Tularemia is a zoonosis caused by Francisella tularensis. Humans of any age, sex, or race are universally susceptible to this systemic infection. Tularemia is primarily a disease of wild animals and persists in contaminated environments, ectoparasites, and animal carriers. Human infection is incidental and usually results from interaction with biting or blood-sucking insects, contact with wild or domestic animals, ingestion of contaminated water or food, or inhalation of infective aerosols. The illness is characterized by various clinical syndromes, the most common of which consists of an ulcerative lesion at the site of inoculation, with regional lymphadenopathy and lymphadenitis. Systemic manifestations, including pneumonia, typhoidal tularemia, meningitis, and fever without localizing findings, pose a greater diagnostic challenge.
Etiology and Epidemiology
Tularemia is common in Arkansas, Oklahoma, Missouri, and South Dakota; these states account for more than half of all reported cases in the United States. Small outbreaks in higher-risk populations (e.g., professional landscapers cutting up brush, mowing, and using a leaf blower) have been reported from the island of Martha's Vineyard in Massachusetts. Although the irregular distribution of cases of tularemia makes worldwide estimates difficult, increasing numbers of cases have been reported from the Scandinavian countries, Eastern Europe, and Siberia.
With rare exceptions, tularemia is the only disease produced by F. tularensis—a small (0.2 μm by 0.2–0.7 μm), gram-negative, pleomorphic, nonmotile, non-spore-forming bacillus. Bipolar staining results in a coccoid appearance. The organism is a thinly encapsulated, nonpiliated strict aerobe that invades host cells. In nature, F. tularensis is a hardy organism that persists for weeks or months in mud, water, and decaying animal carcasses. Dozens of biting and blood-sucking insects, especially ticks and tabanid flies, serve as vectors. Ticks and wild rabbits are the source for most human cases in endemic areas of the southeastern and Rocky Mountain states. In Utah, Nevada, and California, tabanid flies are the most common vectors. Animal reservoirs include wild rabbits, squirrels, birds, sheep, beavers, muskrats, and domestic dogs and cats. Person-to-person transmission is rare or nonexistent. Tularemia is more common among men than among women.
The four subspecies of F. tularensis are tularensis, holarctica, novicida, and mediasiatica. The first three of these subspecies are found in North America; in fact, subspecies tularensis has been isolated only in North America, where it accounts for >70% of cases of tularemia and produces more serious human disease than other subspecies (although, with treatment, the associated fatality rate is <2%). The progression of illness depends on the infecting strain's virulence, the inoculum size, the portal of entry, and the host's immune status. F. tularensis is a class A bioterrorism agent.
Ticks pass F. tularensis to their offspring transovarially. The organism is found in tick feces but not in large quantities in tick salivary glands. In the United States, the disease is carried by Dermacentor andersoni (Rocky Mountain wood tick), D. variabilis (American dog tick), D. occidentalis (Pacific Coast dog tick), and Amblyomma americanum (Lone Star tick). F. tularensis is transmitted frequently during blood meals taken by embedded ticks ...