Tularemia: A Brief Overview

December 31, 2006

Francisella tularensis is anonsporulating, nonmotile,aerobic gram-negative coccobacillusthat is usually transmittedto humans frominfected rabbits and other small animalsvia ticks, fleas, or deer flies orby direct contact

Francisella tularensis is anonsporulating, nonmotile,aerobic gram-negative coccobacillusthat is usually transmittedto humans frominfected rabbits and other small animalsvia ticks, fleas, or deer flies orby direct contact.

EPIDEMIOLOGY
Although tularemia was commonin the United States before World WarII, the incidence of this infection hasdeclined and in recent years has remainedbetween 0.05 and 0.15 casesper 100,000. The peak incidence is inJune, September, and December.

CLINICAL COURSE
The incubation period averages3 to 5 days, but ranges from 1 to 21days. There are a number of possibleclinical presentations, depending onthe route of entry, virulence of theparticular organism, and the host'simmune status.

The most common presentationsare ulceroglandular and typhoidal disease.The first makes up 21% to 87%of cases in the United States and isthe result of skin or mucosal inoculation.Patients present with localizedenlargement and tenderness oflymph nodes and one or more painfululcerative skin lesions (Figure). Thesecond type of presentation resultsfrom aerosol exposure and is notablefor fever, headache, prostration,cough, and substernal pain, withoutlymphadenopathy.

Pneumonia can occur with eitherof these presentations but is mostcommon with the typhoidal form.1 Itcan also occur as a primary diseaseprocess. Primary tularemic pneumonia and the typhoidal form of tularemiaare the most likely presentationsfollowing intentional release ofF tularensis. An elevated creatinephosphokinase level, myoglobinuria,renal failure, and adult respiratorydistress syndrome may complicatethe course of tularemia.

DIAGNOSIS
The diagnosis of tularemia isusually made by using serologicmethods (tube agglutination, microagglutination,and ELISA) at least severalweeks after the onset of illness.F tularensis is fastidious in vitro, andcultivation of the causative agent ishazardous for laboratory personnel.PCR assays are not yet widely available.2Perhaps the biggest impedimentto a rapid diagnosis is the lackof clinical suspicion for tularemia in apatient with either primary pneumoniaor typhoidal disease and no apparenttraditional animal exposures.Streptomycin is the drug of choiceand gentamicin is an effective alternative.3 Approximately 35% of untreatedpatients die of this disease.Person-to-person spread of tularemiais extremely rare; respiratoryprecautions need not be observed.4