Infectious diseases

Francisella tularensis

Tularemia is a highly infectious disease caused by the bacterium Francisella tularensis. Infections in humans are not contagious and most often result from contact with infected wildlife, ingestion of or contact with contaminated water, or bites from ticks and other arthropods that have fed on infected wildlife. Aerosol transmission is another way humans can become infected. Disease is expressed in different clinical forms, and varies in severity depending on the virulence of the organism, dose, and site of inoculums. 

Tularemia has a broad geographic distribution in the Northern Hemisphere and is more restricted elsewhere. Reported human cases of tularemia within the USA were greatest during the 1930s and 1940s, averaging over 1,000 cases per year. Since then, the yearly average has fallen below 200; however, the past importance of tularemia as a human disease, considerations of tularemia as a biological weapon and several recent major epidemics have caused renewed interest in this disease.

Clinical features: Most cases of naturally occurring tularemia are ulceroglandular disease, involving an ulcer at the inoculation site and regional lymphadenopathy. Lymphadenopathy can take a significant period to resolve, even with treatment. Other presentations (oculoglandular and oropharyngeal disease) may occur. Occasionally patients with tularemia present with a non-specific febrile systemic illness (typhoidal tularemia) without evidence of a primary inoculation site. Pulmonary disease can occur naturally (pneumonic tularemia), but is uncommon.

Diagnosis:The diagnosis of tularemia in humans is supported by a variety of assays and clinical evaluations readily interpreted by physicians. However, in a small number of the cases, diagnosing tularemia during early stages of the disease may be difficult because of the multiple clinical syndromes presented.

The diagnosis of tularemia by culture can be challenging because the organism grows poorly on routine culture medium. The low sensitivity of culture methods together with the lack of standardization in PCR methodology for the direct identification of the pathogen make serological assay the most widely used tool for the diagnosis of tularemia. Levels of antibody may be measurable within 1 week after infection, although significant levels usually appear in 2 weeks. Antibody levels against F. tularensis can remain detectable for years.

Treatment: Tularaemia responds well to antibiotic therapy and the mortality rate of the more acute forms of the disease is reduced significantly if the patient receives suitable antibiotics.

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