Tularemia is a caused by the bacterium Francisella tularensis. The two most common subspecies are called Type A, which is the more virulent type and is associated with rabbits, and Type B, which is associated with aquatic rodents. Tularemia-like disease has been reported throughout history in humans in Europe, Japan, and the United states, but 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.
Tularemia is a zoonotic disease, meaning that it can be transmitted from infected animals to humans by certain vectors (listed below), inhalation, or ingestion of contaminated food or water. Humans are most often exposed when skinning or dressing infected rabbits. 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. Tularemia is listed as a possible bioterrorism agent because it is highly infectious and has the potential to cause severe illness and death in humans.
F. tularensis has been reported in a broad range of host species including mammals, reptiles, birds, fish, and invertebrates. Rabbits 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. The most commonly infected species in North America are cottontail rabbits, black-tailed jackrabbits, snowshoe hares, beavers, and muskrats. Tularemia is occasionally reported in domestic cats and occasionally in domestic dogs.
Exposure to infected rabbits accounts for 90% of human infections in the United States, the majority of these being from cottontail rabbits.
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.
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. 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.
The disease lasts about 2-10 days in infected 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.
At necropsy animals that died of acute tularemia have pale spots throughout the 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. Less sensitive species are more likely to be chronically infected and will deteriorate in body condition.
Diagnosis is confirmed by isolating F. tularensis from the infected animal’s blood or tissues.
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. Beaver and muskrat trappers are at a greater risk of waterborne exposure. Rabbit hunters and rodent trappers should be aware of the potential risk and should take precaution by wearing gloves, practicing simple hygiene, and cooking meat thoroughly (a meat thermometer should read at least 165° F). It is also important that trappers seek medical attention when a possible exposure occurs. People should also use insect repellent and promptly remove ticks to help prevent becoming infected.