Digging up the Black Death: What Caused the Black Plague, and Are We In Danger?

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Home / Digging up the Black Death: What Caused the Black Plague, and Are We In Danger?
The black rat can be infected with the black plague and spread the disease via fleas. Image by the CDC

The black rat can be infected with the black plague and spread the disease via fleas. Image by the CDC

Archaeologists in London have found remains of victims of the black plague, dating back to 1348-1350.

Between 1347 and 1351, the “Great Pestilence” or Black Death, as it was later called, swept across Europe and killed millions of people.

Now, however, archaeologists are finding out what they thought they knew about the black plague isn’t actually right.

Plagues: Pneumonic,  Bubonic, and Septicemic

The plague is a bacterial disease caused by Yersinia pestis, which generally effects wild rodents, like rats. According to the CDC, if you were bitten by an infected flea, you would experience one of the three clinical presentations:

  1. Bubonic infection, which includes a rapid onset of fever, painful and swollen lymph glands (buboes).
  2. Pneumonic infection, in which you’d experience high fever, chills, cough, bloody sputum, and “overwhelming pneumonia” – Pneumonic means “of the lungs.”
  3. Septicemic infection, in which you’d have fever, exhaustion, hemorrhagic or thrombotic phenomena, progressing to gangrene on the skin. Septicemia is an infection of the blood.

People who are bitten by infected fleas generally develop the bubonic form of the plague. However, if the bacteria reaches the lungs, it can develop into pneumonic plague and can be spread via person to person by coughing. According to the WHO, if the bubonic plague is diagnosed early, treatment with antibiotics can be successful. However, if the victim develops pneumonic plague, he or she can die within 24 hours of contracting the disease.

The Plague: What We Thought We Knew

According to the Centers for Disease Control and Prevention (CDC), it’s the bite of infected rodent fleas that causes plague transmission. However, transmission can also occur from handling infected animal tissues (dead rats), inhaling droplets from infected cats or dogs, and rarely, contact with a person who has pneumonic plague.

Gangrene is one of the symptoms of the plague. Image by CDC/ Christina Nelson, MD, MPH

Experts thought that people contracted the bubonic plague from infected fleas alone. However, with the high mortality rates, and new information from the finding of the mass grave sites, some scientists are beginning to think that it wasn’t the fleas that spread this dangerous disease.

New Plague Theory

Archaeologists who examined the remains beneath the Charterhouse Square have concluded that perhaps, the rats and fleas aren’t all to blame. The reasoning? The black death arrived in Britain in the fall of 1348 and by late spring of 1349 it had killed six out of every ten people in London.

According to The Guardian, scientists extracted the DNA of the disease from the largest teeth and compared it to the current black death that has killed 60 people in Madagascar. They found that the DNA from the 14th century black death is almost identical to the one in Madagascar. So why did the black plague kill so many people in 1349 and so few now, if the strain is no more virulent now than it was in 1348?

Researchers now are suggesting that the black death was not bubonic, but pneumonic, meaning that it was transmitted via airborne particles, such as when a person’s sneezes or coughs. According to the WHO, pneumonic plague is one of the most deadly infectious diseases; even with treatment, mortality rates are high.

Dr. Tim Brooks, whose documentary, Secret History: The Return of the Black Death, airs today, also now believes, after examining these skeletons, that the victims also suffered from rickets, malnutrition, and anemia, making them more susceptible to getting sick.

Interview with the CDC

Decoded Science had the opportunity to interview Dr. Ken Gage, chief of the Flea-Borne Diseases Activity in the Bacterial Diseases Branch, Division of Vector-Borne Diseases.
I asked Dr. Gage about the recent articles stating the the bubonic plague is actually pneumonic and he explained that although it’s possible that the fleas may not have been the primary source of transmission, it doesn’t necessarily follow that they weren’t involved at all. He tells us,

First, it is important to remember that historical records of the disease during this period almost always described cases of what was clearly bubonic plague, which is to say that these cases experienced swollen lymph nodes in the groin, armpits or around the neck. Although a small percentage of those cases which involved axillary (armpit) buboes could have been acquired as a result of handling infected animals or coming into hand contact with infectious human body fluids or materials contaminated with these fluids, most cases of bubonic plague are acquired as a result of being bitten by an infectious flea near the affected lymph node (bubo).

Why doesn’t the person-to-person transmission theory work? Dr. Gage tells us,

The problem with proposing that transmission occurred primarily from person to person through an airborne route is – how can one then account for the large number of bubonic cases described in these historical descriptions? Because the course of illness in primary pneumonic plague progresses so rapidly, persons with this form of the disease are very unlikely to live long enough to develop the swollen lymph nodes (buboes) described so frequently in accounts of disease from the Black Death.

