- Tularemia is caused by the gram-negative intracellular bacterium Francisella tularensis.
- Wild rabbits, hares, and rodents are particularly susceptible to disease and often die in large numbers during outbreaks.
- Humans can become naturally infected through several routes. Although transmission occurs primarily through arthropod bites, exposure can also occur through skin contact with wild rabbits or rodents as well as inhalation of contaminated aerosols.
- Symptoms in humans vary with the route of infection, but the most common form of disease occurs after handling an infected animal or following an insect bite. Clinical findings include skin ulceration and regional lymphadenopathy.
- Prevention relies upon preventing exposure to the organism. Wear gloves when handling sick or dead animals, especially wild rabbits and rodents, like prairie dogs.
What is tularemia?
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 (7,12):
- F. tularensis subsp. tularensis (Type A) is pathogenic for rabbits and endemic to North America.
- 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.
What species are most commonly affected by tularemia?
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 (12).
Tularemia has also been reported in various rodent species (8, 9). 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 (11). Tularemia outbreaks have also occurred among hamsters purchased from pet stores. There is also a case report of an American child developing tularemia after being bitten by a pet hamster (4). Tularemia in humans has also been traced back to wild-caught, commercially traded prairie dogs (Cynomys spp.) (2,15,17).
How do humans contract tularemia?
Transmission of F. tularensis occurs primarily through arthropod bites, including the dog tick (Dermacentor variabilis), wood tick (D. andersoni), lone star tick (Amblyomma americanum), and biting flies, like the deer fly (Chrysops spp.) (12).
Contact with infected animals through skin or inhalation of contaminated aerosols is another common route of transmission. Animal related-disease transmission is most commonly caused by exposure to wild rabbits or rodents (Fig 1) (12, 16), however disease transmission has also been traced back to other species, like sheep and cats (10,14).
”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” (12,16).
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 (13).
What are the signs of tularemia in humans?
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 (13) (Fig 2).
Pneumonic tularemia, the most serious form of disease, occurs secondary to inhalation of the organism or when ‘lesser’ forms go untreated (13). Although tularemia is a potentially fatal, multisystemic disease, most human infections can be treated with the judicious use of antibiotics (13).
How do I prevent exposure to tularemia?
Prevention of tularemia relies upon preventing exposure to the organism. Wear gloves when handling sick or dead animals, especially wild rabbits and rodents, such as prairie dogs, muskrats (Ondatra zibethicus), and beavers (Castor canadensis) (8,9). The Centers for Disease Control and Prevention has also warned the general public to avoiding mowing over sick or dead animals. Hunters should also cook game meat completely before eating (1,3,6).
To prevent exposure to ticks, use insect repellent as needed and wear long pants, long sleeves, and long socks (3). And of course, ticks should be removed promptly with fine-tipped tweezers (3). Since contaminated water is another potential source of transmission, do not drink untreated surface water (3).
References and further reading
- Agger WA. Tularemia, lawn mowers, and rabbits’ nests. J Clin Microbiol. 2005;43(8):4304; author reply 4304-5. doi: 10.1128/JCM.43.8.4304-4305.2005. PMID: 16082010; PMCID: PMC1233993.
- Avashia SB, Petersen JM, Lindley CM, et al. First reported prairie dog-to-human tularemia transmission, Texas, 2002. Emerg Infect Dis. 2004;10(3):483-6. doi: 10.3201/eid1003.030695. PMID: 15109417; PMCID: PMC3322778.
- Centers for Disease Control and Prevention. Tularemia. December 13, 2018. CDC website. Available at https://www.cdc.gov/Tularemia/. Accessed January 29, 2022.
- Centers for Disease Control and Prevention. Tularemia associated with a hamster bite–Colorado, 2004. MMWR Morb Mortal Wkly Rep. 2005;53(51):1202-3. PMID: 15635290.
- Cherry CC, Kwit NA, Ohms RE, et al. Tularemia (Francisella tularensis) in a black-tailed prairie dog (Cynomys ludovicianus) colony. J Wildl Dis. 2019;55(4):944-946. Epub 2019 Mar 28. PMID: 30920904.
- Djordjevic-Spasic M, Potkonjak A, Kostic V, Lako B, Spasic Z. Oropharyngeal tularemia in father and son after consumption of under-cooked rabbit meat. Scand J Infect Dis. 2011;43(11-12):977-81. doi: 10.3109/00365548.2011.592988. Epub 2011 Jul 8. PMID: 21736510.
- Foley JE, Nieto NC. Tularemia. Vet Microbiol. 2010;140(3-4):332-8. doi: 10.1016/j.vetmic.2009.07.017. Epub 2009 Aug 8. PMID: 19713053.
- Gabriele-Rivet V, Ogden N, Massé A, et al. Eco-epizootiologic study of Francisella tularensis, the agent of tularemia, in Québec wildlife. J Wildl Dis. 2016;52(2):217-29. doi: 10.7589/2015-04-096. Epub 2016 Mar 11. PMID: 26967133.
- Ganoe LS, Brown JD, Yabsley MJ, Lovallo MJ, Walter WD. A review of pathogens, diseases, and contaminants of muskrats (Ondatra zibethicus) in North America. Front Vet Sci. 2020;7:233. doi: 10.3389/fvets.2020.00233. PMID: 32478106; PMCID: PMC7242561.
- Gerhold RW, Jessup DA. Zoonotic diseases associated with free-roaming cats. Zoonoses Public Health. 2013;60(3):189-95. doi: 10.1111/j.1863-2378.2012.01522.x. Epub 2012 Jul 26. PMID: 22830565.
- Gyuranecz M, Dénes B, Dán A, et al. Susceptibility of the common hamster (Cricetus cricetus) to Francisella tularensis and its effect on the epizootiology of tularemia in an area where both are endemic. J Wildl Dis. 2010;46(4):1316-20. doi: 10.7589/0090-3558-46.4.1316. PMID: 20966287.
- Kim DY, Reilly TJ, Schommer SK, Spagnoli ST. Rabbit tularemia and hepatic coccidiosis in wild rabbit. Emerg Infect Dis. 2010;16(12):2016-7. doi: 10.3201/eid1612.101013. PMID: 21122253; PMCID: PMC3294546.
- Nigrovic LE, Wingerter SL. Tularemia. Infect Dis Clin North Am. 2008;22(3):489-504, ix. doi: 10.1016/j.idc.2008.03.004. PMID: 18755386.
- O’Toole D, Williams ES, Woods LW, et al. Tularemia in range sheep: an overlooked syndrome? J Vet Diagn Invest. 2008;20(4):508-13. doi: 10.1177/104063870802000417. PMID: 18599860.
- Petersen JM, Schriefer ME, Carter LG, et al. Laboratory analysis of tularemia in wild-trapped, commercially traded prairie dogs, Texas, 2002. Emerg Infect Dis. 2004;10(3):419-25. doi: 10.3201/eid1003.030504. PMID: 15109407; PMCID: PMC3322795.
- Ryan-Poirier K, Whitehead PY, Leggiadro RJ. An unlucky rabbit’s foot? Pediatrics. 1990;85(4):598-600. PMID: 2314975.
- Zeidner NS, Carter LG, Monteneiri JA, et al. An outbreak of Francisella tularensis in captive prairie dogs: an immunohistochemical analysis. J Vet Diagn Invest. 2004 Mar;16(2):150-2. doi: 10.1177/104063870401600210. PMID: 15053367.
Pollock C. Tularemia in rabbits and rodents. March 21, 2013. LafeberVet Web site. Available at https://lafeber.com/vet/zoonotic-concern-tularemia-in-rabbits-and-rodents/