Yersinia pestis

Yersinia pestis is the bacterium that is responsible for causing three forms of plague: bubonic, septicemic, and pneumonic. These plagues have historically been among the most devastating diseases to strike humanity. Several pandemics throughout human history have been traced back to the Yersinia pestis bacterium. These include the Justinian plague that began in 541 CE and killed 25 million people over a 200-year period of reoccurrence; the Black Death, which is estimated to have killed up to 60 percent of Europe's population in the fourteenth century; and the modern plague, which killed 10 million people in China in the mid-nineteenth century.

Plague has had a huge impact on the course of human history. For instance, historians have sought to link episodes of plague to the fall of the Roman Empire, the successful Anglo-Saxon invasion of Britain, and the rise of the Renaissance and industrial eras of Europe. Plagues resulting from exposure to Yersinia pestis remain an ongoing threat to humans, especially in densely populated areas. Roughly 1,000 to 2,000 people are diagnosed with bubonic plague on an annual basis, although the actual number of cases is likely higher. According to the World Health Organization (WHO) in 2022, bubonic plague has a mortality rate of 30 to 60 percent if left untreated, while pneumonic plague is always fatal unless treated.

Background

Yersinia pestis is often initially transmitted through the bites of fleas found on rodents, although it may also be spread through contact with infected fluid or tissue, or through the ingestion of infected droplets spread through coughing. It may be transmitted to rodents, insects, and humans and other carnivores. Rats are considered to be among the most common hosts for this bacterium. In this capacity, rats are called reservoir hosts, meaning that it is believed that Yersinia pestis is able to be maintained among various rat populations without decimating their numbers. This type of regular circulation of the bacterium within the rodent populations between epidemics is called the enzootic cycle of Yersinia pestis.

The bubonic plague is a vector-borne illness, which means that it must have a living host to successfully transmit the disease between animals. The agent of this transmission is most typically a type of flea called Xenopsylla cheopis, or the tropical rat flea. While other types of fleas—such as the human flea, Pulex irritans—can transmit plague, Xenopsylla cheopis is the most common transmitter because of a unique biological trait. Xenopsylla cheopis is the only known flea with a digestive system that becomes blocked by the presence of Yersinia pestis. When this occurs, the flea's gastrointestinal system stops working. This prevents the flea from being able to ingest the blood it needs for survival. In order to feed, the flea regurgitates the bacteria blocking its system into the host's system after a bite, thereby infecting the host. Other fleas generally transmit the disease through infected mouthparts rather than through the transmission of infected blood. The Yersinia pestis bacterium does not have an adverse effect on most host flea species because their body temperatures are too low to allow the bacteria to activate their harmful properties.

In the United States, Yersinia pestis is most commonly found among certain species of wild rodents such as ground squirrels, prairie dogs, rock squirrels, and rats. Occasionally, the Yersinia pestis bacterium will begin to proliferate among its host species. While the triggers behind these epidemics are not fully understood, they may be caused by changes in environment, such as cool summers that follow wet winters. Similarly, areas that have poor sanitation and overcrowding that allow rodent populations to proliferate are more prone to epidemics. During die-offs of host species (which is called a plague epizootic), the loss of their primary hosts causes fleas to search for other food sources. Cats are particularly susceptible to plague due to their increased interactions with infected rodents. Once infected, cats and other animals are able to spread Yersinia pestis bacteria through droplets of moisture passed through coughing. In addition, the bacteria may be spread through contact with the skin or flesh of infected animals.

Overview

The Yersinia pestis bacterium first attacks the lymph nodes of a human victim. Once it reaches the lymph nodes, it multiplies rapidly. Lymph nodes are a vital component of the immune system. The Yersinia pestis's endotoxins (a type of molecule found in bacteria) triggers the immune system of the host's body. This causes these organs to swell into painful lumps called buboes. As the immune system seeks to fight the infection, the body develops fever, chills, and severe muscle aches. If left untreated or treated improperly, this infection—called bubonic plague—can spread to the rest of the body, potentially causing death within a few days. In medieval Europe, doctors would often seek to burst these buboes; however, this had few positive effects and served only to spread potentially infectious fluids. Ninety percent of victims of bubonic plague victims survive if treated properly with antibiotics.

Septicemic plague occurs when Yersinia pestis bacteria spread from lymph nodes to the bloodstream. This results in the development of a type of blood infection called septicemia. Septicemic plague can also be contracted when a victim is exposed to Yersinia pestis–tainted fluids or meat. This form of plague causes diarrhea and abdominal pain, followed by the appearance of severe bruises and blood in the urine, mouth, nose, and rectum. This is the result of the body's inability to clot properly. The unclotted blood then leaks into the skin and organs of the body. Seventy to eighty percent of septicemic plague victims survive.

Pneumonic plague is the rarest form of Yersinia pestis–borne plagues. Whereas bubonic plague is a disease of the lymphatic system, pneumonic plague is a respiratory illness. It occurs when someone breathes in droplets of tainted moisture that have been released by someone already infected with pneumonic plague. Victims of bubonic or septicemic plague may sometimes develop pneumonic plague in their lungs. Pneumonic plague results in symptoms of fever, chills, chest pain, breathing problems, vomiting, and blood in the lungs. Pneumonic plague has a very high mortality rate.

In the United States, an average of seven cases of plague are reported on an annual basis, with a range of zero to seventeen cases reported annually between 2000 and 2022. Most of these occur in rural regions of the American West. No human cases of plague east of the Mississippi River have been reported since 1970, and only fifteen fatalities were recorded between 2000 and 2022. The last US epidemic of the plague occurred between 1924 and 1925 in Los Angeles.

Internationally, a 1994 epidemic in India killed close to 10 million people. In 2017, Madagascar experienced a pneumonic plague epidemic. The Madagascar Ministry of Public Health reported that between August 1 and November 22, 2017, there were 2,348 cases of plague, including 202 fatalities. According to WHO, by 2021, both pneumonic and bubonic plague were endemic in the country, with 200 to 400 cases of mostly bubonic plague annually. In Madagascar, plague season usually occurs between September and April each year.

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