Tularemia is a highly infectious zoonotic disease caused by several subspecies of the gram-negative bacterium Francisella tularensis (Table 286-1). Francisella tularensis is a small, aerobic, nonmotile gram-negative bacterium first identified in 1911 by Dr. Edward Francis, after an outbreak of plague-like disease in rodents in Tulare County, California. Infection has been reported in humans since 1914. In the United States, nearly all human cases of tularemia are caused by the F tularensis subspecies tularensis (type A, 66%) or F tularensis subspecies holarctica (type B, 34%).
TABLE 286-1FRANCISELLA TULARENSIS SUBSPECIES ||Download (.pdf) TABLE 286-1FRANCISELLA TULARENSIS SUBSPECIES
|Subspecies ||Geographic Distribution ||Clinical Characteristics |
|tularensis (type A) ||North America (70%) ||More virulent; lagomorph exposure |
|holarctica (type B) ||North America (30%), Europe, Siberia, Japan ||Less virulent; rodent exposure |
|mediasiatica ||Central Asia, some parts of former Soviet Union ||Little data are known about this subspecies |
|novicida ||North America ||Not recognized by all antibody-based testing |
PATHOGENESIS AND EPIDEMIOLOGY
Mammals provide the primary reservoir for F tularensis, including ground squirrels, rabbits, hares, voles, muskrats, water rats, and other rodents. Human infection typically occurs after handling infected animals or after a bite from an arthropod vector. In the United States, biting flies and ticks are the primary arthropod vectors. In Europe and the former Soviet Union, ticks and mosquitoes have been reported to transmit infection. Infection can also occur after ingestion of contaminated food or water or after inhalation of the organism from decaying animal carcasses, contaminated straw, or other sources. There have been several large waterborne outbreaks of tularemia in Europe and the former Soviet Union. The largest airborne outbreak of tularemia was reported among farmers in Sweden in the 1960s, attributed to the aerosolization of organisms from rodent-infested hay. There has been no documented person-to-person transmission of tularemia.
Once the organism gains access into the body, it begins to multiply and then spreads to the local lymph nodes. Once in the lymph nodes or tissues (eg, skin, liver, spleen, lungs) necrosis begins (eg, ulceration of the skin at the tick bite site) and true granulomas develop, often leading to microabscess or abscess formation. The caseating granulomas that form can be indistinguishable from those caused by infections with Mycobacterium tuberculosis. The severity of illness is determined by the virulence of the organism, the inoculum size, the portal of entry of the organism, and the immune system of the host.
Infections with F tularensis are found only in the Northern Hemisphere. In the United States, cases are reported from the eastern seaboard, Arkansas, Missouri, Oklahoma, and the central mountain regions. Other endemic areas include Eurasia, particularly the former Soviet Union, Japan, and the Scandinavian countries. Tularemia is not a World Health Organization (WHO) reportable disease. The incidence of disease is believed to have decreased significantly around the world in the ...