Tularemia is a bacterial septicemia that affects >250 species of wild and domestic mammals, birds, reptiles, fish, and people. It is listed as a category A bioterrorism agent due to the potential for airborne dissemination.
The causative bacterium, Francisella tularensis, is a nonsporulating, gram-negative coccobacillus that is antigenically related to Brucella spp. It is a facultative intracellular parasite that is killed rapidly by heat and proper disinfection but survives for weeks or months in a moist environment. It is fastidious in growth but can be cultured readily. There are 2 types of organisms based on their biochemistry and virulence. Type A has been found predominantly in North America and is more virulent; in people, the mortality rate may be 5–7% if untreated. Type A has been further subdivided into distinct subpopulations, A1 and A2, which differ with respect to clinical severity. Type B is less virulent and is most commonly isolated from aquatic animals and water-associated infections in North America and Eurasia. Both types have been isolated from arthropod vectors.
Epidemiology and Transmission
In domestic animals, sheep are the primary host, but clinical infection has been reported in dogs, pigs, and horses. Cats are at increased risk due to predatory behavior and appear to have an increased susceptibility, while cattle appear to be resistant. Little is known of the true incidence and spectrum of clinical disease in domestic animals. Important wild animal hosts include cottontail and jackrabbits, beaver, muskrat, meadow voles, and sheep in North America, and other voles, field mice, and lemmings in Europe and Asia.
Natural foci of infection exist in North America and Eurasia, where the organism circulates between arthropod vectors and various mammals, birds, reptiles, and fish. Although found in every state except Hawaii, tularemia is most often reported in the southcentral and western USA (eg, Missouri, Oklahoma, South Dakota, and Montana).
Tularemia is a classic zoonosis, capable of being transmitted by aerosol, direct contact, ingestion, or arthropods. Inhalation of aerosolized organisms (in the laboratory or as an airborne agent in an act of bioterrorism) can produce a pneumonic form. Direct contact with, or ingestion of, infected carcasses of wild animals (eg, cottontail rabbit) can produce the ulceroglandular, oculoglandular, oropharyngeal (local lesion with regional lymphadenitis), or typhoidal form. Immersion in or ingestion of contaminated water can result in infection in aquatic animals. Ticks can maintain infection transstadially and transovarially, which makes them an efficient reservoir as well as a vector. Recognized vectors in the USA include Dermacentor andersoni (the wood tick), Amblyomma americanum (the lone star tick), D variabilis (the dog tick), and Chrysops discalis (the deer fly).
The most common source of infection for people and herbivores is the bite of an infected tick, but persons who dress, prepare, or eat improperly cooked wild game are also at increased risk. Dogs, cats, and other carnivores may acquire infection from ingestion of an infected carcass. A few case reports have implicated cats as a source of infection in people.
The incubation period is 1–10 days. In sheep and most mammals, the disease is characterized by sudden onset of high fever, lethargy, anorexia, stiffness, reduced mobility, or other signs associated with septicemic disease. Pulse and respiratory rates are increased. Coughing, diarrhea, and pollakiuria may develop. Prostration and death may occur in a few hours or days. Sporadic cases are best recognized by signs of septicemia. Outbreaks in untreated lambs may have up to 15% mortality. Subclinical cases may be common.
The most consistent lesions are miliary, white to off-white foci of necrosis in the liver and sometimes in the spleen, lung, and lymph nodes. Enlargement of the liver, spleen, lung, and lymph nodes is common. Organisms can be readily isolated from necropsy specimens by use of special media. Risk of infection during necropsy or to laboratory personnel is significant; special procedures and facilities are essential.
Tularemia must be differentiated from other septicemic diseases (especially plague) or acute pneumonia. When large numbers of sheep show typical signs during periods of heavy tick infestation, tularemia or tick paralysis (see Tick Paralysis) should be suspected. Tularemia should be considered in cats with signs of acute lymphadenopathy, malaise, oral ulcers, and history of recent ingestion of wild prey.
Diagnosis of acute infection is confirmed by culture and identification of the bacterium, direct or indirect fluorescent antibody test, or a 4-fold increase in antibody titer between acute and convalescent serum specimens. A single titer of ≥1:80 by the tube agglutination test is presumptive evidence of prior infection. When tularemia is suspected, laboratory personnel should be alerted in order to reduce the risk of laboratory-acquired infection.
Treatment and Control
Streptomycin, gentamicin, chloramphenicol, and tetracyclines are effective at recommended dose levels. Gentamicin should be continued for 10 days. Because tetracycline and chloramphenicol are bacteriostatic, they should be continued for 14 days to minimize the risk of relapse. Early treatment should prevent death loss. Control is difficult and is limited to reducing tick infestation and to rapid diagnosis and treatment. Prolonged treatment may be necessary because many organisms are intracellular. Efforts are currently underway to develop a safe and effective vaccine. Recovery confers long-lasting immunity.
Last full review/revision March 2012 by Barton W. Rohrbach, VMD, MPH, DACVPM