Tularemia

This fact sheet was updated 12/6/20.

Other names: Rabbit fever, Hare plague, Lemming fever, Deerfly fever, Ohara’s disease, Yato-byo

Cause

Tularemia is a highly infectious, zoonotic disease caused by the bacterium Francisella tularensis. In North America, the two most common subspecies are called Type A, which is the more virulent type and is associated with rabbits and squirrels, and Type B, which is associated with aquatic rodents such as muskrats and beavers. Tularemia-like disease has been reported throughout history in humans in Europe and Asia. The bacterium was first isolated in the United States in 1911. F. tularensis is now recognized as the cause of a multi-systemic, sometimes fatal, disease of humans and animals.

Significance

Tularemia can be transmitted from infected animals to humans by vectors, direct contact, inhalation, or ingestion of contaminated food or water. Humans are most often exposed when skinning or dressing infected animals. In rare cases, human infections have been reported after running over rabbits with a lawnmower. Laboratory workers, veterinarians, landscapers, farmers, hunters, trappers, and cooks have increased risk of exposure to tularemia. F. tularensis is classified as a “tier 1 select agent” and a potential bioweapon by the federal government because it lends itself to aerosolization, is highly infectious and can cause severe illness and death in humans. All cases of tularemia must be reported to state and federal agencies.

While F. tularensis can cause mortality in wild animals, the effect of this disease on a species or population is unknown.

Species Affected

F. tularensis has been reported in a broad range of host species including mammals, reptiles, birds, fish, and invertebrates. Rabbits, which account for 90% of all human tularemia infections in the United States, and rodents are most commonly infected. The most important rabbit hosts are hares and cottontail rabbits, and the most important rodent hosts include water voles, muskrats, lemmings, hamsters, and red-backed voles. Overall, the most commonly reported cases in North America involve cottontail rabbits, black-tailed jackrabbits, snowshoe hares, beavers, and muskrats.

Distribution

Tularemia is present throughout North America, including the continental United States, Alaska, Canada, and Mexico. It has also been reported in central and western Europe, Africa, and several Asian countries including Russia and Japan. Though tularemia is found primarily in the Northern hemisphere, in 2011, Australia reported its first cases of the disease. 

Transmission

Tularemia can be transmitted via the bite of blood-feeding arthropods including ticks, fleas, flies, and mosquitoes that carry the bacteria. The disease is also transmitted via inhalation of infected droplets or dust contaminated with infected tissues, feces, or urine. Transmission can occur through direct skin contact with sick or dead animals, their blood or tissues. F.tularensis can survive for weeks in soil, water and dead animals. Consumption of undercooked infected animal tissue, and contaminated food or water can also result in disease. Waterborne outbreaks have been reported and are associated with aquatic rodents. 

Clinical Signs

The disease lasts about 2-10 days in infected animals. Though an acute fever is a common sign in both humans and animals, clinical signs are rarely observed in wildlife because the infected animals are usually found dead. Highly susceptible animals exhibit severe lethargy followed by death. Infected animals are often slow and may be captured easily. Less susceptible animals may have local inflammation or enlarged lymph nodes. Less sensitive species are more likely to be chronically infected and will deteriorate in body condition. 

Diagnosis

In wildlife, tularemia is generally a post-mortem diagnosis. At necropsy animals that died of acute tularemia have pale spots in one or all of liver, bone marrow, and spleen. The lungs are often fluid filled and may have dark or firm areas. The liver and spleen may be enlarged. Diagnosis is confirmed by isolating F. tularensis from the infected animal’s blood or tissues. 

Treatment

Antibiotics can be used to treat tularemia. However, it is not economically feasible or practical to treat animals in the wild.

Management

Tularemia occurs naturally in wild rabbit and rodent populations. Increased rabbit and tick population densities can lead to an increase in the occurrence of the disease. Wearing long sleeve shirt and pants, using insect repellent and promptly removing any ticks can help prevent infections. Beaver and muskrat trappers are at risk of waterborne exposure and should be cautioned against drinking from streams. Rabbit hunters and rodent trappers should be aware of the potential risk and should take precaution by wearing gloves, practicing proper hygiene, and cooking meat thoroughly to at least 165° F. It is also important to seek medical attention immediately when a possible exposure occurs as delay in treatment can cause serious disease and death. 

Suggested Reading

Images

Tularemia distribution in the United StatesReported human cases of Tularemia in the U.S. 2003-2012
Liver of a muskrat infected with TularemiaLiver of a muskrat infected with Tularemia. Photo by Walt Cottrell, Pennsylvania Game Commission