Why does the plague still plague people?

Conor Meehan: Throughout human history, various infectious diseases have taken over the role of the “deadliest diseases” to attack humans.

In the last century alone, the human population has experienced multiple pandemics: COVID, HIV, and various flu epidemics, to name a few. Some have lasted for centuries and continue to this day, like tuberculosis. Others are often considered consigned to history.

Before the 20th century, the deadliest disease in Europe and its surrounding areas was the bubonic plague. There have been three major pandemics of this disease in the last 1,500 years.

The first epidemic occurred from the 5th to the 7th century and claimed the lives of approximately 15 million people in the Mediterranean basin and severely affected the Byzantine, Sassanid and Roman Empires.

In the 14th century, Europe experienced a second, much larger epidemic, known as the Black Death. More than 50 million people died from the disease, which was about 50% of the entire population of Europe.

The third wave of this pandemic occurred worldwide in the 19th and 20th centuries and claimed the lives of another 30 million people worldwide, many of them in China and India.

However, since the 1960s, the number of cases has declined dramatically, and bubonic plague is not often considered a state-of-the-art disease. Despite this, a modern case was recently reported in the United States, which has renewed interest in the disease.

Although bubonic plague is no longer common in many parts of the world, it still occurs in circumscribed geographic areas and can spread within communities if the right set of conditions occurs.

Bubonic plague, or plague for low, is caused by a bacterium called Yersinia pestis. There are three types of plague caused by this pathogen, each with a different part of the body as the primary site of infection: pneumonic plague is mainly based in the lungs, septicemic plague is mainly based in the blood, and bubonic plague is mainly in the lymph nodes.

Although one form of the disease can develop into another during the course of infection, the form of the disease a person experiences generally depends on the method of infection.

Bubonic plague is a form of Y. pestis infection that is spread by fleas living on miniature animals, mainly rodents such as the house rat and the field rat. These rodents are reservoirs of the bacteria: they show little or no symptoms but can transmit the bacteria to others, including humans.

This transmission from rodents to humans occurs via fleas. These insects bite rats and then can jump and bite humans, injecting the plague bacterium into the human's lymphatic system. The bacteria then travel through this system to the lymph nodes and infection begins.

The main symptom of bubonic plague is swollen lymph nodes, usually in the neck, groin, thighs, and armpits. These swollen nodes, called buboes, can cause the tissue around them to turn black and die. They can also burst, releasing pus into the tissue.

Other symptoms include fever, headaches, and vomiting, and the pathogen can spread to other parts of the body, such as the lungs and blood, causing other forms of plague. Bubonic plague kills 30-60% of people, while pneumonic and septicemic plague are always fatal if left untreated.

So why was it so perceptible hundreds of years ago, but barely heard of today? The key combination of vector (flea), reservoir (rodent), and bacterium (Y. pestis) occurs together and in close contact with humans.

Before the 19th century, people believed that disease spread primarily through miasma: harmful forms of air. It wasn’t until the 1880s that people realized that microscopic organisms transferred between people, animals, and the environment could cause disease.

Under control

As a result, sanitary conditions have improved in many parts of the world, making it possible to isolate rodents from humans and break the cycle of plague transmission.

The discovery of antibiotics, especially fluoroquinolones, in the 1960s contributed to a further decline in the incidence of plague, as it was now possible to provide appropriate treatment for all its forms.

Today, we still see cases of plague in specific hotspots, primarily in Asia, Africa, and South America. The Democratic Republic of the Congo, Peru, and Madagascar are the countries with the most cases.

Madagascar alone has dozens of cases a year, with larger outbreaks in 2014 and 2017 (the latter with more than 2,000 cases). Dense forest areas are home to many of the rodents, and human contact with these ecosystems is driving these state-of-the-art outbreaks.

Plague will likely never be eradicated. Because of the sophisticated web of transmission from fleas, rodents, and humans, finding, controlling, and treating all of these aspects is nearly impossible.

However, thanks to proper animal handling, separation of natural reservoirs from humans, and rapid and effective treatment, the number of plague cases is decreasing year by year, and there is increasing hope that the number of cases will drop to a negligible level.

Conor Meehan, Assistant Professor of Microbial Bioinformatics, Nottingham Trent University

This article was reprinted from Conversation under the Creative Commons license. Read original article.