Plague is an acute (rapid onset and short course) contagious illness caused by bacteria called Yersinia pestis. These bacteria primarily infect rodents and their fleas; humans are incidentally infected by bites from infected fleas. In recent years, rodent and flea control has reduced the incidence of plague, and prompt treatment with antibiotics has reduced the mortality. However, around 1000 to 3000 cases of human plague occur internationally each year, mainly in developing countries. Most cases occur in impoverished rural environments that are heavily rat-infested.
Most cases of plague are caused by bites from fleas infected with Y pestis. Once the flea bites, the bacteria migrate to the nearby lymph nodes and multiply. If untreated, the bacteria then enter the bloodstream and can invade distant organs.
Plague is occasionally transmitted via handling infected animals or by inhaling respiratory droplets spread by the cough of a person with plague pneumonia.
The most common form of human plague is bubonic plague. After an incubation period of 2 to 6 days, there is a sudden onset of high fever, chills, headache, muscle aches, joint aches, lethargy, abdominal pain, and diarrhoea. A papule (small bump), pustule (pus-filled blister), ulcer, or eschar (scab) may develop at the site of the flea bite (10% of cases). Within a few days the regional lymph nodes become swollen and extremely tender. These are called buboes and usually arise in the groin, armpit, or neck. The skin over the buboes becomes red, hot, swollen, and tense. Enlargement of the buboes can lead to rupture and discharge of malodorous pus. The liver and spleen may be enlarged and tender.
If treated at this stage, most symptoms improve in 2 to 5 days. The buboes can remain enlarged and tender for one or more weeks. If untreated, septicaemic plague can develop.
Septicaemic plague can occur when Y pestis infection spreads directly through the bloodstream without evidence of a bubo. More commonly, untreated bubonic plague can lead to invasion of the bloodstream by the bacteria. Gastrointestinal symptoms such as nausea, vomiting, abdominal pain, and diarrhoea may initially predominate. This condition can rapidly progress to disseminated intravascular coagulation, which presents as bleeding into the skin (purpura) and other tissues, and gangrene of the extremities, hence the term,
the black death. The patient may also develop profoundly low blood pressure, multi-organ failure, and acute respiratory distress syndrome (severe breathing difficulties). The fatality rate is high.
Pneumonic plague is the most lethal and least common form of plague. It can arise as a consequence of untreated bubonic or septicaemic plague, or it can result directly from inhaling infectious respiratory droplets or other materials. In the case of the latter, the incubation period is usually about 2 days. There is an abrupt onset of fever, chills, muscle aches, joint aches, dizziness, and lethargy. By the second day the patient has a cough with bloody sputum and shortness of breath. Acute respiratory distress syndrome develops, which can lead rapidly to death.
About 50-60% of patients with untreated bubonic plague die. Untreated septicaemic or pneumonic plague is almost always fatal. Even with appropriate antibiotic treatment, about 10-20% of patients with bubonic plague die, and 50% of patients with pneumonic plague die.
- Blood, lymph node aspirates, and sputum samples, as appropriate, should be taken for culture and microscopic examination. Wright-Giemsa, Wayson, or Gram stains may reveal Y pestis.
- Rapid dipstick tests have been validated for field use to quickly screen for Y pestis antigen in patients in endemic countries.
- Blood tests taken during the early and late stages of infection can be examined to detect antibodies to Y pestis.
- Antibiotic treatment should begin as soon as possible after laboratory specimens are taken. Streptomycin is the antibiotic of choice. Gentamicin, tetracycline, and chloramphenicol are also effective.
- Buboes may require surgical drainage.
- Admission to an intensive care unit is required for supportive therapy.
- Preventive strategies include avoidance of areas with known endemic plague, precautions against flea bites (such as DEET insect repellent, insecticides, and protective clothing), and avoidance of diseased or dead animals.
- Persons with suspected pneumonic plague must be isolated and strict infection control precautions must be used.
- A safe and effective vaccine is not available for humans.
- Antibiotic prophylaxis can be given to people who have had probable exposure to the disease.
- Lupi O, Madkan V, Tyring SK. Tropical dermatology: bacterial tropical diseases. J Am Acad Dermatol. 2006 Apr;54(4):559-78.
On DermNet NZ:
Books about skin diseases:
See the DermNet NZ bookstore