PLAGUE : CLINICAL ASPECTS
The incubation period for bubonic plague is 2 to 7 days after the flea bite. Onset is marked by fever and the painful bubo, usually in the groin (bubo is from the Greekboubon for “groin”) or, less often, in the axilla. Without treatment, 50 to 75% of patients progress to bacteremia and die in Gram-negative septic shock within hours or days of development of the bubo. About 5% of victims develop pneumonic plague with mucoid, then bloody sputum. Primary pneumonic plague has a shorter incubation period (2 to 3 days) and begins with only fever, malaise, and a feeling of tightness in the chest. Cough, production of sputum, dyspnea, and cyanosis develop later in the course. Death on the second or third day of illness is common, and there are no survivors without specific therapy. A terminal cyanosis seen with pneumonic plague is responsible for the term Black Death. Even today, plague pneumonia is almost always fatal if appropriate treatment is delayed more than a day from the onset.
Gram smears of aspirates from the bubo typically reveal bipolar-staining Gram-negative
bacilli. An immunofluorescence technique is available in public health laboratories for immediate identification of smears or cultures. Y. pestis is readily isolated on the media used for other members of the Enterobacteriaceae (blood agar, MacConkey agar), although growth may require more than 24 hours of incubation. The appropriate specimens are bubo aspirate, blood, and sputum. Laboratories must be notified of the suspicion of plague to avoid delay in the bacteriologic diagnosis and to guard against laboratory infection.
Streptomycin is the treatment of choice for both bubonic and pneumonic plague, because its effectiveness has been proven. Tetracycline, chloramphenicol, and trimethoprim– sulfamethoxazole are alternatives. Timely treatment reduces the mortality of bubonicplague below 10%. Of the 31 human cases of plague reported in the United States in 1984, 6 (19%) died.
Urban plague has been prevented by rat control and general public health measures such as use of insecticides. Sylvatic plague is virtually impossible to eliminate because of the size and dispersion of the multiple rodent reservoirs. Disease can be prevented by avoidance of sick or dead rodents and rabbits. Eradication of fleas on domestic pets, which have been known to transport infected fleas from wild rodents to humans, is recommended in endemic areas. The continued presence of fully virulent plague in its sylvatic cycle poses a risk of extension to the urban cycle and epidemic disease in the event of major disaster or social breakdown. Chemoprophylaxis with tetracycline is recommended for those who have had close contact with a case of pneumonic plague. It is also used for the household contacts of a case of bubonic plague, because they may have had the same flea contact. A formalin-killed plague vaccine once used for those in high-risk occupations is no longer available.