This organism is maintained in numerous and diverse mammalian (rabbits, hares, and rodents) and tick reservoirs. In addition, a rodent-mosquito cycle has been described.

Small and medium-sized mammals are the principal natural reservoirs for F tularensis. Examples include (see References: Dennis 1998, Gelman 1961, Hopla 1974):

  • Lagomorphs (rabbits, hares) (predominantly North America, Europe, Japan)
  • Aquatic rodents (beaver, muskrats, water voles)
  • Field voles
  • Water and wood rats
  • Squirrels
  • Lemmings (former Soviet Union, Sweden, Norway)
  • Meadow and field mice (predominantly former Soviet Union)

Humans, other mammalian species (eg, cats, dogs, cattle, primates), and some species of birds, fish, and amphibians are incidental hosts.

  • A serologic survey of 91 cats in Connecticut and New York found that 12% had antibodies to F tularensis (see References: Magnarelli 2007).
  • An outbreak of tularemia in commercially distributed prairie dogs was recognized in the United States in 2002 (see References: CDC 2002: Outbreak of tularemia among commercially distributed prairie dogs, 2002; Petersen 2004). Serologic testing of potentially exposed persons demonstrated that one person (an animal handler) had a positive F tularensis titer of 1:128 on initial testing that subsequently declined to 1:32 on follow-up testing at 4 and 6 months, suggesting prairie dog–to-human transmission (see References: Avashia 2004).
  • Outbreaks also have occurred in nonhuman primates housed outdoors. In a German primate facility, 18 of 35 cynomolgus monkeys (Macaca fasicularis) contracted tularemia within a 2-year period; 6 of the animals died (see References: Matz-Rensing 2007).

Information from studies conducted on Martha's Vineyard suggests that F tularensis can persist in the environment and that persons can acquire infection by engaging in activities that lead to aerosolization (such as lawn mowing, weed-whacking, and using a power blower) (see References: Feldman 2001, Feldman 2003). An analysis of sera from a variety of mammals on Martha’s Vineyard found that skunks and raccoons were frequently seroreactive (49% of skunks tested and 52% of raccoons), whereas white-footed mice, cottontail rabbits, deer, rats, and dogs were much more likely to be seronegative (see References: Berrada 2006).

During the fall and winter of 2003, F tularensis was identified on several filters from a biodetection air-monitoring system in Houston, Texas (see Oct 10, 2003, CIDRAP News story). An investigation conducted at that time supported contamination of the filters by naturally occurring F tularensis organisms, although the environmental reservoir was not definitively identified.

Diverse bacteria, including select agents, have been detected in urban air by DNA arrays during a 17-week study of airborne bacterial composition and dynamics in Austin and San Antonio, Texas. Although taxonomic clusters containing organisms closely related to Francisella were found, F tularensis was never encountered (see References: Brodie 2007).

In a study of natural outbreaks of human tularemia in two locations in Sweden, genotyping of F tularensis subsp holarctica isolates was used in conjunction with information obtained from patients to investigate the epidemiology and geographic spread of disease. Strong, highly localized spatial associations were found between F tularensis subpopulations and places of disease transmission, indicating likely point sources of infection. Most patients were infected from mosquito bites. Disease clusters were linked to recreational areas near water, and identical subtypes were present throughout the tularemia season and persisted over different years (see References: Svensson 2009). As with the studies on Martha's Vineyard, these findings support persistence of F tularensis in the environment over time.

F tularensis appears to survive within Acanthamoeba (relatively ubiquitous protozoa), suggesting that these organisms may serve as a reservoir for F tularensis. Survival within Acanthamoeba may provide a mechanism for F tularensis to persist in the environment (see References: Abd 2003).

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