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Human Plague -- United States, 1993-1994

From 1944 through 1993, 362 cases of human plague were reported in the United States; approximately 90% of these occurred in four western states with endemic disease (Arizona, California, Colorado, and New Mexico) (1). During each successive decade of this period, the number of states reporting cases increased from three during 1944-1953 to 13 during 1984-1993 (Figure_1), indicating the spread of human plague infection eastward to areas where cases previously had not been reported. In 1993, health departments in four states reported 10 confirmed cases * of human plague to CDC; one case has been confirmed during 1994 **. This report summarizes information about the 11 cases of human plague reported during 1993-1994 and describes epidemiologic and epizootic trends of plague in the United States.

In 1993, the 10 confirmed cases of human plague were reported from New Mexico (six cases), Colorado (two), Texas (one), and Utah (one) (Table_1). Persons with plague infection were aged 22-96 years (median: 55.5 years); five were aged greater than or equal to 67 years. Six cases occurred among men. Five cases occurred during June-August, three during March-May, and two during September- November. Seven persons were exposed at their homesites, and one (a veterinarian) was exposed at work; exposure sites could not be determined for two cases. Seven cases were bubonic plague; two, primary septicemic; and one, primary pneumonic. Nine of the 10 patients recovered with antibiotic therapy; one patient died (Table_1).

For three patients, the probable mode of transmission was flea bite (based on the presence of an inguinal bubo or a recollection of flea bites). Two patients (including the veterinarian) were infected by domestic cats with visible signs of plague infection (i.e., oral lesions and a swollen tongue). For five cases (including the fatal case), the probable mode of transmission could not be determined; however, evidence of plague infection in local animal populations was detected in association with three of these cases (Table_1).

In 1994, plague infection has been confirmed in a 56-year-old resident of Inyo County, California, who had onset of illness on January 1 (the first report in California of a human plague case during winter since 1928) (Table_1). The patient lived in a county where plague was known to be endemic. In addition, he had recently worked in a subterranean gold mine and slept in a cabin at the minesite; signs of rodent activity were found in the mine shaft and the cabin outbuildings.

Reported by: SB Werner, MD, R Murray, DrPH, K Reilly, DVM, M Madon, MS, M Jay, DVM, C Smith, MS, B Wilson, MS, J Wang, MS, RJ Jackson, MD, California Dept of Health Svcs; R Hurd, J Levesque, MD, Inyo County Health Dept, Independence, California. J Pape, MS, T Davis, MS, RE Hoffman, MD, State Epidemiologist, Colorado Dept of Health. R Stoneberg, Montana Dept of Fish, Wildlife, and Parks; TA Damrow, PhD, State Epidemiologist, Montana State Dept of Health and Environmental Sciences. C Montman, Albuquerque Environmental Health Dept; T Brown, MS, P Reynolds, M Tanuz, New Mexico Environment Dept; M Eidson, DVM, CM Sewell, DrPH, State Epidemiologist, New Mexico Dept of Health. S Lanser, MPH, R Tanner, MPA, CR Nichols, MPA, State Epidemiologist, Utah Dept of Health. W Kramer, Nebraska State Dept of Health. TG Murnane, DVM, BN Hicks, PV Thomas, DVM, J Buck, J Taylor, MPH, GM Moore, MS, Texas Dept of Health; J Wood, City of Dallas, C Vaughn, MS, C Haley, MD, R Farris, MD, Dallas County Health Dept, Dallas. DR Akin, P Bohan, MS, R Enscore, MS, J Sarisky, MPH, L Courtois, Office of Environmental Health, Indian Health Svc. L Handegard, MT Billings, D Virchow, NE Scottsbluff, Animal and Plant Health Inspection Svc, Animal Damage Control, US Dept of Agriculture. RJ Andrascik, Theodore Roosevelt National Park, National Park Svc, North Dakota. Div of Field Epidemiology, Epidemiology Program Office; Bacterial Zoonoses Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The findings in this report emphasize the increasing importance of two related trends in the epidemiology of human plague in the United States: 1) increased peridomestic transmission and 2) the role of domestic cats as sources of human infection. Peridomestic transmission is especially important in the most highly plague-endemic states of Arizona, Colorado, and New Mexico, where rapid suburbanization has resulted in increasing numbers of persons living in or near active plague foci. Domestic cats that are permitted to roam freely in areas where plague occurs in rodents are at increased risk for infection and, therefore, increase the risk for peridomestic transmission to humans. Before 1977, domestic cats were not reported as sources of human plague infection; however, since 1977, cats have been identified as the source of infection for 15 human plague cases. In addition, the proportion of human plague cases with primary pneumonic plague has been substantially higher among cat-associated cases (four of 15 cases) than among cases for which cats were not sources of infection (one of 236 cases). Persons working in veterinary practices should be warned of the risks associated with handling plague-infected cats. Four of the 15 cat-associated cases occurred in veterinarians or their assistants. In addition, CDC recommends that veterinary personnel wear gloves and eye protection and take appropriate respiratory precautions (2) when examining sick cats in or from plague-endemic areas, especially cats with lymphadenopathy, oral lesions, or pneumonia.

