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Tularemia (Rabbit Fever; Deer-Fly Fever)

Francisella tularensis (formerly Pasteurella tularensis), the bacterial agent for tularemia (PDF), is a small, gram-negative nonmotile coccobacillus. Jellison Type A strain (Francisella tularensis biovar tularensis) has an LD50 in rabbits of fewer than 10 bacteria, while Type B strains (biovar palaearctica) have an LD50 of greater than 100,000,000 in rabbits.

Tularemia (PDF) occurs throughout North America. In the USA, it occurs in all months of the year. Incidence may be higher in adults in early winter during rabbit hunting season and in children during the summer when ticks and deer flies are abundant. Francisella tularensis tularensis organisms, restricted to North America, are common in rabbits (cottontail, jack, and snowshoe), and are frequently transmitted by tick bite. Biovar palaearctica strains are commonly found in mammals other than rabbits in North America.

The reservoirs for the virulent strain include rabbits, hares, and various hard ticks. The organism is transmitted through the bite of arthropods, including the wood tick, the dog tick, the lone star tick, and, less commonly, by the deer fly. Other methods of infection include inoculation of skin, conjunctival sac, or oropharyngeal mucosa with contaminated water, blood or tissue while handling carcasses of infected animals; by handling or ingesting insufficiently cooked meat of infected animal hosts; by drinking contaminated water; by inhalation of dust from contaminated soil, grain, or hay; rarely, from bites of carnivores whose mouth presumably was contaminated from eating an infected animal.

The clinical manifestations are related to the route of introduction and the virulence of the disease agent. Most often, it presents as an indolent ulcer at the site of introduction of the organism, together with swelling of the regional lymph nodes. There may be no apparent primary ulcer, but only one or more enlarged and painful lymph nodes that may suppurate. Ingestion of organism in contaminated food or water may produce painful pharyngitis (with or without ulceration), abdominal pain, diarrhea, and vomiting.

Inhalation of infectious material may be followed by pneumonic involvement or a primary septicemic syndrome, with a 30%-60% case-fatality rate if untreated (typhoidal type). Bloodborne organisms may localize in the lung and pleural spaces (pleuropulmonary type). The conjunctival sac is a rare route of introduction that results in a clinical disease of painful purulent conjunctivitis with regional lymphadenitis. Pneumonia may complicate all clinical types, and requires prompt identification and specific treatment to prevent a fatal outcome.

The two biovars (tularensis and palaearctica) cause human disease of different severity. Francisella tularensis biovar tularensis organisms are more virulent, with an untreated case-fatality rate of 5%-15% primarily due to typhoidal or pulmonary disease. With appropriate antibiotic treatment, the case-fatality rate is negligible. Francisella tularensis biovar palaearctica organisms are less virulent and, even without treatment, produce few fatalities.