaptation to the human population," he says.
It seems that a closely related bacterium, found in dogs, rabbits and many other animals, mutated. That change turned the bacterium into one that spreads easily from person to person and causes that agonizing cough.
The genetic analysis suggests that event happened several thousand years ago. But for whatever reason, the highly contagious disease remained hidden for thousands of years before it burst out as a full-fledged human disease.
That has implications for controlling the disease today.
The bacterium continues to evolve, and one consequence is the current vaccine isn't as effective as it used to be. Parkhill says it's time to use this new genetic information to tune up the vaccine.
"For influenza we come out with a new vaccine every year or two," Parkhill says. Fortunately, pertussis changes much more slowly, so it's much easier to keep up with it.
The current vaccine has been highly successful. Though there have been a few notable outbreaks, the federal Centers for Disease Control and Prevention estimates that the vaccine has prevented 54 million cases of pertussis in the United States since 1994.
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