• Since plague (Yersinia pestis) was introduced into the United States in the San Francisco Bay area in 1900, there have been a total of 941 confirmed human cases recorded through the year 2000.1 Of that total, at least 402 (42%) were fatal. An urban outbreak in Los Angeles in 1923-1924 resulted in 37 deaths in 39 cases, all in one residential neighborhood.
• The fatality rate has undergone a rapid decline since the widespread use of antibiotics beginning about 1960. By the end of the 20th century, the fatality rate declined to 15% of confirmed cases. Since 1985, there have been no more than 15 cases in the United States in any year. Only the pneumonic form of plague is transmissible from person to person. The last case of human-to-human transmission of plague in the United States occurred in the Los Angeles outbreak in 1924.2
• Worldwide, an average of 1,700 cases of plague have been reported annually for the past 50 years.
• The epidemiology of plague following its use as a biological weapon would differ substantially from that of naturally occurring infection. Intentional dissemination of plague most likely would occur via an aerosol of Y. pestis, resulting in a pneumonic plague outbreak. The size of the outbreak would depend on several factors, including the quantity of biological agent used, characteristics of the strain, environmental conditions, and methods of aerosolization.
• Indications that plague had been artificially disseminated would be the occurrence of cases in locations not known to have enzootic infection, among individuals with no known risk factors (e.g., animal contact), and the absence of prior rodent deaths (historically, rats die in large numbers prior to human outbreaks, precipitating the movement of the infesting flea population from the rats to humans).
• Following a deliberate attack, aerosolized inhaled Y. pestis bacilli would cause primary pneumonic plague, with the time from exposure to the development of first symptoms being one to six days.
• The sudden appearance of a large number of previously healthy patients with fever, cough, shortness of breath, chest pain, and a fulminant course leading to death immediately should suggest the possibility of pneumonic plague or inhalational anthrax. The presence of hemoptysis in this setting would strongly suggest plague.
• Parenteral forms of the antimicrobials streptomycin or gentamicin are recommended. In a mass-casualty setting, intravenous or intramuscular therapy may not be possible, so oral therapy, preferably with doxycycline, tetracycline, or ciprofloxacin, should be administered
Infection control measures
• National infection control guidelines recommend the use of disposable surgical masks to prevent transmission via respiratory droplets.
• Other respiratory droplet precautions (gown, gloves, and eye protection) also should be used by people caring for pneumonic plague cases.
• Patients with pneumonic plague should be isolated until they have had at least 48 hours of antibiotic therapy and shown clinical improvement.
• If large numbers of patients make isolation impractical, pneumonic plague patients may be cohorted. Patients should wear surgical masks while they are being transported.
• Hospital rooms should receive terminal cleaning consistent with standard precautions; clothing and linens contaminated with the body fluids of pneumonic plague patients should be disinfected per hospital protocol.
• Laboratories should observe biosafety level 2 conditions. Activities with a high potential for aerosol or droplet production (centrifuging, grinding, vigorous shaking, animal studies) require biosafety level 3 conditions. It should be noted that a plague case reported in Michigan in 1901 occurred in a lab worker who was examining infected rats from an ongoing outbreak in San Francisco.
• Bodies should be handled with routine strict precautions. Aerosol-generating procedures (bone-sawing associated with surgery or post-mortem examinations) should be avoided.
• There is no evidence to suggest that environmental decontamination following an aerosol release is warranted. Y. pestis is very sensitive to sunlight and heating and does not survive long outside its host.
1. Brown TL, Reynolds PJ. A Manual for the Investigation of Plague Cases in New Mexico. Santa Fe, NM: New Mexico Environment Department; March 2001.
2. Inglesby TV, Dennis DT, Henderson DA, et al. Plague as a biological weapon: Medical and public health management. JAMA 2000; 283:2,281-2,290.