Tularemia is caused by the Gram-negative intracellular bacterium Francisella tularensis
Tularemia is a highly pathogenic disease of animals and humans that has been reported throughout the northern hemisphere including North America, Europe, and Asia. In the United States, naturally occurring infections have been reported in all states except Hawaii.
There are several defined species and subspecies (Foley 2010, Kim 2010):
- F. tularensis subsp. tularensis (Type A) is pathogenic for rabbits and endemic to North America (Fig 1).
- F. tularensis subsp. holarctica (Type B) and F. tularensis supsp. mediasiatica are less pathogenic for rabbits. F. tularensis subsp. holarctica is also endemic to the US.
- F. tularensis subsp. novicida is sometimes identified as a separate, albeit closely related, species F. novicida.
Humans can become naturally infected through several routes
Transmission of F. tularensis occurs primarily through bites from arthropods, including the dog tick (Dermacentor variabilis), the wood tick (D. andersoni), the lone star tick (Amblyomma americanum), and biting flies like the deer fly (Chrysops spp.) (Kim 2010).
Of course contact with infected animals is another common route of transmission through skin contact or inhalation of contaminated aerosols. Animal related-disease transmission is most commonly caused by exposure to wild rabbits or rodents (Kim 2010), however disease transmission has also been traced back to other species like sheep and cats (O’Toole 2008, Gerhold 2012).
”A wide variety of case reports have been published describing unique incidences of rabbit–human transmission, including a lawn mower aerosolizing rabbit nests along with their occupants, consumption of undercooked rabbit meat, and contact with a “lucky” rabbit’s foot” (Kim 2010)
Human infections have also been traced back to the ingestion of contaminated water. This route of transmission appears to be much more common in Europe.
Rabbits, hares, and rodents are particularly susceptible to disease and often die in large numbers during outbreaks
Although many wild and domestic animals can be infected, the wild rabbit is most often implicated in tularemia outbreaks. In fact, tularemia is often referred to as “rabbit fever”. Francisella tularensis can cause profound septicemia, and infected animals are frequently found dead or dying (Kim 2010).
Tularemia has also been reported in various rodent species. Although not a main reservoir of F. tularensis, outbreaks in free-ranging common hamsters (Cricetus cricetus) are believed to be a source of human tularemia infection in Europe (Gyuranecz 2010). Tularemia outbreaks have also occurred among hamsters purchased from pet stores. There is a case report of an American child developing tularemia after being bitten by a pet hamster (CDC 2005). Tularemia in humans has also been traced back to wild-caught, commercially traded prairie dogs (Cynomys spp.) (Alcala Minagorre 2004, Avashia 2004, Peterson 2004, Zeidner 2004).
Symptoms in humans vary with the route of infection
The most common form of disease, called ulceroglandular tularemia, occurs after handling an infected animal or following an insect bite. Disease is manifested by skin ulceration accompanied by regional lymphadenopathy (Fig 2). Pneumonic tularemia, the most serious form of disease, occurs secondary to inhalation of the organism or when ‘lesser’ forms go untreated. Although tularemia is a potentially fatal, multisystemic disease, most human infections can be treated with the judicious use of antibiotics.
Refer to the Centers for Disease Control or CDC Web site for more information on tularemia in human patients.
Wear gloves when handling sick or dead animals
Prevention of tularemia relies upon preventing exposure to the organism:
- Wear gloves when handling sick or dead animals, especially wild rabbits and rodents like prairie dogs.
- Use insect repellent as needed.
- The CDC has also warned the general public to avoiding mowing over dead animals.