Microbial Disease Report: The Plague

The Plague is one of the most recognizable and culturally significant diseases in the world. Plague (or Yersinia Pestis) has become so ubiquitous that the term colloquially refers to any contagious disease that spreads rapidly and is incredibly deadly. Yet, even given its great impact on humanity, many people today still do not understand this disease. In this report, I would like to clarify what the Plague is in more detail including: the different forms it can take, its various signs and symptoms, its history and current threat, and the available methods we have to treat it. Following the end of the report, I will also seek to answer a couple of my own questions regarding the disease.

The Plague is caused by a bodily infection by the bacteria, Yersinia Pestis. It is classified as a gram-negative bacterium that typically takes the shape of a coccobacillus (or oblong sphere). Yersinia Pestis is a facultative anaerobic organism which means that it can survive with or without the presence of oxygen. An infection of Yersinia Pestis can manifest in three different ways depending on the organ system that it invades.

Bubonic plague is the most common variation of the disease. Eighty percent of cases are characterized as Bubonic, yet despite its prevalence, this type of plague infection has a mortality rate of 50% if left untreated, which is the lowest mortality rate of the three. Bubonic is characterized as an infection of the lymph nodes by the Yersinia Pestis bacterium. These swollen lymph nodes form large abscesses known as Buboes which form on the surface of the skin usually near the groin. The most common way to contract Bubonic plague is to be bit by a flea infected with Yersinia Pestis and having the bacteria travel through the lymphatic system until it reaches a nearby lymph node and begins to grow. Rodents tend to be the carriers of the infected fleas, but they have been found on other animals as well. If the immune system cannot stop the disease from continuing to grow, then it can overpower the nodes and pass eventually pass into the bloodstream where it can cause secondary infections in the lungs (known as Pneumonic plague) or in the blood (known as Septicemic plague).

Symptoms of the Bubonic plague can develop within 2 to 6 days after initial exposure to the bacteria. These symptoms can include: the painful swollen buboes, high fever and chills, muscle pain and fatigue, headaches, and even seizures. The symptoms will usually manifest as defensive measures by the immune system in order to try to isolate and destroy the bacteria, but infected body tissues can often be overtaken without the use of antimicrobial assistance.

Pneumonic plague is the rarest form of the disease and the second most deadly with 90% of those that contract it likely to die if left untreated. It is also the most contagious variation of the disease. Pneumonic plague is when the Yersinia Pestis bacteria infects the lungs. This can happen as a secondary infection from Bubonic plague, Septicemic plague, or from another individual who has contracted Pneumonic plague. This happens when an infected individual coughs and transmits the bacteria into the air where someone else can inhale it and subsequently contract it.

The symptoms of Pneumonic plague tend to appear 1 to 4 days after a patient’s first exposure. These symptoms will generally consist of chest pain, high fever and chills, headaches, severe cough, difficulty breathing, and bloody sputum (or coughing up blood). High fever and chills as well as headaches stem from the immune response much in the same way that those with bubonic plague do. The respiratory symptoms, however, are caused by the infection directly interfering with the lungs. The lungs will attempt to cough out the irritant, but the bacteria remain which explains symptoms of chest pain, severe cough, and bloody sputum.

Septicemic Plague is not quite as rare as Pneumonic plague, but it is easily the most dangerous manifestation with a mortality rate of 100% if left untreated. This variation occurs when Yersinia Pestis bacteria infects the blood. This can be caused by an infected animal’s blood or meat entering into the bloodstream via a wound or by a secondary infection from either a plague-infected bubo or lung.

Septicemic Plague is so dangerous that it can often kill an infected individual before symptoms even begin to show. If symptoms do appear, they include abdominal pain, fever, nausea and vomiting, diarrhea, and various forms of bleeding (such as blood in the urine, blood coming from the mouth, nose, or rectum, and bleeding under the skin). These bleeding symptoms stem from the plague-infected blood’s inability to properly clot. This can then produce severe breathing issues, signs of shock, and the eventual death of various organ tissue.

The Plague was first isolated in Hong Kong in 1894, but it has a long history of causing devastating loss of human life long before that. The Plague has three great pandemics to its name: the Justinian Plague of 541 CE, the Black Death of 1347, and the Third Pandemic of 1894. These three pandemics have been estimated to have killed 165 million people in total. So deadly is this disease that it even has a history of being used as a biological weapon. But, despite its destructive power, modern cases of The Plague are exceedingly rare in this country with those in contact with rodents of the rural western United States the most at risk. Sadly, this is a different story at the global level. An increasing majority of cases of Plague are found in sub-Saharan Africa, Madagascar, and Peru. Based on recent research, there is also some evidence that as local rodent populations become more diverse in their species, the intensity and presence of human plague will increase alongside it. This information could be intensely useful in the suppression of future outbreaks of the disease.

The primary method of treating The Plague is through intravenous administration of antibiotics. Vaccinations have also been created, but the CDC usually reserves the vaccines for special circumstances due to the relative rarity of the disease. If it is diagnosed early enough and properly treated, the average mortality rate of all three forms of plague can be reduced to just 10%; however, the threat of a drug resistant form of Yersinia Pestis is very real with a few recent cases of it appearing in Madagascar. Luckily, research to better understand the metabolic structures of Yersinia Pestis in hopes to develop future antiplague therapeutics is currently being conducted.

The Plague is one of the most terrifying and influential diseases of the common era. While for many it is simply seen as a remnant of the past due to our modern advances in medicine, the Plague still takes the lives of thousands every year and the risk of an outbreak of antibiotic resistant Yersinia Pestis remains looming in the background. We have made great strides in the treatment of the Plague, but continued research into the bacteriology and epidemiology of the disease will be increasingly necessary in order to protect humans from future possible threats and further reduce casualties from the plague.