Plague

Plague is an infection caused by the gram-negative bacterium Yersinia pestis, which has been responsible for a number of human epidemics, most notably the Black Death in the Middle Ages. Plague is an infection of small rodents and mammals, predominantly the rat, and is endemic in many parts of the world, for which the World Health Organisation reports about 1,000 to 3,000 naturally occurring cases every year.

Transmission from rodent to man usually relies on a flea vector, although cases of pulmonary transmission from household pets have been described. Transmission by feeding fleas leads to the characteristic swelling of the draining lymph nodes of the bite, followed by septicaemic illness. This presentation is the classic bubonic plague. One of the complications of bubonic plague is pneumonia, which is highly contagious by airborne transmission from person to person. In pneumonic plague the symptoms of a respiratory infection develop first followed by an acute onset septicaemic illness. In the military context, the likely exposure is via the inhalation route, as a Biological Warfare Agent (BWA), and therefore protection against pneumonic plague is of paramount importance.

The onset of plague often presents as flu-like symptoms, making diagnosis very difficult. The different forms of plague have different mortality patterns. Untreated bubonic plague is fatal in approximately 60% of cases. If appropriate treatment is commenced within 24 hours of exposure, the death rate is reduced to approximately 16%. However, pneumonic plague is much more severe and untreated this form of the disease is almost universally fatal and, even if treatment is started immediately, the death rate remains high.