Surveillance for plague in rodent and rodent-consuming carnivore populations during the 1990s indicates that plague has spread eastward to counties in areas (e.g., eastern Montana, western Nebraska, western North Dakota, and eastern Texas) believed to be free of this disease since widespread animal surveillance began in the 1930s (3-5). For example, the potential for human plague cases in eastern Texas was demonstrated in 1993 when an infected roof rat (Rattus rattus) and two infected fox squirrels (Sciurus niger) were identified in Dallas. Animal surveillance was initiated in the Dallas metropolitan area to monitor plague in local rodent and carnivore populations as a sentinel of increased risk for plague among humans. The continued expansion of human plague in the United States (Figure_1) underscores the need to enhance plague surveillance and to increase efforts to prevent, detect, and control human plague.

Epizootic plague activity usually peaks during or immediately after years with cooler temperatures and more rain than usual. Such conditions occurred during 1991-1993 in the highly plague-endemic areas of Arizona, Colorado, and New Mexico, as well as in the western Great Plains region, *** and resulted in large populations of many plague-susceptible rodent species -- including deer mice, the principal reservoir of hantavirus in the western United States (6).

Nearly all fatal plague cases in the United States result from delays in seeking treatment and in making the proper diagnosis. The person with fatal plague in 1993 received medical care 6 days after onset of illness and died within 4-6 hours of seeking care at a hospital. Because of similarities in clinical features of plague and the recently discovered hantavirus pulmonary syndrome (HPS) (7), diagnosis of plague may be further complicated. In 1993, HPS was suspected in a person with secondary pneumonic plague; as a result, the patient was transported to a regional medical center in another state for specialized care. At this facility, plague was diagnosed, and the patient recovered with antibiotic treatment. Increasing public and physician awareness about plague can assist in prompt diagnosis and treatment.

Efforts to prevent plague should include public education about risk factors for exposure, methods to prevent plague, and the signs and symptoms of infection; surveillance of rodent populations; and use of insecticides, and occasionally rodenticides, to control populations of fleas and rodents, respectively. Control measures should be undertaken when surveillance indicates epizootic activity among rodent populations.


  1. Craven RB, Maupin GO, Beard ML, Quan TJ, Barnes AM. Reported cases of human plague infections in the United States, 1970-1991. J Med Entomol 1993;30:758-61.

  2. CDC. Draft guidelines for preventing the transmission of tuberculosis in health-care facilities, 2nd edition; notice of comment period. Federal Register 1993;58:52843-54.

