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Tularemia—United States, 1990-2000
Arch Dermatol. 2002;138:988-989.
TULAREMIA IS a zoonotic disease caused by the gram-negative coccobacillus Francisella tularensis. Known also as "rabbit fever" and
"deer fly fever," tularemia was first described in the United States in 1911
and has been reported from all states except Hawaii. Tularemia was removed
from the list of nationally notifiable diseases in 1994, but increased concern
about potential use of F. tularensis as a biological
weapon led to its reinstatement in 2000. This report summarizes tularemia
cases reported to CDC during 1990-2000, which indicate a low level of natural
transmission. Understanding the epidemiology of tularemia in the United States
enables clinicians and public health practitioners to recognize unusual patterns
of disease occurrence that might signal an outbreak or a bioterrorism event.
Tularemia characteristically presents as an acute febrile illness. Various
clinical manifestations can occur depending on the route of infection and
host response, including an ulcer at the site of cutaneous or mucous membrane
inoculation, pharyngitis, ocular lesions, regional lymphadenopathy, and pneumonia.
A diagnosis of tularemia can be laboratory-confirmed by culture of F. tularensis from clinical specimens or by a fourfold titer change
of serum antibodies against F. tularensis. Presumptive
diagnosis can be made by detecting F. tularensis
antigens with fluorescent assays or by a single elevated antibody level.1 For purposes of national surveillance, confirmed
and probable tularemia cases are defined as clinically compatible illness
with confirmatory or presumptive laboratory evidence of F. tularensis infection, respectively. Before September 1996, because
of ambiguity in the case definition, some cases of tularemia might have been
considered confirmed by fluorescent assay alone. Case status is determined
at the state level. For the purposes of this report, any case reported to
CDC was assumed to have laboratory evidence of infection. Similar results
were obtained when the analysis was limited to cases with documented confirmed
or probable status.
During 1990-2000, a total of 1,368 cases of tularemia were reported
to CDC from 44 states, averaging 124 cases (range: 86-193) per year; 807 cases
(59%) were reported as confirmed and 85 cases (6%) were reported as probable;
the status of 476 cases is unknown. Most (91%) unclassified cases were reported
during 1990-1992; all cases during 1990-1991 and 54% of cases from 1992 were
not classified. The number of cases reported annually did not decrease substantially
during the lapse in status as a notifiable disease during 1995-1999, but an
increase in reporting occurred during 2000, when notifiable status was restored.
Four states accounted for 56% of all reported tularemia cases: Arkansas (315
cases [23%]), Missouri (265 cases [19%]), South Dakota (96 cases [7%]), and
Oklahoma (90 cases [7%]).
County of residence was available for 1,357 reported cases. Among the
3,143 U.S. counties, 543 (17.3%) reported at least one case during 1990-2000.
The counties with the highest number of reported cases were located throughout
Arkansas and Missouri, in the eastern parts of Oklahoma and Kansas, in southern
South Dakota and Montana, and in Dukes County, Massachusetts (the island of
Martha's Vineyard).
During 1990-2000, the average annual incidence of tularemia reported
using 1995 population estimates was highest in persons aged 5-9 years and
in persons aged  75 years. Males had a higher incidence in all age categories.
Incidence was highest among American Indians/Alaska Natives (0.5 per 100,000),
compared with 0.04 per 100,000 among whites and  0.01 per 100,000 among
blacks and Asians/Pacific Islanders. Of the 936 cases reported with date of
onset, 654 cases (70%) reported onset during May-August, but cases were reported
in all months of the year.
Reported by:
E Hayes, MD, S Marshall, MPH, D Dennis, MD, Div of Vector-borne Infectious
Diseases, National Center for Infectious Diseases; K Feldman, DVM, EIS Officer,
CDC.
CDC Editorial Note:
The number of tularemia cases reported annually has decreased substantially
since the first half of the 1900s. The incidence was highest in 1939, when
2,291 cases were reported2 and remained
high throughout the 1940s. The number of cases declined substantially in the
1950s and 1960s to the relatively constant number of cases reported since
that time.
In the United States, most persons with tularemia acquire the infection
from arthropod bites, particularly tick bites, or from contact with infected
mammals, particularly rabbits. Historically, most cases of tularemia occurred
in summer, related to arthropod bites, and in winter, related to hunters coming
into contact with infected rabbit carcasses. In recent years, a seasonal increase
in incidence has occurred only in the late spring and summer months, when
arthropod bites are most common. Outbreaks of tularemia in the United States
have been associated with muskrat handling,3
tick bites,4-5 deerfly bites,6 and lawn mowing or cutting brush.7
Sporadic cases in the United States have been associated with contaminated
drinking water8 and various laboratory exposures.9 Outbreaks of pneumonic tularemia, particularly
in low-incidence areas, should prompt consideration of bioterrorism.10
The high incidence of tularemia among males and among children aged
<10 years might be associated with increased opportunity for exposure to
infected ticks or animals, less use of personal protective measures against
tick bites, or diagnostic or reporting bias. The high incidence among American
Indians/Alaska Natives might be associated with their increased risk for exposure;
outbreaks of tularemia have been reported on reservations in Montana and South
Dakota, where a high prevalence of tularemia infection was found in ticks
and dogs.4-5
The findings in this report are subject to several limitations, including
underreporting and the lack of documented laboratory confirmation for all
cases. Surveillance for tularemia could be improved by documenting laboratory
confirmation of diagnosis and by including additional data (e.g., clinical
presentation, exposure history, and outcome).
Following a dramatic decline in the second half of the 20th century,
the incidence of tularemia in the United States remains low. The epidemiologic
characteristics described in this report provide a background against which
unusual patterns of disease occurrence, including bioterrorism events, may
be recognized more quickly.
Acknowledgement
This report was based on data contributed by state and local health
departments.
MMWR. 2002;51:181-184.
3 figures omitted.
REFERENCES
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1. CDC. Case definitions for infectious conditions under public health surveillance. MMWR Recomm Rep. 1997;46(RR-10):1-55.
2. Jellison WL. Tularemia in North America, 1930-1974. Missoula, Montana: University of Montana, 1974.
3. Young LS, Bicknell DS, Archer BG, et al. Tularemia epidemic: Vermont, 1968. Forty-seven cases linked to contact
with muskrats. N Engl J Med. 1969;280:1253-60.
4. Schmid GP, Kornblatt AN, Connors CA, et al. Clinically mild tularemia associated with tick-borne Francisella tularensis. J Infect Dis. 1983;148:63-7.
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5. Markowitz LE, Hynes NA, de la Cruz P, et al. Tick-borne tularemia: an outbreak of lymphadenopathy in children. JAMA. 1985;254:2922-5.
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6. Klock AE, Olsen PF, Fukushima T. Tularemia epidemic associated with the deerfly. JAMA. 1973;226:149-52.
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7. Feldman KA, Enscore R, Lathrop S, et al. Outbreak of primary pneumonic tularemia on Martha's Vineyard. N Engl J Med. 2001;345:1601-6.
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8. Jellison WL, Epler DC, Kuhns E, Kohls GM. Tularemia in man from a domestic rural water supply. Public Health Rep. 1950;65:1219-26.
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9. Overholt EL, Tigertt WD, Kadull PJ, et al. An analysis of forty-two cases of laboratory-acquired tularemia. Am J Med. 1961;30:785-806.
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10. Dennis DT, Inglesby TV, Henderson DA, et al. Tularemia as a biological weapon: medical and public health management. JAMA. 2001;285:2763-73.
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