Rabbit Fever (or) Tularemia


Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium Francisella tularensis. Symptoms may include fever, skin ulcers, and enlarged lymph nodes. Occasionally, a form that results in pneumonia or a throat infection may occur. The bacterium is typically spread by ticks, deer flies, or contact with infected animals. It may also be spread by drinking contaminated water or breathing in contaminated dust. It does not spread directly between people. Diagnosis is by blood tests or cultures of the infected site.

Depending on the site of infection, tularemia has six characteristic clinical variants: ulceroglandular (the most common type representing 75% of all forms), glandular, oropharyngeal, pneumonic, oculoglandular, and typhoidal. The incubation period for tularemia is one to 14 days; most human infections become apparent after three to five days. In most susceptible mammals, the clinical signs include fever, lethargy, loss of appetite, signs of sepsis, and possibly death. Nonhuman mammals rarely develop the skin lesions seen in people. Subclinical infections are common, and animals often develop specific antibodies to the organism. Fever is moderate or very high, and tularemia bacilli can be isolated from blood cultures at this stage. The 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.

The bacteria can penetrate into the body through damaged skin, mucous membranes, and inhalation. Humans are most often infected by tick/deer fly bite or through handling an infected animal. Ingesting infected water, soil, or food can also cause infection. 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 Francisella tularensis can live both within and outside the cells of the animal it infects, meaning it is a facultative intracellular bacterium. It primarily infects macrophages, a type of white blood cell, and thus is able to evade the immune system. The course of disease involves the spread of the organism to multiple organ systems, including the lungs, liver, spleen, and lymphatic system. The course of disease is different depending on the route of exposure. Mortality in untreated (before the 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.

The laboratorial isolation of F. tularensis requires special media such as buffered charcoal yeast extract agar. It cannot be isolated in the routine culture media because of the need for sulfhydryl group donors (such as cysteine). The microbiologist must be informed when tularemia is suspected not only to include the special media for appropriate isolation, but also to ensure that safety precautions are taken to avoid contamination of laboratory personnel. Serological tests (detection of antibodies in the serum of the patients) are available and widely used. Cross reactivity with Brucella can confuse interpretation of the results, so diagnosis should not rely only on serology. Molecular methods such as PCR are available in reference laboratories.

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