1. IDENTIFICATION

Also known as: Rabbit fever, Deer-fly fever, Ohara disease, Francis disease.  Tularemia is a zoonotic bacterial disease with a variety of clinical manifestations related to the route of introduction and the virulence of the disease agent. Most often it presents as an indolent (i.e., slow to heal) ulcer at the site of introduction of the organism (see picture), together with swelling of the regional lymph nodes (ulceroglandular type). There may be no apparent primary ulcer, but only one or more enlarged and painful lymph nodes that may suppurate (glandular type). Ingestion of organisms in contaminated food or water may produce a painful pharyngitis (with or without ulceration), abdominal pain, diarrhea and vomiting (oropharyngeal type). 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); bloodbome 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 (oculoglandular type). Pneumoma may complicate all clinical types and requires prompt identification and specific treatment to prevent a fatal outcome.

Two biovars (biotypes) with differing pathogenicity cause human disease. Jellison type A 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. Jellison type B organisms are less virulent and, even without treatment, produce few fatalities. Clinically, because of buboes and/or severe pneumonia, tularemia may be confused with plague, as well as many other infectious diseases, including staphylococcal and streptococcal infections, catscratch fever and sporotrichosis.

Diagnosis is most commonly made clinically and confirmed by a rise in specific serum antibodies that usually appear in the second week of the disease. Cross-reactions occur with Brucella species. Examination of ulcer exudate, lymph node aspirates and other clinical specimens by FA test may provide rapid diagnosis. Diagnostic biopsy of acutely infected lymph nodes should be done only under the cover of specific antibiotic treatment since it will often induce bacteremia. The causative bacteria can be cultured on special media such as cysteine-glucose blood agar or by inoculation of laboratory animals with material from lesions, blood or sputum. The biovars are differentiated by their chemical reactions; type A organisms ferment glycerol and convert citrulline to ornithine. Extreme care must be exercised to avoid laboratory transmission of highly infectious aerosolized organisms; hence, culture identification is performed only in reference laboratories and most cases are diagnosed serologically.