  3. Eskey CR, Haas VH. Plague in the western part of the United States. Public Health Bulletin 1940;254.

  4. Caten JL, Kartman L. Human plague in the United States, 1900- 1966. JAMA 1968;205:81-4.

  5. Barnes AM. Surveillance and control of bubonic plague in the United States. In: Edwards MA, McDonnel U, eds. Animal disease in relation to conservation. New York: Academy Press, 1982:237-70.

  6. CDC. Hantavirus infection -- southwestern United States: interim recommendations for risk reduction. MMWR 1993;42(no. RR-11).

  7. CDC. Update: outbreak of hantavirus infection -- southwestern United States, 1993. MMWR 1993;42:495-6.

* A case of human plague is considered to be confirmed when 1) a bacterial culture is identified as Yersinia pestis by biochemical testing and bacteriophage typing or 2) there is a fourfold rise in antibody titers to the F-1 antigen of Y. pestis. 

** Provisional data. 

*** West North Central, West South Central, and Mountain regions.

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TABLE 1. Confirmed human plague cases -- United States, 1993-1994 *
                                                   Clinical form                        mode of                        Epidemiologic/
State/County          Date          Age (yrs)/    (Bubo location)/    Exposure        transmission                       epizootic
  of residence      of onset           Sex         Recovery status      site           or source                         findings
New Mexico/
  Sandoval       March 13, 1993        44/M       Bubonic (axillary)/   Home         Scratch of infected     Infected woodrats and woodrat fleas
                                                  Recovered                          cat                      recovered at exposure site; probable
                                                                                                              rock squirrel die-off a few months before
                                                                                                              patient became ill.

Texas/Kent       April 24, 1993        96/F       Bubonic (cervical)/   Home         Undetermined            Infected fleas recovered from rabbit
                                                  Recovered                                                   captured near house; probable woodrat die-
                                                                                                              off; persons visiting patient's house
                                                                                                              bitten by fleas; patient had trapped
                                                                                                              rodents in house.

  Boulder        May 19, 1993          31/F       Primary pneumonic/    Work/        Inhaled infectious       None.
                                                  Recovered             Veterinary   aerosol while
                                                                        Office       examining infected

New Mexico/
  Rio Arriba     June 28, 1993         71/F       Bubonic (axillary)/   Home         Undetermined             Infected flea pool recovered from deer
                                                  Recovered                                                    mouse trapped near patient's home;
                                                                                                               patients's cat disappeared a few days before
                                                                                                               patient became ill.

New Mexico/
  Bernalillo     July 4, 1993          68/M       Septicemic/Died       Home         Undetermined             Rock squirrel epizootic near patient's home;
                                                                                                               patient's dog was seropositive.

New Mexico/
  San Juan       July 24, 1993         22/F       Bubonic (axillary)/   Undeter-     Undetermined             None.
                                                  Recovered             mined

New Mexico/
  Rio Arriba     August 8, 1993        35/M       Bubonic (inguinal)/   Home         Flea bite                None.

  La Plata       August 17, 1993       40/M       Bubonic (inguinal)    Home         Flea bite                Rock squirrel epizootic near home; two family
                                                  Recovered                                                    dogs were seropositive.

Utah/Salt Lake   October 2, 1993       67/M       Bubonic (axillary)/   Undeter-     Undetermined             None.
                                                  Recovered             mined

New Mexico/
  Santa Fe       October 3, 1993       73/F       Septicemic/           Home         Flea bite                Rock squirrel epizootic near home: plague-
                                                  Recovered                                                    infected rabbit found dead near home; rabbit
                                                                                                               infected with plague-infected rock squirrel
  Inyo           January 1, 1994       56/M       Septicemic/           Undeter-     Undetermined             12 dogs and 3 cats living at or near the
                                                  Recovered +           mined                                  patients's home were seronegative; evidence
                                                                                                               of rodent activity found at or near patient's

* Data for 1994 are provisional.
+ Four weeks following recovery and discharge from the hospital, the patient died from an acute myocardial infarction; he had a history of heart disease.

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