Tularemia pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Tularemia is caused by the bacterium Francisella tularensis found in animals (especially rodents, rabbits, and hares). Francisella tularensis (F. tularensis) is a tiny, pleomorphic, nonmotile, gram-negative, facultative intracellular coccobacillus (0.2 to 0.5 μm by 0.7 to 1.0 μm). It is a fastidious organism and may require cysteine supplementation for good growth on general laboratory media.
Pathogenesis
Transmission
- Bacteremic phase initiates the spread of disease into reticuloendothelial tissue.
- This evokes an imunnological reaction resulting in a flurry of tumor necrosis factor alpha, interleukin-10 and 12, as well as IFN-gamma.
- T-cells become involved as the disease progresses.
- Studies show that T-cells are necessary in stopping the reaction, but not controlling it.
- A successful tularemia infection is the ultimate result of the disease's ability to survive within macrophage host cells.
Mechanism of infection
- Francisella tularensis is one of the most infectious bacteria known; fewer than ten organisms can cause disease leading to severe illness.
- The bacteria penetrate into the body through damaged skin and mucous membranes, or through inhalation.
- Humans are most often infected by tick bite or through handling an infected animal. Ingesting infected water, soil, or food can also cause infection.
- Tularemia can also be acquired by inhalation; hunters are at a higher risk for this disease because of the potential of inhaling the bacteria during the skinning process.
- Tularemia is not spread directly from person to person.
- Francisella tularensis is an intracellular bacterium, meaning that it is able to live as a parasite within host cells.
- It primarily infects macrophages, a type of white blood cell. It is thus able to evade the immune system.
- The course of disease involves spread of the organism to multiple organ systems, including the lungs, liver, spleen, and lymphatic system.