Plague

Syndromes and synonyms: Pestis, pest, bubonic plague, pneumonic plague, black plague, black death.

Agent: Yersinia pestis, a Gram-negative bacillus. Y. pestis is considered a potential biological warfare agent.

Yersinia pestis (Pasteurella pestis) is the bacterium responsible for the bubonic plague.

Reservoir: Rodents, principally rats and sylvatic ground squirrels: marmots, susliks, and prairie dogs. Rabbits, camels, carnivores, and domestic cats may also be infected. Cats can also develop pneumonic plague. The desert regions of Central Asia contain endemic plague foci where the great gerbil is the main host.

Vector: Fleas, especially the rat flea (Xenopsylla cheopis) and possibly human flea (Pulex irritans).

Transmission: By flea bite for the bubonic form, by the respiratory route for the pneumonic form; handling carcasses or eating meat of infected animals. Person-to-person transmission occurs through the bite of fleas (bubonic form) or respiratory droplets (pneumonic form).

Hand of a plague patient displaying acral gangrene. Gangrene is one of the manifestations of plague and the
origin of the term Black Death given to plague throughout the ages.

📖 Introduction to Epidemiology 

Cycle: There are different cycles, including a sylvatic rodent- flea cycle, a commensal rodent-flea cycle, and a cycle of pneumonic transmission in humans. Y. pestis can survive in the environment, mainly in rodent burrows in a sylvatic cycle. In case an infected flea feeds on a commensal rodent (rat), a rodent-flee-rodent cycle starts. When the rodents dies, their fleas move to alternative hosts, possibly humans. If humans develop pneumonic plague, the infection can be transmitted from person to person via respiratory droplets.

Incubation period: 1-4 days for pneumonic, 2-7 days for bubonic plague.

Clinical findings: Sudden onset of fever, chills, headaches, body aches, sore throat, vomiting and nausea. Bubonic plague is most common, producing swollen, painful and eventually suppurating lymph nodes (buboes) which are usually inguinal. In the septicemic form, Y. pestis spreads through the bloodstream usually affecting the lungs, ending in fatal endotoxic shock and DIC. The CFR of the bubonic form is 40-70%; of pneumonic and septicemic plague in the absence of prompt treatment, nearly 100%.

Diagnostic tests: Microscopy of stained smear from a bubo, sputum or CSF shows characteristic 'safety-pin' shape; serology (IFA; ELISA); rapid dipstick test. Culture takes about 4 days.

Therapy: Streptomycin, tetracyclines, and sulfonamides are standard; alternatives are gentamicin and fluoroquinolones. Chloramphenicol in cases of plague meningitis. Treatment should be started within 18 hours of onset. Buboes may need to be drained.
Prevention: A killed vaccine is available for laboratory workers, but is not recommended for use in epidemics. In buildings or rodent burrows, flea control with insecticide should be followed by rat destruction (killing rats liberates fleas); rat control in ports and ships. Chemoprophylaxis of pneumonic plague contacts. Pneumonic cases should be isolated.

Epidemiology: There are an estimated 1,000-3,000 human cases per year, but there is considerable underreporting and underdiagnosis. The last plague pandemic of 1894 started in Hong Kong establishing many endemic foci world wide. New foci continue to arise, as was seen in Algiers, in 2003. Warm springs and wet summers have increased the plague prevalence in the great gerbil in Kazakhstan. Similar climatic conditions may have resulted in past plague pandemics. Infection control and antibiotics can decrease plague morbidity and mortality but plague cannot be eradicated as it is widespread in wild rodents. There has been a large shift in case load from Asia to Africa, with more than 90% of cases occurring in Africa. The most common form is bubonic plague, but outbreaks of pneumonic plague still occur. Plague is possibly more common in Africa as poor rural communities in Africa live in close proximity to rodents, which are widely hunted and eaten in plague-endemic areas.

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