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DEFINITION

Tularemia is a zoonotic disease caused by the bacterium Francisella tularensis. Human infection is rare but widespread and can be life-threatening. Sources of human infection include arthropod bites, agricultural aerosols, contaminated food or water, and contact with tissues of infected animals. Clinical diagnosis of tularemia can be challenging as the disease manifestations are diverse, with up to six distinct clinical syndromes. The prognosis is favorable when effective antimicrobial treatment is initiated early; however, complications are common if treatment is delayed.

ETIOLOGY

F. tularensis is a small (0.2 × 0.2–0.7 μm), aerobic, nonmotile, non-spore-forming, gram-negative coccobacillus. The bacterium is dependent on the invasion of host cells in vivo to multiply and cause disease. Genetically, it is not closely related to other known human pathogens. F. tularensis can enter the human body through the skin, mucous membranes, or respiratory tract. The infectious dose is low, with inhalation of 25 or fewer organisms sufficient to cause illness. Historically, F. tularensis was developed as a biological weapon and is currently classified as a Tier 1 select agent (Chap. S2). Two subspecies of F. tularensis, subsp. tularensis (hereafter referred to as type A) and subsp. holarctica (hereafter referred to as type B) cause human tularemia in the United States.

EPIDEMIOLOGY

Image not available. Tularemia occurs widely throughout the Northern Hemisphere. The disease is nationally notifiable in the United States, and cases have been reported in all U.S. states except Hawaii (Fig. 165-1). States located in the south-central and midwestern regions—specifically, Arkansas, Kansas, Missouri, and Oklahoma—account for a disproportionate number of cases. Despite a substantial decrease during the mid-twentieth century, U.S. case counts have remained relatively stable since 1970 (Fig. 165-2). During the 10-year period from 2006 through 2015, 93–314 cases were reported annually (average, 147 cases). The year 2015 marked a substantial increase in cases in Colorado, Wyoming, South Dakota, and Nebraska, with >100 cases reported among residents of these four states. The incidence was highest in Wyoming (35.8 cases per 1 million population), far higher than the national average of 1 case per 1 million population in 2015.

FIGURE 165-1

Yearly reported tularemia cases in the United States, 1950–2015.

FIGURE 165-2

Map displaying reported U.S. tularemia cases in 2000–2015. One dot was plotted randomly within the county of residence for each reported case.

In nature, F. tularensis is maintained by arthropod and animal hosts. The bacterium is transmitted among animal hosts by arthropod bite or by direct exposure to contaminated materials in the environment. F. tularensis can infect and cause illness in an exceedingly broad variety of animals, having been isolated from >100 species, including domestic animals (cats and dogs). However, lagomorphs (wild hares and cottontail ...

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