Plague

Plague is an infectious disease caused by the bacterium Yersinia pestis. It manifests in three primary forms: bubonic, septicaemic, and pneumonic plague. Each form affects different parts of the body and displays distinct symptoms.

Signs and Symptoms

Bubonic Plague

Bubonic plague is the most common form. It typically begins when a flea bites a human, introducing the bacteria into the tissue. Y. pestis can reproduce inside cells and spread through the lymphatic system, causing acute lymphadenitis. This results in swollen lymph nodes known as buboes, which are often hemorrhagic or necrotic.

Swollen inguinal lymph glands on a person infected with the bubonic plague. The swollen lymph glands are termed buboes from the Greek word for groin, swollen gland: bubo.

Septicaemic Plague

Septicaemic plague occurs when the bacteria enter the bloodstream, causing disseminated intravascular coagulation (DIC) and ischaemic necrosis. This form of plague can cause a red and/or black patchy rash and hemoptysis/hematemesis.

Septicaemic plague resulting in necrosis.

Pneumonic Plague

Pneumonic plague infects the lungs, causing symptoms such as coughing, chest pain, and hemoptysis. It can be spread through airborne droplets and has a rapid disease course, with untreated cases often resulting in close to 100% mortality.

Cause

The primary vector for Y. pestis is the Oriental rat flea (Xenopsylla cheopis), which transmits the bacteria through bites. The bacteria circulate in animal reservoirs, particularly rodents.

The Oriental rat flea (Xenopsylla cheopis) engorged with blood after a blood meal. This species of flea is the primary vector for the transmission of Yersinia pestis, the organism responsible for bubonic plague in most plague epidemics in Asia, Africa, and South America.

A child bitten by a flea infected with the bacterium Yersinia pestis. Y. pestis, a member of the family Yersiniaceae, has caused the bite to become ulcerated.

Diagnosis

Laboratory testing is required to definitively diagnose plague. Y. pestis can be identified through microscopy and culturing from samples of blood, sputum, or aspirate from buboes. Polymerase chain reaction (PCR) tests and rapid diagnostic tests detecting the F1 capsule antigen (F1RDT) may also be used for faster results. However, these tests need to be interpreted within the epidemiological context and confirmed by further testing.

Prevention

Vaccination

The first plague vaccine was developed by Waldemar Haffkine in 1897. Routine vaccination is not generally recommended, except for individuals at high risk, such as laboratory personnel working with resistant strains or people engaged in field operations in enzootic plague areas.

Early Diagnosis and Prophylaxis

Early diagnosis and treatment are very important in reducing transmission. Pre- and post-exposure prophylaxis may be considered for individuals at risk, such as healthcare providers or those in close contact with infected persons or animals.

Treatments

Various forms of plague respond well to antibiotic therapy if diagnosed in time. Common antibiotics used include streptomycin, chloramphenicol, tetracycline, gentamicin, and doxycycline. Drug resistance remains a concern, with some cases reported in Madagascar.

Epidemiology

Approximately 600 cases of plague are reported annually, with most cases occurring in the Democratic Republic of the Congo, Madagascar, and Peru. Historical outbreaks, such as the Black Death in the 14th century, resulted in millions of deaths. Recent DNA studies suggest the origin of modern Yersinia pestis strains dates back to 1338 in Kyrgyzstan.

Distribution of plague-infected animals 1998.

Biological Weapon

Historically, plague has been used as a biological weapon, with infected corpses catapulted into besieged cities. During World War II, Japan developed weaponized plague, and both the United States and the Soviet Union conducted experiments on pneumonic plague as a biological weapon. Modern concerns persist about the potential use of aerosolized pneumonic plague.

Self-assessment MCQs (single best answer)

Which bacterium causes plague?

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What is the most common form of plague?

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How is the Oriental rat flea scientifically known?

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What characteristic symptom is associated with bubonic plague?

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Which form of plague affects the lungs?

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What is a common symptom of septicaemic plague?

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Which diagnostic method could provide rapid results for identifying Yersinia pestis?

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Who developed the first plague vaccine?

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Which antibiotic is commonly used to treat plague?

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In which countries are most modern cases of plague reported?

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