Tularemia is an uncommon but potentially serious bacterial zoonosis caused by Francisella tularensis. Transmission to humans occurs through arthropod bites, ingestion of contaminated food or water, inhalation of contaminated aerosols, or handling of infected animal tissues. Clinical manifestations are variable and depend on the route of inoculation, the dose, and the virulence of the organism. Most commonly, the disease presents in humans as an indolent ulcer at the site of cutaneous inoculation accompanied by regional lymphadenitis (ulceroglandular form). Other forms include glandular, oculoglandular, oropharyngeal, gastrointestinal, septic, and pneumonic tularemia. Although uncommon, tularemia occurs widely in temperate and subarctic regions of North America and Eurasia. Currently in the United States, fewer than 200 cases are reported annually.1 F. tularensis is classified as a Tier 1 Select Agent and has been evaluated as a potential weapon by several countries.
F. tularensis is a small (0.2 by 0.2–0.7 µm), nonmotile, pleomorphic, gram-negative coccobacillus. Although nonsporulating and strictly aerobic, the bacterium is a hardy saprophyte that can survive in water, moist soil, and in decaying animal carcasses. In the laboratory, F. tularensis is fastidious, slow growing, and requires cysteine, cystine, or other sulfhydryl containing media.
Isolates of F. tularensis can be divided into several subspecies based on virulence testing, biochemical reactions, PCR, and epidemiologic features. Two subspecies account for most human illness: F. tularensis subspecies tularensis (Jellison Type A) and F. tularensis subspecies holarctica (formerly paleartica, Jellison Type B). Type A strains have an LD50 in rabbits of fewer than ten organisms and are generally considered more virulent. They are found almost exclusively in North America. Type B strains have an LD50 of more than 107 organisms in rabbits and are found in both North America and Eurasia. Further discrimination of isolates can be achieved using a variety of molecular assays, including pulsed-field gel electrophoresis, multiple-locus variable-number tandem repeat analysis, single-nucleotide polymorphisms, and whole-genome microarray.2,3
LIFE CYCLE OF F. TULARENSIS
F. tularensis has been isolated from more than 100 species of wild mammals, at least 9 species of domestic animals (including cattle, dogs, cats), 25 species of birds, amphibians, fish, and more than 50 species of arthropods. However, many of these animals may be colonized or infected only incidentally. Actual maintenance cycles, although incompletely defined, appear to differ among F. tularensis subspecies. Type A F. tularensis is believed to occur primarily in rabbits with transmission by ticks (Demacentor spp, Amblyomma spp, Ixodes spp) and tabanid or deer flies (Cysops discalis). Type B infections are often associated with aquatic environments and rodents such as beaver, muskrats, and voles. Transmission among these animals may occur through ingestion of contaminated water, soil, or food. Epidemiological studies have also implicated mosquitoes as a potential mode of Type B transmission to humans in Europe.4 F. tularensis has been shown ...