Tularemia

Basics

Description

Tularemia is an acute febrile illness caused by the small aerobic gram-negative pleiomorphic intracellular coccobacillus Francisella tularensis:
- Organism is highly infectious
- Person-to-person transmission has not been reported
Humans become infected through different environmental exposures:
- Bites from infected tick, deer fly, mosquito, or other infected insect
- Direct contact with infectious animal tissue or fluid
- Contact with or ingestion of contaminated food, water, or soil
- Inhalation of infected aerosols (e.g., cutting grass with power mowers, which may aerosolize the organism)
- Prevention should focus on insect repellents, avoiding untreated surface water, using gloves when handling animals such as rabbits, prairie dogs, rodents, and cooking meat thoroughly
The 4 major strains of the bacterium have different virulence and geographic location:
- Two subspecies cause human infection in North America: F. tularensis subspecies tularensis (type A, more virulent) and F. tularensis subspecies holartica (type B, less virulent). In Europe, type B predominates
Natural hosts:
- Lagomorphs and other rodents
- Found in species of wild animals (rabbits, hares, ticks, muskrats, beavers, mice), domestic animals (sheep, cattle, cats), ticks, and water and soil contaminated by infected animals
Natural vectors:
- Ticks
- Biting flies
- Mosquitoes
- Wild rabbits
- Rodents
Weaponization of tularemia was accomplished during the Cold War:
- Because of its virulence and ability to be aerosolized, it remains a potential biological agent for mass destruction
Laboratory technicians handling culture specimens are at high risk:
- F. tularensis cultures should be manipulated only in a biosafety level 3 facility

Etiology

Individuals who spend time outdoors in endemic areas are at higher risk:
- Farmers
- Hunters
- Forest workers
- Lawnmowers/landscapers
- Those who handle animal carcasses are at highest risk (taxidermists and butchers)
- Those in contact with polluted waters
- Two-thirds of cases occur in males
Although tularemia can occur worldwide, it is endemic in the Northern Hemisphere:
- Reported nationwide except in Hawaii
- States with the highest incidence include Missouri, Arkansas, Kansas, South Dakota, and Oklahoma
- Increased incidence in Colorado, Nebraska, South Dakota, and Wyoming
- Few hundred cases annually in the U.S., although probably underreported
- Peak season is June–October
Mortality is 5–15%. Appropriately treated patients have mortality as low as 1%.