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Description and Distribution
Tularemia is a disease of rabbits and rodents caused by the bacterium Francisella tularensis. The organism can be transmitted by a variety of ectoparasites and by contact with environmental contamination. Mammals (including man), birds and perhaps reptiles are susceptible to infection.
There is evidence that tularemia in man may have occurred in the nineteenth century in the U.S., Norway, Russia, and Japan before the isolation of causative organisms from California ground squirrels in 1911. Since its identification, tularemia has been reported from all of the continental U.S. (except Vermont), Canada, Mexico, Venezuela, Ecuador and Colombia. The disease has also been reported from Japan, Russia, Turkey, Israel, Scandinavia, central and western Europe, Italy, Thailand, and Tunisia. It has not been detected in Australia or in the British Isle.
In Michigan, tularemia has been found to occur in two forms: terrestrial and aquatic. Both are rare, but have been found throughout the state, including islands in the Great Lakes. The disease has been diagnosed in muskrats, beaver, cottontail rabbits, snowshoe hares and a great horned owl. It usually occurs as an epizootic of varying magnitude, but isolated cases do occur. The disease appears to be most prevalent in the spring, but it may occur at any time of the year.
Transmission and Development
Transmission of tularemia is accomplished directly by bloodsucking arthropods, mechanically by means of contaminated mouth parts, or by contamination of the host's skin by arthropod discharges. A variety of arthropods may be involved; mites, ticks, flies, midges, blackflies, fleas, mosquitoes and lice. The organism may also be transmitted by contact with infected vertebrates, by inhalation of feces-contaminated dust or ingestion of insufficiently cooked infected carcasses. In addition, waterborne epidemics have been reported, and F. tularensis has been found in ponds where there were epidemics among beavers in the northwestern U.S. Human infections are usually the result of dressing or skinning infected rabbits. In the U.S., rabbits are the source of infection in 90% of human cases, 70% of which result from contacts with the genus Sylvilagus. Jackrabbits are an important source of infection in some areas but are a minor factor nationally. Snowshoe hares comprise less than 1% of the source of human infection.
Clinical Signs and Pathology
Clinical manifestations of tularemia in wildlife are not always evident or clearly recognized. Furthermore, the opportunities to observe these signs in natural outbreaks among wild animal populations are extremely limited; when infected animals are found, they are usually moribund or dead.
Tularemic hares and cottontails have been observed to behave oddly, to run slowly, and to be captured easily. Infected rabbits appear to be tame or in a stupor; they do not raise their heads or carry their front feet well; they rub their noses and forefeet into the ground. They have recurrent muscle spasms, and stagger for a few yards between spasms. Clinical signs in experimentally infected red foxes were loss of appetite, diarrhea, and noisy labored breathing.
In general, the gross and histopathologic lesions of tularemia in mammals resemble those of bubonic plague, or paratuberculosis, in rodents. Typical gross lesions are white foci or spots scattered throughout the liver (see illustration), spleen and lymph nodes, varying in size from pinpoint to large irregular conglomerate foci several millimeters in diameter. The spleen and liver may be dark bluish-red and enlarged.
Lesions in naturally infected muskrats and beaver may include excessive clear dark fluid in the peritoneal and thoracic cavities; petechiae or minute hemorrhages in the subcutaneous fat; congestion and pneumonia in the lungs, congestion in the liver and spleen; and focal necrosis in the spleen, liver, kidneys and mesenteric lymph nodes.
Diagnosis of tularemia is based upon gross and microscopic lesions and must be confirmed by isolation and identification of the causative organism from tissues or blood of the suspect animal.
Treatment and Control
Although broad-spectrum antibiotics such as tetracycline or chloramphenicol are effective, streptomycin is the most satisfactory drug for treatment of tularemia. Strains resistant to streptomycin, but not to tetracycline or chloramphenicol, have been encountered. It is not economically feasible to treat animals in the wild, and enzootic tularemia of rabbits and rodents cannot be eradicated.
At times, tularemia kills large numbers of wild animals. However, the effect of this disease on a species or population is not known. Since the disease can be fatal to humans it is of public health significance. Sportsmen and sportswomen should be cautioned against drinking from streams in enzootic areas. Hunters should be very suspicious of "lazy" rabbits which are killed easily, and rubber gloves should be worn when dressing rabbits. Any suspicious acting rabbits should not be consumed. The meat of normal acting animals should be thoroughly cooked.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.