Tularemia overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Tularemia is a serious infectious disease caused by the bacterium Francisella tularensis. The disease is endemic in North America, and parts of Europe and Asia. The primary vectors are ticks and deer flies, but the disease can also be spread through other arthropods. Animals such as rabbits, prairie dogs, hares and muskrats serve as reservoir hosts. The disease is named after Tulare County, California.
Historical Perspective
Franciscella tularensis was discovered in 1911 during an outburst of rabbit fever, when the disease killed a large number of ground squirrels in the area of Tulare Lake in California. Scientists determined that tularemia could be dangerous to humans: a human being may catch the infection after contacting an infected animal. The ailment soon became frequent with hunters, cooks and agricultural workers.[2]
Pathophysiology
Causes
Francisella tularensis is one of the most infective 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. It has been contracted from inhaling particles from an infected rabbit ground up in a lawnmower (see below). 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. The course of disease is similar regardless of the route of exposure. Mortality in untreated (pre-antibiotic-era) patients has been as high as 50% in the pneumoniac and typhoidal forms of the disease, which however account for less than 10% of cases.[1] Overall mortality was 7% for untreated cases, and the disease responds well to antibiotics with a fatality rate of about 2%. The exact cause of death is unclear, but it is thought be a combination of multiple organ system failures.
Classification
Differential Diagnosis
Epidemiology and Demographics
Risk Factors
Natural History, Complications & Prognosis
The disease has a very rapid onset, with headache, fatigue, dizziness, muscle pains, loss of appetite and nausea. Face and eyes redden and become inflamed. Inflammation spreads to the lymph nodes, which enlarge and may suppurate (mimicking bubonic plague). Lymph node involvement is accompanied by a high fever. Death may result.