What is pneumonic plague and how does it spread?

The World Health Organization has asked Russia for information on a worker's death at a Siberian plague research laboratory. Russian officials say those who contacted the researcher tested negative for pneumonic plague, and investigations continue into the cause of death.

Pneumonic plague is a severe lung infection caused by the bacterium Yersinia pestis. It ranks as the most virulent form of plague, alongside bubonic plague, which affects lymph nodes, and septicaemic plague, which affects the blood. Symptoms include shortness of breath, chest pain and coughing up blood. It occurs less commonly than bubonic plague but carries greater severity.

Humans contract plague through bites from infected fleas or by handling infected animals. Pneumonic plague can also spread human-to-human via infected respiratory droplets, though this typically requires close contact. The infection can also develop when septicaemic plague spreads to the lungs.

The WHO reports pneumonic plague has an incubation period as short as 24 hours. Without diagnosis and rapid treatment, it is nearly always fatal, according to experts. Dr Malick Gibani, Clinical Associate Professor in Bacterial Vaccinology at Imperial College London, stated some textbooks describe untreated pneumonic plague as having 100% mortality. However, the infection responds well to antibiotics if treatment begins early.

Global plague cases across all three types range between 1,000 to 3,000 annually, with most occurring in central Africa. Cases also develop in Russia, India, China and the United States. Prof Piero Olliaro at the University of Oxford noted that around 200 rodent species can harbour Yersinia pestis, with reservoirs found across the Americas, Eurasia and Africa.

In the Russian incident, authorities issued a statement saying tests showed "no microorganisms associated with the patient's professional activities." The Centers for Disease Control said there was "no indication of a broader threat to the United States." The UK's Health and Safety Authority said the risk to Britain's population was "very low."