Yersinia Pestis

This plague-causing bacterium has been affecting people for at least 5,000 years.

Yersinia pestis is a type of disease-causing bacteria that causes all three forms of plague — bubonic, septicemic, and pneumonic.

Bubonic plague is widely known as the disease behind the devastating "Black Death" of the European Middle Ages that killed up to 60 percent of the European population, according to the Centers for Disease Control and Prevention (CDC).

The name of the bacteria comes from doctor and bacteriologist Alexandre Yersin, who discovered that Y. pestis causes plague in the mid-1890s while studying a plague epidemic in Hong Kong.

Numerous Yersinia species are known to exist today, but only two species besides Y. pestis — Y. pseudotuberculosis and Y. enterocolitica — cause disease in people, according to a 2003 report in the American Journal of Clinical Pathology.

Yersinia Pestis in Rodents and Fleas

Yersinia pestis is an obligate parasite, meaning that it cannot reproduce without a host.

Rodents are the primary hosts of the bacteria, which is spread through fleas.

When a flea feeds on an infected rodent, such as a rat, it swallows Y. pestis.

The bacteria multiplies in the gut of the flea and produces an enzyme that clots the blood the flea ingests, preventing the blood from moving past the flea's esophagus.

The flea is unable to fully ingest blood, so every time it tries to feed on an animal, it ends up regurgitating the blood — which has become newly infected with Y. pestis — back into the animal, infecting it.

Rodents and fleas generally serve as long-term reservoirs of Y. pestis, because the bacteria generally can't circulate quickly enough in these animals to cause massive die-offs.

But sometimes the infection rate can surge, killing many rodents — in the southwestern United States, this spike is more likely to occur in cooler summers that follow wet winters, the CDC notes.

When the infected fleas' food source dies off in this way, the hungry insects seek out new animals to bite — particularly cats, dogs, and people.

Transmission to People

When people are infected with plague bacteria, it's usually through bites from fleas.

If Y. pestis travels to the lymphatic system from the bite wound, it causes bubonic plague. If it travels to the bloodstream, it causes septicemic plague.

People can also catch bubonic and septicemic plague by handling the tissues or bodily fluids of infected animals.

For example, you can become infected if you accidentally cut yourself while skinning a freshly caught rabbit or other animal.

Pneumonic plague, which develops when Y. pestis infects the lungs, can spread to people through infected droplets.

This means that if a person or pet with pneumonic plague coughs or sneezes into the air, you can catch the bacteria — and get pneumonic plague — if you inhale those infected droplets.

Yersinia Pestis History

Yersinia pestis has caused three major plague pandemics in recorded history:

  • The Justinian Plague, which began in 541 A.D.
  • The Black Death, which began in 1334
  • The Modern Plague, which began in 1860

Though Y. pestis still causes plague today, the strain of bacteria is different from the (probably extinct) strain that caused the Black Death, according to research published in 2011 in the Proceedings of the National Academy of Sciences.

What's more, the Y. pestis strain behind the Black Death is different from the strain behind the Justinian Plague, which probably also went extinct before the second pandemic took place, according to a 2014 study in The Lancet Infectious Diseases.

Other recent research has begun to establish when the bacteria first evolved into existence.

A 2015 study in the journal Cell reported that Y. pestis has affected people for as long as 5,000 years.

Also in 2015, a study in the Journal of Medical Entomology reported the discovery of a piece of amber encasing a 20-million-year-old flea that had bacteria attached to it; that bacteria resembles Y. pestis in appearance and may be related to it.

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