Tularemia -- United States, 1990-2000

Morbidity and Mortality Weekly Report. 2002;51(9) 

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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 (Figure 1), 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) (Figure 2).

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 (Figure 3). 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.

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