The Black Death: Symptoms and Diagnosis of The Plague

From the Lecture Series: The Black Death — The World's Most Devastating Plague

By Dorsey Armstrong, Ph.D., Purdue University

In the late 1800’s, multiple outbreaks of plague popped up in China and India. While these outbreaks were not as severe as either the 6th-century Plague of Justinian or the 14th-century Black Death, they were still pretty scary. Estimates suggest that somewhere between 50,000–125,000 people were infected, and 80 percent of those who contracted plague would die from it.

What resulted from these 19th century outbreaks was not just the horrors of disease, but also a modern understanding of what the plague is, and how it spreads.

Image of a flea carrying the plague
Xenopsylla Chepsis (Oriental Rat Flea)

The Plague Bacillus is Identified

The year is 1894. Two scientists working in Hong Kong—a Japanese student named Shibasaburo Kitasato and a Swiss-French student named 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 plague, but Yersin’s description of the bacterium was more thorough and accurate, so to him went the naming honors of this newly discovered pathogen. From 1894 on, the bacterium that causes plague has been called Yersinia pestis in his honor.

This is a transcript from the video series The Black Death: The World’s Most Devastating Plague. Watch it now, on The Great Courses Plus.

A few years after isolating and identifying the bacillus, Yersin identified rats as the prime carrier of the disease, and 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 plague is zoonotic: like smallpox and some other diseases, it originates in animals, and then somehow jumps from animals and infects the human population.

Learn more about the enormity of the Black Death’s impact on the medieval world

How The Plague Travels

Here’s how it works. 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 doesn’t mean that it’s infective.

The way that fleas become infective is due to a feature of their alimentary system—they have not only a stomach, or a ventriculus, but also 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, the nourishment doesn’t pass to the ventriculus as quickly or as easily as it would if a flea were feeding on a non-infected rodent; in fact, what happens is that 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. And where does it go? Well, it goes directly into the system of whatever the flea is feeding on—and if it’s a human being, and the flea has jumped there from a black rat, then the starving flea will aggressively regurgitate and feed.

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 a food source that it would not typically have chosen—and in this case, that source was humans.

Learn more about how it spread through medieval Europe with such astonishing speed and virulence

What Are Symptoms of the Plague?

So, let’s say you’ve been bitten by an infected and infective rat flea; what happens next?

Well, in most people, large swollen areas develop around the lymph nodes, usually at the neck, groin, and armpits. These lumps are called buboes, and it is from this word that we get the most common name we use for the Black Death: the bubonic plague. In instances of the bubonic form of the plague, human-to-human transmission seems to be almost impossible, although it may have occurred in some instances when doctors or caretakers tried to effect a cure by lancing the buboes.

First person accounts of this process from the period indicate that the pus that came out when this operation was performed was disgusting not only in appearance but also, and particularly, in terms of the smell. A few accounts relate that the doctor and others in the room were so overcome by the stench that they often fainted or vomited. But again, this seems to be the one form of plague that you might, just might, survive.

Now, here’s the good news—if you get the bubonic form of the plague, you have around an 18 percent chance of surviving. While that doesn’t sound terribly encouraging, it’s much better than the zero percent chance you have of surviving the two other forms, pneumonic and septicemic.

Learn more about how the plague traveled by sea across the Mediterranean

Pneumonic Plague

Pneumonic plague was the second most common form of plague, and, as its name suggests, in this case, Yersinia pestis has set up shop in the sufferer’s respiratory system, rather than in the lymphatic system, as is the case with the bubonic form. It starts, usually, with a patient zero who’s been infected with the bubonic form of the disease, which then makes its way from their lymphatic system into their respiratory system.

What is terrifying about this form of plague—and that’s not to say that the other forms aren’t also terrifying—but what’s uniquely terrifying about this form of plague is that it is now easily transmissible from person to person. A doctor or friend or relative taking care of someone infected with the pneumonic form of plague is going to be coming in contact with blood, sputum, saliva, all containing the bacterium, and they will usually themselves become infected.

Also horrifying—the way you died was usually because you were drowning in your own blood. Now the good news here is that from onset of symptoms to death is usually just two days. The bad news—the suffering is intense, and there’s less than one percent survival rate, and even that statistic might be a tad optimistic. I don’t want to contemplate how long those two days must seem to a person who is dying of pneumonic plague.

Learn more about the most advanced community in medieval Europe, dealt with the crippling effects of the plague

A scanning electron micrograph depicting a mass of Yersinia pestis bacteria in the foregut of an infected flea
A scanning electron micrograph depicting a mass of Yersinia pestis bacteria in the foregut of an infected flea

Septicemic Plague

The third and least common form of plague is that known as septicemic, which is an infection of the blood.

Like pneumonic plague, this form can start out as bubonic and then the infection can move to a different bodily system. When plague bacteria enter the bloodstream, they cause something known as disseminated intravascular coagulation, or DIC. In these instances, tiny blood clots start to form throughout the body, which results in something called localized ischemic necrosis, which is just a fancy way of saying that portions of your body tissue start to die off due to lack of circulation.

I know this sounds super pleasant, and it gets better: if you’ve got septicemic plague, and it’s pretty well advanced, your blood starts to lose the ability to clot properly. If your blood doesn’t clot, it starts to seep into other parts of your body, like your skin and internal organs. This produces red and black patchy rashes and bumps on the skin that look rather like lots and lots of pimples, but all over your body. Most scholars think that these visible indicators are what medieval people meant when they said of a dead person that he or she bore “the sign of the plague”.

A final common sign of advanced septicemic plague is the vomiting of blood. But, taking the optimists’ perspective, here’s the good news: if you’ve contracted septicemic plague, you could die within 24 hours of showing symptoms. In some cases, people are reported as having been feeling fine at nine am, not so good at noon, and dead at four.

Learn more about subsequent occurrences of plague across Europe following the Black Death of the 14th century

Does The Plague Still Exist Today?

Every year there are about 5–10 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. 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.

Common Questions About the Plague

Q: Is the plague still a threat that exists?

The plague does still exist and is becoming a bigger threat in recent years. In fact, in the U.S. alone, the plague is seen around 7 times each year and growing.

Q: How did the plague start?

The plague is thought to have begun in Central Asia on the Silk Road, where it eventually set sail on rats with infected fleas on ships leaving Crimea.

Q: What is it like to die by the plague?

Death by the plague is systematic. It begins in the lymph nodes, which swell painfully, and it then travel to the lungs. At this point the body becomes overwhelmed by the toxins, septic shock officially begins, and the body begins to kill itself by allowing the blood vessels to leak, which lowers blood volume. Organ failure happens fairly quickly and painfully after this.

Q: Can the plague be treated or cured?

The plague can be treated with multiple antibiotics. In cases which are treated, the mortality rate is around 10%, and without treatment, the mortality rate is around 30-90% or near fatal.

This article was updated on 12/6/2019

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