Rabbit Fever Tularemia

Tularemia is widespread among wild animals in the United States, involving species as diverse as rabbits, muskrats, and bobcats. Many human cases are acquired when people are skinning animals that appear to be free of disease. The causative organism enters through unnoticed scratches or by penetration of a mucous membrane. The disease can also be acquired from the bites of flies and ticks and by inhalation of the causative organism. It is potentially a disease transmitted by bioterrorism.

Symptoms

Tularemia is characterized by development of a skin ulceration and enlargement of the regional lymph nodes 2 to 5 days after a person is bitten by a tick or insect or handles a wild animal. The usual symptoms of fever, chills, and achiness that occur in many other infectious diseases are also present in tularemia. Symptoms usually clear in 1 to 4 weeks, but sometimes they last for months.

Causative Agent

Tularemia is caused by Francisella tularensis, a non-motile, aerobic, Gram-negative rod that derives its name from Edward Francis, an American physician who studied tularemia in the early 1900s, and from Tulare County, California, where it was first studied. The organism is unrelated to other common human pathogens and is unusual in that it requires a special medium enriched with the amino acid cysteine in order to grow.

Pathogenesis

Typically, F. tularensis causes a steep-walled ulcer where it enters the skin (figure 28.4). Lymphatic vessels draining the area carry

28.3 Bacterial Diseases of the Lymph Nodes and Spleen 721

28.3 Bacterial Diseases of the Lymph Nodes and Spleen 721

Lymfonodes Rabbit

Figure 28.4 Ulceroglandular Form of Tularemia in a Muskrat Trapper

The healing ulcer above the patient's left eyebrow is the site of entry of Francisella tularensis.

Figure 28.4 Ulceroglandular Form of Tularemia in a Muskrat Trapper

The healing ulcer above the patient's left eyebrow is the site of entry of Francisella tularensis.

the organisms to the regional lymph nodes. These nodes then become large and tender, and they may become filled with pus and drain spontaneously. Later, the organisms spread to other parts of the body via the lymphatics and blood vessels. Pneumonia, which occurs in 10% to 15% of the cases, occurs when the organisms infect the lung from the bloodstream or by inhalation. Tularemic pneumonia has a mortality rate as high as 30%, most pneumonias occurring in individuals who work with the organism in laboratories. Francisella tularensis, like Mycobacterium tuberculosis, is ingested by phagocytic cells and grows within them. This may explain why tularemia persists in some people despite the high titers of antibody in their blood. Cell-mediated immunity is responsible for ridding the host of this infection, as it is with other pathogens that can live intracellularly. Both delayed hypersensitivity and serum antibodies quickly arise during infection, so that even without treatment over 90% of infected people survive. ■ cell-mediated immunity, p. 394 ■ delayed hypersensitivity, p. 449

Epidemiology

Tularemia occurs among wild animals in many areas of the Northern Hemisphere, including all the states of the United States except Hawaii. In the eastern United States, human infections usually occur in the winter months, as a result of people skinning rabbits. Hence, the common name, "rabbit fever." Hunters and trappers in various parts of the country, however, have contracted the disease from muskrats, beavers, squirrels, deer, and other wild animals. The animals are generally free of illness. In the West, infections mostly result from the bites of infected ticks and deer flies and, thus, usually occur during the summer. Generally, 150 to 250 cases of tularemia are reported each year from counties across the United States (figure 28.5).

Prevention and Treatment

Rubber gloves and goggles or face shields are advisable for people skinning wild animals; remember that the bacteria can enter

Ocular Tularemia
Figure 28.5 Reported Cases of Tularemia—United States, 1990-2000

the body via mucous membranes. Insect repellants and protective clothing help guard against insect and arachnid vectors. It is a good practice to inspect routinely for ticks after exposure to the out-of-doors and to remove them carefully. A vaccine is available for laboratory workers, veterinarians, trappers, game wardens, and others at high risk for infection. Most cases of tularemia are effectively treated with tetracycline or gentamicin. The main features of this disease are summarized in table

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Responses

  • valerie
    What is the causative agent in rabbit fever?
    3 years ago

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