Zoonotic concern: Tularemia in Rabbits and Rodents

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).

Figure 1. Tularemia can be transmitted to humans while handling infected rabbits and other potential reservoir hosts. Photo credit: Jim via Flickr Creative Commons. Click image to enlarge.

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).

Figure 2. A skin ulcer develops where Francisella tularensis enters the body. Photo credit: ‘Consequentially’ via Wikimedia Commons. Click image to enlarge.

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