Background and Symptoms
Plague is an
acute, bacterial disease caused by Yersinia pestis, a
Gram-negative bacteria that is transmitted from rodent to rodent
by infected fleas. The most common mode of transmission
Y. pestis to humans is by the bite of infectious fleas,
especially the Oriental rat flea (X. cheopis).2,4
Less frequently, infection is caused by 1) direct contact
with infectious body fluids or tissues while handling an
infected animal or 2) inhaling infectious respiratory droplets.
Signs and symptoms include fever, headache, myalgia (muscle
aches), malaise, shaking chills, prostration, and
gastrointestinal symptoms. Several antibiotics may be used for
prophylactic therapy in persons exposed to Y. pestis,
although streptomycin is the drug of choice.2,4 An
inactivated vaccine is also available in the United States.4
A prairie dog may carry fleas that
are infected with plague bacteria. Photo courtesy of the CDC.
The three principal
clinical presentations of plague are bubonic, septicemic, and
pneumonic.4 Bubonic is the most common, with an
incubation period from 2-6 days, and is characterized by the
development of an acute regional lymphadenopathy (swollen lymph
nodes), or bubo. The case-fatality rate for untreated
individuals is 50-60%. A septicemic form of
the disease occurs when Y. pestis invades and multiplies
in the bloodstream. This can occur secondarily to the bubonic
form or develop without detectable lymphadenopathy.
Pneumonic plague is the least common, but most fatal, form of
the disease. It can develop either as a secondary complication
of septicemia or result from inhalation of infectious
respiratory droplets expelled from a human or animal that has
the pneumonic form. The incubation period for primary pneumonic
plague is 1-3 days.
This is an example of a bubo, or enlarged
lymph node that may occur as a result of infection with bubonic
plague. Photo courtesy of the CDC.
of plague in humans usually involve exposure to house rats and
their fleas. Risk for plague in humans is greatest when
epizootics cause high mortality in rat populations, thereby
forcing hungry, infected rat fleas to seek alternative hosts
such as humans.4 The last rat-borne epidemic in the
United States occurred in Los Angeles in 1924-25.5
Since then, all human plague cases in the U.S. have been
sporadic cases acquired from wild rodents or their fleas or from
direct contact with plague-infected animals. Rock squirrels,
ground squirrels, prairie dogs, wood rats, and chipmunks have
served as sources of human infection.5 Domestic cats
have recently been sources of infection for humans.6,7
engorged with blood. This flea species is the primary vector for
most large plague epidemics in Africa, Asia, and South America.
Courtesy of the CDC.
In the United States, the majority of human cases are reported
from Arizona, California, Colorado, and New Mexico.6,7
During the period 1970-1995, 341 cases of plague in humans were
reported to the Centers for Disease Control and Prevention
(average: 13 cases per year). Of these cases, 80% occurred in
the southwestern states of Arizona, Colorado, and New Mexico.4
Plague in Florida
The last reported
case of human plague in Florida occurred in 1920 during an
outbreak in Pensacola.1 The disease still remains a
threat, however, because of enzootic foci in the wild rodent
populations of several western states.2 Even though
plague is not currently in Florida, there is always the
possibility that animals infected with Y. pestis may be
imported into areas of the state that have suitable flea vectors
(including Xenopsylla cheopis).3
Bigler WJ and
Janowski DD. Bubonic plague in Florida??! Epi Update (Florida
Department of Health, Bureau of Epidemiology).
Benenson AS (ed.).
Control of Communicable Diseases Manual, Sixteenth Edition.
United Book Press, Baltimore. 1995: 353-358.
Layne JN. Fleas (Siphonaptera)
of Florida. Florida Entomologist. 1971; 54(1):35-51. March
CDC. Prevention of
Plague: Recommendations of the Advisory Committee on
Immunization Practices (ACIP). Morbidity and Mortality Weekly
Report. December 13, 1996;45 (no. RR-14).
on Plague. Internet address (accessed September 2005):
plagueUnited States, 1993-1994. Morbidity and Mortality Weekly
Report. April 8, 1994; 43(13):242-246.
CDC. Fatal human
plague Arizona and Colorado, 1996. Morbidity and Mortality
Weekly Report. July 11, 1997; 46(27):617-620.