Chapter 16.8 : Tularemia—Francisella tularensis

Abstract:

Francisella tularensis is a tiny (0.2- to 0.5- by 0.7- to 1.0-μm), pleomorphic, nonmotile, fastidious, gram-negative, facultative intracellular coccobacillus. F. tularensis can be divided into two subspecies, F. tularensis subsp. tularensis (type A) and F. tularensis subsp. holarctica (type B), based on virulence testing, 16S sequencing, biochemical reactions, and epidemiological features. Type A and type B strains are highly infectious (e.g., require only 50 to 100 organisms to cause disease) and are the principal agents of tularemia, a zoonotic plague-like disease distributed only in the northern hemisphere ( 3 , 4 ). In the United States, the principal reservoir is the cottontail rabbit (lagomorph), but the disease may also be carried and transmitted by a variety of terrestrial and aquatic mammals, such as beavers, ground squirrels, muskrats, and other rodents ( 3 , 4 ). Transmission to humans can occur through the handling of infected animals; through the bites of ticks, mosquitoes, or deerfly vectors; or by ingestion of contaminated stream water. F. tularensis subsp. novicida is infrequently identified as a cause of human disease.