Tularemia (also known as rabbit fever) is caused by Francisella tularensis, an aerobic gram-negative coccobacillus. Tularemia is ubiquitous in the northern hemisphere between 30 and 71° N latitude. Some 100 species of wild mammals, 25 species of birds, several species of fish and amphibians, and more than 50 arthropods have been found to be naturally infected. Humans are extremely susceptible. Humans acquire the infection either after direct contact with the bodily fluids of animal carriers or, more significantly, through ticks and deer flies. Water contaminated by voles, beavers, and muskrats has been responsible for outbreaks. Laboratory workers and farm workers may be at risk of infection from aerosolization. A number of cases spread by cat bites have been reported. Person-to-person transmission is not known to exist.
The CDC has classified tularemia as a Class A bioterrorism agent due to its ease of dissemination, morbidity, and ability to infect with as few as 10 bacterial organisms. The former Soviet Union developed weaponized antibiotic- and vaccine-resistant strains of F. tularensis. Francisella tularensis is so infective that exposure to an open culture plate can cause human infection. Tularemia is not directly contagious person to person. Aerosol dissemination of F. tularensis has been projected to result in the abrupt onset of large numbers of cases of acute nonspecific febrile illness with pleuropneumonitis as the predominant finding.
Tularemia has an extremely variable presentation. The incubation period may range from a few hours to 21 days, with a mean of 4.5 days. Typically, a patient has an abrupt onset of fever, headache, chills and rigors, myalgia (especially the low back), coryza, and sore throat. In 42% of patients, a pulse-temperature dissociation has been observed. The cough may be dry or slightly productive, and there may be substernal pain or tightness. Signs of pneumonia may or may not be obvious. Nausea, vomiting, and diarrhea may also occur.
Francisella tularensis usually produces a marked reaction at the portal of entry. There are 7 major clinical patterns: glandular, ulceroglandular, oculoglandular, typhoidal, pneumonic, oropharyngeal, and septicemic. Any form of tularemia can be complicated by hematogenous spread resulting in secondary pleuropneumonia, sepsis, or meningitis (rare).
Postexposure prophylaxis is not recommended for natural exposure but can be used for patients with aerosol exposure who are identified early in the incubation period (due to the short incubation period of inhalational tularemia). Isolation is not recommended for tularemia patients due to the lack of human-to-human transmission. However, because of the risk of secondary arthropod transmission, ambient ticks, fleas, lice, and mosquitoes should be controlled.
The geographic distribution includes North America, Europe, the former republics of the USSR, Japan, and Spain. Tularemia is present throughout the United States but is most prevalent in Missouri, Arkansas, Oklahoma, Massachusetts, South Dakota, and Kansas. Prevalence is greatest from June through August (more tick-related infections) and in the fall (during hunting season).
Hunters, game wardens, trappers, and campers are particularly susceptible. Animals known to have transmitted tularemia include rabbits (most common), foxes, squirrels, skunks, muskrats, beavers, voles, and even fish. Other routes of infectivity include contact with contaminated water or mud and aerosol droplets.