Saturday, March 12, 2005
Bubonic plague, which was long thought to be the version that gave the "black death" is moniker, is transmitted by flea vomit. I kid you not.
A flea bites an infected host, drinks its blood, and ingests the plague pathogen, a nasty bacteria known as yersinia pestis to infectious disease gurus and medieval historians. Under certain conditions that can accelerate in damp, cold weather, the yersinia pestis living in a flea's gut will suddenly proliferate and fill up the digestive system of the flea. The next time such a "blocked" flea bites somebody and attempts to drink blood, there will be no room for the incoming food and the flea will regurgitate (that would be the delicate term for "vomit") the now infected blood back into the bitten victim. It has long been thought that it was by this method that plague raced through the population of Europe during the late Middle Ages, precipitating a demographic disaster that transformed European society.
We are apparently still experiencing its effects today. According to a recent article in the Journal of Medical Genetics($), the descendants of plague survivors may preferentially carry a gene that seems to confer immunity to HIV. The Nature article describing the study also exposes weaknesses in the long-held view that bubonic plague was the driving disease behind the Black Death.
Duncan admits that his theory is difficult to prove. But he argues that the outbreaks are easier to explain if one assumes that plague was passed directly from person to person as a virus, rather than the 'bubonic plague' that was caused by bacteria carried by rats and their fleas. "Rats are absolutely in the clear for Europe," he argues.
The article does not say whether Duncan believe that pneumonic plague, which is transmitted from human-to-human by coughing -- was more common than generally thought by historians.
Not only is this news interesting to people working to unravel the puzzle of HIV, it should rock the world of medieval historians who have long thought that they understood the pathological basis of the most significant demographic event in European history.
I must admit, I've never heard this before, but it's very interesting. Although I'm still confused as to how a bacterial infection could produce immunity for a viral infection.
More convincing is the evidence that shows we don't really know what caused the plague.
I first heard of the plague/HIV link on the PBS program "Secrets of the Dead" in an episode called "The Mystery of the Black Death". Evidently the delta 32 mutation is recessive and alters the surface of white blood cells. In an individual with two copies of the mutated gene, the white blood cell surface has no receptors for viral OR bacterial agents. Hence, this is not immunity, which requires an immune response, but insusceptibility.
I have also seen the theory (somewhere) that the Black Death spread too far too quickly to be plague, and that it might have been a bovine-derivative disease, such as anthrax. Can't find the source, though.
I remember reading about this in 'In the Wake of the Plague: The Black Death and the World It Made'
by Norman Cantor. I think he said that the immunity (or insuseptibility) surivives in about fifteen percent of the population, all descendants of European plague survivors. There's tons of other interesting stuff in the book. He also said that no epidemiologist has ever been able to account for the patterns and speed of the 1347 outbreak with respect to *any* disease or combination of diseases. Conventional wisdom says that it was a combination of bubonic and anthrax. It's not a new book, though, I'm sure more work has been done.
Grace, you're right! That was where I read the theory that the black death may not have been bubonic plague. I read Cantor's book a few years ago and lent it to somebody. I'm not sure I ever got it back...
Take a look at the book review in this week's New Yorker. I thought I was seeing double...
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