A Sampling of Plague Throughout Time

By Hannah Steinberg ’16

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Facebook trends, Saturday October 31, 2015.

On Halloween day the number one trending topic on Facebook was “Crook County, Oregon: Teenage Girl Diagnosed With Bubonic Plague.” What is scarier and more holiday-appropriate than this? A young girl in the year 2015 contracted one of history’s deadliest diseases. A bacterium that wiped out a third to half of Europe’s population in the fourteenth century is alive and living here in the United States.


It turns out that nowadays plague, a disease caused by the bacterial species Yersinia pestis, infects about seven Americans every year, and over one thousand people worldwide. The most common form of plague is bubonic plague, which is characterized by the presence of buboes, or swelling of lymph nodes that appear like large blisters. With the application of modern-day antibiotics, the disease is treatable if caught early, and about 90% of those treated survive.

A flea infected with Y. pestis (National Institute of Allergies and Infectious Diseases)
A flea infected with Y. pestis (National Institute of Allergies and Infectious Diseases)

Plague is transmitted by fleas that live on rodents, and unlike many vector-borne diseases, it kills all of its host species (fleas, rodents, and humans). Y. pestis grows in a flea until it forms a biofilm in its foregut, which restricts food from entering the stomach, and causes the flea to starve. Because the flea is so hungry, it will bite any warm-blooded animal it can sink its teeth into, and it will bite a lot. This intense biting by starving fleas is what transmits most versions of plague (a rarer type, pneumonic plague, can be transmitted directly between humans through an infected person’s cough).

But is the Y. pestis alive today infecting teenage girls in Oregon really the same killer that was responsible for the millions of deaths during the fourteenth century in Europe? The plague that existed in the Middle Ages seems to have behaved differently than it does today in some important ways. One of the most revealing differences between ancient and modern plagues is their incubation times. The incubation period (the time between infection and symptoms) of plague now is rarely longer than six days. Primary sources from the fourteenth century however, show that travelers could trek for weeks from an infected area and not fall ill until their arrival in a previously uninfected community. This prompted some European cities to establish quarantines of newcomers, and these isolations were more effective if they lasted forty days as opposed to thirty days, suggesting that the incubation period of Y. pestis was up to seven times longer than it is now. The plague of medieval times has appeared so different to some microbiologists that they have even argued that the Black Death was caused by a different species all together.

Two studies published in 2010 and 2011, however, debunked any claims denying Y. pestis as the pathogen involved in the Black Death. These studies both involved excavating bodies from mass gravesites known to hold victims of the Black Death. The researchers extracted DNA from the skeletons’ bones and teeth and found concrete evidence that the victims were indeed infected with Y. pestis, but not necessarily the Y. pestis that exists today. The 2010 study found two strains of the bacterium present in different skeletons, and the 2011 paper discovered a third. None of the strains identified were previously described, and likely no longer exist.

Some genomic differences were reported between these ancient strains and modern ones, and more disparities likely exist, as the researchers could not assemble whole genomes from the ancient DNA. We do not know if the genomic differences identified thus far account for the discrepancies in virulence that are observed. It would not be a stretch, however, to speculate that the Y. pestis strains that were responsible for the Black Death had some sort of molecular difference from modern strains that would account for the changes we see in transmission and incubation.

Recent work has shown that at least 4,000 years before the Black Death another, older strain of Y. pestis was commonly infecting humans in a very different way than is observed today. New DNA evidence from millennia-old human teeth from Asia and Europe show that Bronze Age Y. pestis lacked crucial virulence genes that are needed to cause the buboes characteristic of bubonic plague. Furthermore, they lacked the genes necessary to infect fleas, and were thus likely transmitted only between humans.

It is clear that various strains of Y. pestis have presented themselves extremely differently throughout history. In the Bronze Age Y. pestis was transmitted from human to human and lacked the distinctive symptoms of modern plague. Today, plague is usually transmitted when an individual comes in contact with infected fleas, most often in rural, semi-arid areas. And in between these two variants of plague, the disease stormed rapidly through Europe, as well as parts of Asia and Africa, and had an incubation period up to seven times longer than observed today. It is likely that an accumulation of mutations and gene transfers account for the stark differences in the epidemiology of these strains and plague outbreaks throughout history. Taxonomists recognize all three of these pathogenic agents as part of the same species, yet we can ask whether this kind of lumping is beneficial to our understanding of plague throughout time.