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Excerpts have been reprinted with permission from the APHA’s Control of Communicable Diseases Manual (CCDM). Please refer to the CCDM for more complete information.

PLAGUE ICD-9 020; ICD-10 A20

1. Identification – A specific zoonosis involving rodents and their fleas, which transfer the bacterial infection to various animals and to people. Initial signs and symptoms may be nonspecific with fever, chills, malaise, myalgia, nausea, prostration, sore throat and headache. Lymph­adenitis often develops in those lymph nodes that drain the site of the bite, where there may be an initial lesion. This is bubonic plague, and it occurs more often (90%) in lymph nodes in the inguinal area and less commonly in those in the axillary and cervical areas … All forms, including instances in which lymphadenopathy is not apparent, may progress to septicemic plague … Endotoxic shock and disseminated intravascular coagulation (DIC) may occur without localizing signs of infection … Secondary pneumonic plague is of special significance, since respiratory droplets may serve as the source of person-to-person transfer with resultant primary pneumonic or pharyngeal plague …
Untreated bubonic plague has a case-fatality rate of about 50%- 60%. Plague organisms have been recovered from throat cultures of asymptom­atic contacts of pneumonic plague patients. Untreated primary septicemic plague and pneumonic plague are invariably fatal. Modern therapy mark­edly reduces fatality from bubonic plague; pneumonic and septicemic plague also respond if recognized and treated early. However, one report stated that patients who had not received adequate therapy for primary pneumonic plague within 18 hours after onset of respiratory symptoms were less likely to survive.
Visualization of characteristic bipolar staining, "safety pin" ovoid, Gram-negative organisms in direct microscopic examination of material aspirated from a bubo, sputum or CSF are each suggestive, but not conclusive, evidence of plague infection … Diagnosis is confirmed by culture and identification of the causal organism from exudate … or by a 4-fold or greater rise or fall in antibody titre …

2. Infectious agent – Yersinia pestis.

3. Occurrence – Plague continues to be a threat because of vast areas of persistent wild rodent infection … While urban plague has been controlled in most of the world, human plague has occurred in the 1990s in several African countries ... Plague is endemic in China, India, Lao People's Democratic Republic, Mongolia, Myanmar and Viet Nam. In the Americas, foci in northeastern Brazil and the Andean region (Brazil, Ecuador and Peru) continue to produce sporadic cases and occasional outbreaks including an outbreak of pneu­monic plague in Ecuador in 1998 … No person-to-person transmission has occurred in the USA since 1925, although secondary plague pneumonia has occurred in about 20% of bubonic cases in recent years; 5 instances of primary plague pneumonia through cat-to-human transmission have been recorded.

4. Reservoir – Wild rodents (especially ground squirrels) are the nat­ural vertebrate reservoir of plague. Lagomorphs (rabbits and hares), wild carnivores and domestic cats may also be a source of infection to people.

5. Mode of transmission – … The most frequent source of exposure that results in human disease worldwide has been the bite of infected fleas (especially Xenopsylla cheopis, the oriental rat flea). 

Other important sources include the handling of tissues of infected animals, especially rodents and rabbits, but also carnivores; rarely airborne droplets from human patients or household cats with plague pharyngitis or pneumonia; or careless manipulation of laboratory cultures. Person-to-­person transmission by Pulex irritans fleas ("human" flea), is presumed to be important in the Andean region of South America and in other places where plague occurs and this flea is abundant in homes or on domestic animals … In the case of deliberate use plague bacilli would possibly be transmitted as an aerosol.

6. Incubation period – From 1 to 7 days … For primary plague pneumonia, 1-4 days ...

7. Period of communicability – Fleas may remain infective for months under suitable conditions of temperature and humidity … Pneumonic plague may be highly communica­ble under appropriate climatic conditions …

8. Susceptibility – … Immunity after recovery is relative; it may not protect against a large inoculum.

9. Methods of control­

A. Preventive measures: The basic objective is to reduce the likelihood of people being bitten by infected fleas, having direct contact with infective tissues and exudates, or of being exposed to patients with pneumonic plague.

1) Educate the public …
2) Survey rodent populations periodically to determine the effectiveness of sanitary programs and to evaluate the poten­tial for epizootic plague.
3) Control rats on ships and docks and in warehouses …
4) Wear gloves when hunting and handling wildlife.
5) Active immunization with a vaccine of killed bacteria confers some protection against bubonic plague (but not primary pneumonic plague) in most … Immunization of visitors to epidemic localities and of laboratory and fieldworkers han­dling plague bacilli or infected animals is justifiable but should not be relied upon as the sole preventive measure; routine immunization is not indicated for most persons resident in enzootic areas … Commercial plague vaccine is no longer available in the USA.

B. Control of patient, contacts and the immediate environment:

1) Report to local health authority