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The Transmission of Plague: Identification of the Cause - The Great Courses Daily News
In the 19th century in Asia, plague caused death, suffering, and panic. But it is the 19th-century plague—and modern medicine's attempts to understand it—that would ultimately offer some answers about the Black Death and the Plague of Justinian that occurred in the 6th century.
The plague bacterium was identified during the plague outbreak in Asia in the late 19th century. (Image: Kateryna Kon/Shutterstock)
Isolating the Plague Bacterium
In 1894, in Canton, China, a plague outbreak began, which quickly spread to Hong Kong through the constant trade traffic. Estimates suggest that somewhere between 50,000 and 125,000 people were infected in this massive outbreak; 80% of those who contracted the plague would die from it.
Two scientists working in Hong Kong—Shibasaburo Kitasato and Alexandre Yersin—almost simultaneously managed to isolate the cause of plague in the laboratory after careful examination of tissue samples of those infected.
Kitasato was a little quicker in discovering the source of the plague, but Yersin's description of the bacterium was more thorough and accurate. And, from 1894 on, the bacterium that causes plague has been called Yersinia pestis in his honor. A few years after isolating and identifying the bacillus, Yersin identified rats as the prime carrier of the disease.
In 1898, a scientist named Paul-Louis Simond argued conclusively that the disease is transmitted to humans when fleas jump from a rat to a human being and bite that human being. What this means is that the plague is zoonotic: like smallpox and some other diseases, it originates in animals, and then jumps from animals and infects humans.
Many kinds of rodents can carry plague, and the fleas that feed on rats, guinea pigs, and other similar animals, like squirrels, can become infected with plague.
But just because a flea is infected, it doesn't mean that it can cause infection. Fleas become infective due to a feature of their alimentary system. They have something called a proventriculus, which acts as a kind of valve that regulates the food that the flea is ingesting and trying to get to its stomach.
When a flea feeds on a plague-infected rodent, a blockage of bacteria and blood forms in the proventriculus, so that nourishment can't get to the flea's stomach. Now you've got a very hungry flea who starts biting more aggressively and frequently in order to get some nourishment, but the blockage in the proventriculus just gets bigger and bigger. Finally, the flea's system realizes what's happening, and regurgitates the blockage out of the proventriculus.
The plague bacteria is transmitted when a flea bites an infected rat, and subsequently bites a human. (Image: Vera Larina/Shutterstock)
And where does it go? Well, it goes directly into the system of whatever the flea is feeding on, which could be a human being.
Studies conducted in the 1970s suggested that it was crucial that the biting flea be a rat flea, and also that the fleas typically found on humans don't really transmit plague.
The proportion of plague in the blood of an infected human didn't seem to be enough to cause a blockage in the digestive system of fleas that are usually found on people. So, the theory went, you had to have rats as hosts, and then those hosts needed to die, so the rat flea was forced to find an atypical food source, such as humans.
This is why, when you go hiking in the mountains, you sometimes encounter warning signs at trailheads advising you to stay away from animals, especially those of the rodent variety, and especially those that are dead, because a dead animal is no longer a viable host for the fleas that have been feeding on it. And even if they are rat fleas who don't prefer humans, in a pinch, they'll happily jump to a hiker or camper who happens to be in close proximity.
The Evolution of the Plague
Later research into the evidence from the Justinian Plague and the Black Death seems to indicate that most of the infections of these plagues seem to follow the same pattern of the modern plague. There were, of course, some differences, which seem to indicate some changes in the nature of the infection.
However, we can say that although the same bacillus seems to have been primarily responsible for the three plagues, their epidemiology and etiology seem to differ a bit, suggesting that the bacterium itself has undergone some evolutionary shifts at different times in its existence.
For example, in England toward the end of the 14th century, there was a wave of plague that seemed to target only the healthiest people. The elderly and the very young, who were usually the first victims of an outbreak because they were either the weakest or had the least developed immune systems, were spared in this outbreak, but healthy people in their late 20s were almost completely wiped out.
So why isn't plague happening all the time, and everywhere? Why did it show up in such dramatic fashion in the 6th century, disappear for the better part of a millennium, reappear in the 14th century, disappear again, and then, about 600 years later, reappear briefly in the modern period?
Of course, plague still exists today. Every year, there are about 5 to 10 cases in the U.S., and, in 2015 there were at least 15 cases, usually due to people coming into contact with plague-infected rodents in mountain or wilderness areas.
And if the doctors can figure out what it is quickly enough, the plague can usually be easily cured with a course of antibiotics, usually streptomycin or gentamicin, although a few others are sometimes used. Unfortunately, because plague is so rare these days, doctors don't always recognize it when it appears, and there have been some deaths from plague in the U.S. in the last several decades.
It was due to the plague outbreak in India and China at the end of the 19th century that medical science was able to identify the cause of plague and figure out its modes of transmission.
And, doing some historical detective work, scholars seem pretty certain that the two earlier, more devastating outbreaks of the disease in the Western world are a pretty good match for the epidemic that occurred in 1894 in Asia.
Common Questions about the Impact of Plague
Q. Who isolated the plague bacterium?
The plague bacterium was isolated by two scientists working in Hong Kong—Shibasaburo Kitasato and Alexandre Yersin—in 1894.
Q. How does the plague jump from rats to humans?
When a flea feeds on a plague-infected rodent, a blockage of bacteria and blood forms in the proventriculus, so that it bites more aggressively. The flea regurgitates the blockage out of the proventriculus, directly into the system of whatever the flea is feeding on, which could be a human being.
Q. Did the plague remain the same over the centuries?
Although the same bacillus seems to have been primarily responsible for the three plagues, their epidemiology and etiology seem to differ a bit, suggesting that the plague bacterium itself has undergone some evolutionary shifts at different times in its existence.
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