With the exception of a small percentage of cases that might develop axillary buboes (i.e. swollen lymph nodes in the armpits) as a result of handling animals or, perhaps materials contaminated with infectious body fluids from a human victim, bubonic cases are nearly always acquired by flea bite and any explanation of plague spread during the Black Death has to account for this fact.

So, what really happened? According to Dr. Gage,

In my opinion, the most likely solution to this problem is not to look for missing rat fleas, or to overemphasize the role of primary pneumonic plague and airborne transmission, but rather to consider the human flea (Pulex irritans) as the most likely source of human to human transmission in England and other areas of northern Europe affected by the Black Death.

This idea has been proposed by others in the past but perhaps not accepted because the human flea rarely becomes blocked.

Blockage is a term used to describe occlusion of the flea’s gut with a mass consisting of plague bacterium embedded within a biofilm produced by the bacterium itself. This blockage occurs at the point where the flea’s foregut (throat and esophagus) meets its midgut (stomach) in a spine-filled structure called the proventriculus. Blockage of the proventriculus, which commonly occurs in rat fleas but not in many other fleas, causes eventual starvation of the affected flea because ingested blood cannot pass from its foregut (gullet) into the midgut.

As a result of this blockage-induced starvation, blocked fleas will repeatedly try to feed on hosts and in the process can regurgitate portions of the ingested blood meal into the feeding site in an attempt to clear the blockage from their “throats”.

This regurgitation process can result in the flushing of a small part of the blockage, which contains viable plague bacteria, into the bite wound, thereby allowing the bacterium can to invade its mammalian host (rat, other rodents, humans, etc.) and establish an infection in that animal.

Although blockage greatly increases the efficiency of transmission, it is not essential for transmission to occur and it has been shown recently that some fleas that block poorly can actually transmit the disease very efficiently during the first few days (1-4 days) after they take an infectious blood meal.

The efficiency of this so-called early phase transmission is comparable to the efficiency seen for transmission of the disease by blocked rat fleas and modeling studies indicate that such transmission can support the rapid spread of plague. Although these studies were done with a variety of fleas (ground squirrel, prairie dog, cat, mouse and rat fleas) other than Pulex irritans, all fleas tested to date have been able to support at least some level of early phase transmission and it seems likely that this also would be the case for the human flea (Pulex irritans).

If this is indeed true, it would be a much more likely explanation for the rapid spread of plague during the Black Death in many local outbreaks that involved obvious bubonic cases.

As noted above, however, there is no doubt that pneumonic outbreaks also occurred, but these usually occurred in conjunction with considerable numbers of bubonic cases.”

Worldwide Plague Infections: Are We In Danger?

I also asked Dr. Gage about the plague that continues to affect a small number of people worldwide. Are we in any danger of a future pandemic? Dr. Gage responded that the risk is low,

“The vast majority of the world’s plague cases now occur in developing countries where poverty and severe rat infestations in rural villages remain a problem. Most of these cases are bubonic and are acquired through the bites of infectious rat fleas, although occasional outbreaks of pneumonic plague do occur, such as those seen within the past decade in Madagascar, the Democratic Republic of Congo, and Uganda.

Although local epidemics of plague, occasionally involving a few hundred cases, still occur in certain plague-endemic regions in developing countries, the risk of pandemic spread of the disease is no longer considered a risk because of our abilities to control the spread of plague through flea and rodent control, rodent sanitation measures, prophylactic treatment of case contacts, use of personal protective measures to avoid flea bite and the isolation of pneumonic plague patients and utilization of appropriate respiratory precautions and other infection control procedures to prevent spread of the disease among health care staff.

It should be noted that although plague invaded the U.S. in 1900 during the last pandemic, and still occurs in this country, the disease is confined to certain western states where it exists in our native (wild) rodent species and their fleas.

These wild rodent foci of plague in the U.S. are sources for a handful (typically less than 10 cases per year) of human cases… usually as the result of people being bitten by infectious wild rodent fleas, especially a particular species (Oropsylla montana) normally found on rock squirrels or California ground squirrels. Smaller numbers of U.S. cases occur among people who have a history of handling animals, including cats, rabbits, and various wild carnivores infected with Yersinia pestis, the causative agent of plague. “

Black Plague: Rats, Fleas, and Pandemics

The plague is famous for killing millions of people in Europe during the 1300s. Although we’ve always thought it was infected rats and their fleas, some scientists now think that a more dangerous form of the plague spread via airborne particles between the infected people. Dr. Gage at the CDC, however, believes that the rat fleas – and possibly human fleas – contributed.

Human plague infections continue across the western United States, but more so in parts of Africa and Asia. Antibiotic treatment to avoid the development of pneumonic plague is critical, but due to current advancements, we’re in little danger of a global plague pandemic.

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