Tularemia is primarily a specific, infectious disease of rodents and lagomorphs. The causative organism, however, has been isolated from over 100 species of mammals, 9 species of domestic animals, 25 species of birds, 70 species of insects, and several species of fish and amphibians. Humans can become infected by being bitten by infected blood-sucking insects, by handling infected animal carcasses, and by ingesting contaminated water or poorly cooked meat. In humankind, tularemia is an acute, infectious, moderately severe, febrile disease, which has a mortality rate of approximately 7 percent in untreated cases. The causative agent, Francisella (Pasteurella) tularensis, is a tiny gram-negative, pleomorphic coccobacillus requiring special media for isolation and growth. Tularemia is also know as deer-fly fever, Pahvant Valley plague, rabbit fever, Ohara’s disease, yatobyo, and lemming fever.

Etiology and Epidemiology

Two variants (biovars) of the causative organism are recognized. F. tularensis biovar tularensis (type A) has been isolated in nature only in North America and is the most virulent in human beings. The second is designated F. tularensis biovar palaearctica (type B) and is found in all areas where tularemia is endemic in the Northern Hemisphere.

Tularemia is unique in the number of ways in which humans can become infected, and the clinical picture of the disease depends upon the infection route. The most common route is via the skin, either by insect bite or by direct passage through intact skin by contact with infected carcasses or a scratch from an infected animal. Of the numerous insects that transmit the disease, the tick is the most important.