What is pneumonic plague and how does it spread?

The death of a worker at a plague research institute in Russia's Siberian region has raised concerns over pneumonic plague, one of the most severe forms of the disease.

Russian authorities have taken precautionary measures following the death, while the exact cause of the illness has not been publicly confirmed. The incident has also renewed attention on pneumonic plague, which can spread directly from person to person through respiratory particles.

But what exactly is pneumonic plague, and how does it spread?

What is pneumonic plague?

Pneumonic plague is a form of plague that affects the lungs. It is caused by the bacterium Yersinia pestis, which is also responsible for bubonic and septicemic plague.

Bubonic plague is the most common form and usually spreads through the bite of an infected flea. Pneumonic plague, however, can develop when the bacteria reach the lungs or when a person inhales infectious respiratory particles from someone or an animal with pneumonic plague.

It is considered the most serious form of plague because it can progress rapidly and, unlike bubonic plague, can spread directly between people.

How does it spread?

A person with pneumonic plague can release bacteria-containing respiratory particles when coughing or sneezing. Another person can become infected by inhaling these particles.

Such transmission generally requires close and direct contact with an infected person or animal, according to the US Centers for Disease Control and Prevention (CDC).

Plague can also spread through infected flea bites and contact with infected animals or their tissues and bodily fluids. However, these routes generally cause bubonic or septicaemic plague rather than direct person-to-person transmission.

What are the symptoms?

Pneumonic plague can cause sudden fever, headache, weakness, cough, chest pain, difficulty breathing, and rapidly developing pneumonia. The cough may sometimes produce bloody or watery mucus.

Symptoms can appear very quickly. The CDC says the incubation period after inhalation can be as short as one day, while its clinical guidance generally puts it at one to three days.

Why is it considered dangerous?

The disease can deteriorate rapidly if treatment is delayed. According to the World Health Organization (WHO), untreated pneumonic plague can be fatal within 18 to 24 hours after symptoms begin.

However, plague is treatable with antibiotics, and early diagnosis and treatment can significantly improve the chances of survival.

Can pneumonic plague cause an outbreak?

Yes, but person-to-person transmission generally requires close contact with an infected person who has pneumonic plague.

The WHO describes pneumonic plague as especially contagious among the forms of plague because it can spread through respiratory particles. This is why suspected cases may lead to isolation of patients, monitoring of close contacts and preventive antibiotics for people who have been exposed.

Is pneumonic plague the same as the Black Death?

Pneumonic plague is caused by the same bacterium, Yersinia pestis, that caused the Black Death, but the two terms do not refer to exactly the same thing.

The Black Death was a historic pandemic in which plague caused massive deaths across Europe and other regions. Today, plague remains present in some parts of the world, but modern antibiotics and public-health measures make it a treatable disease when detected early.

How can it be prevented?

Prevention focuses on avoiding contact with infected animals and their tissues, protecting against flea bites, and taking precautions around people suspected of having pneumonic plague.

Patients with suspected pneumonic plague should be isolated, while close contacts may be monitored and given preventive antibiotics.