A disease first described in a California ground squirrel around 1913, tularemia is also known as "rabbit" or "deerfly fever". The disease is caused by a gram-negative bacterium by the name of "Francisella tularensis". The bacterial septicemia may affect over 50 different species of wild and domestic mammals, birds, reptiles, fish, and even man.

There are five subspecies of F. tularensis, of which two are considered to be clinically significant in humans. The tularensis subspecies is highly infective for rabbits, humans, and cats. The holarctica subspecies is most commonly associated with rodents and aquatic mammals such as muskrats, beaver, voles, lemmings, and prairie dogs. The subspecies novicida is found in the US, Australia, and Canada, and though it is less infectious, it may still can cause a tularemia-like illness.

Tularemia most commonly affects wild lagomorphs (rabbits and hares) and rodents. Domestic rabbits have rarely been infected. Infections may also occur in birds, fish, amphibians, and reptiles, but are relatively uncommon. Carnivores require high doses of the bacteria to become infected, although wild carnivores may serve as reservoirs of infection. Domestic animals and humans are considered to be accidental hosts. Outbreaks have been reported in sheep, commercially bred mink, beaver, and fox.

The average number of reported human cases in the United States ranges from 100-200 yearly. Most human exposures are associated with areas high in insect vector activity and rabbit hunting.

Ticks are the most important vector of F. tularensis. Deerflies, horseflies, mosquitoes, sucking lice, and biting flies may also transmit the bacteria. The bacteria can survive in mud, soil, decaying animal carcasses, or water for long periods of time (weeks to months). Transmission may occur through insect bites, inhalation of aerosolized bacteria, eating or drinking contaminated food or tissues, and handling infected animals or their tissues. The manufacture of Tularemia as an effective aerosol weapon is possible but would require considerable sophistication.

Sheep, domestic cats, and the sale of wild-caught prairie dogs have all caused human infections.

Clinical signs in rabbits, hares, and rodents are not well-described. Affected animals are most often found dead. These animals may exhibit weakness, fever, ulcers, swollen lymph nodes, and abscesses. Death usually occurs in 8 to 14 days.

Tularemia is so highly infectious that only 10 - 50 organisms may cause disease. Due to the highly infectious nature of tularemia, it has long been feared that it could be used as an agent in bioterrorism.

Tularemia is typically a seasonal disease in sheep coinciding with large tick infestations. Clinical signs in sheep include elevated temperatures, a stiff gait, reduced mobility, diarrhea, frequent urination, weight loss, and difficulty breathing. Affected sheep may isolate themselves from the flock. Death is most common in young animals. Pregnant ewes may abort.

Horses may become infected when exposed to extensive tick infestations. Horses will have a fever, difficulty breathing, incoordination, and may appear depressed.

Cats may have an elevated temperature, be depressed, have swollen lymph nodes, abscesses, ulcerations of the mouth and tongue, gastroenteritis, enlarged liver or spleen, icterus (jaundice), anorexia, weight loss, pneumonia, shock, and death.

In dogs, clinical illness is typically inapparent or mild. Puppies are more likely to develop clinical signs than are adult dogs. Affected animals may be depressed, have an elevated temperature, there may be a mucopurulent discharge from the nose and/or eyes, pustules at the sites of contact, swollen lymph nodes, and anorexia (loss of appetite).

In humans, the incubation period is typically 3 to 5 days, but may range from 1-21 days. Patients may exhibit fever, chills, headache, muscle soreness, joint pain, diarrhea, and vomiting. People may also have a dry cough, bloody sputum, difficulty breathing, and progressive weakness. Two forms of the disease are possible in people. The Ulceroglandular form is most commonly seen syndrome, encompassing 75 to 85% of reported cases. In the ulceroglandular form of the disease, an ulcer is evident at the site of entry, usually on the fingers or hands; swollen lymph nodes occur and may drain pus and scar.

The second syndrome produces a pleuropneumonia (an infection in the pleural cavity or the lining of the chest cavity as well as the lungs), meningitis, shock, and death in untreated cases or in immunocompromised individuals. The overall case fatality rate in humans is <2%. Tularemia is not known to be spread from person-to-person contact, therefore infected individuals need not be isolated.

Diagnosis may be made through serum testing for a bacterial titer to Tularemia. Often paired serum samples (blood samples taken days to weeks apart) are necessary to indicate an active infection verses exposure. A significant increase in the bacterial titer between the two samples would indicate an acute infection. The bacteria may also be cultured or detected by florescent antibody testing. Tularemia is a reportable disease in the US.

The key to prevention is tick control. Insect repellants containing DEET are recommended for use in people. Wild game should be thoroughly cooked before consumption. In endemic areas, handling of dead and dying animals should be avoided. Gloves should be worm when handling wild game, and if gloves are not available, you should wash your hands often using soap and warm water.

Treatment is usually successful when the infection is caught early. Streptomycin and doxycycline are the antibiotics of choice for treating both wild and domestic animals. In human infections, streptomycin has been the preferred drug. Fluoroquinolones, gentamycin, and chloramphenicol have been used as alternative drugs. Prolonged treatment regimens may be necessary since many organisms are found intracellular (within the body cells).


References:

Antech Diagnostics News. “Tularemia”. July 2007.

Kahn, Cynthia, Editor. The Merck Veterinary Manual. 9th Edition, 2005. Pp.553-555.

“Key Facts about Tularemia”. CDC. Fact Sheet. October 7, 2003.