Tularemia (also known as rabbit fever) is caused by Francisella tularensis, a pleomorphic aerobic gram-negative coccobacillus that normally resides in a wide variety of animals. Humans acquire the infection either after direct contact with the bodily fluids of animal carriers or through insect vectors (ticks) or contaminated food and water. A number of cases spread by cat bites have been reported. Cases of oropharyngeal tularemia from harvesting and processing infected grapes for wine have been reported. Tularemia infection has been acquired via organ transplantation. Person-to-person transmission has not been reported. Thus isolation is not recommended for tularemia patients, although due to 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. Hunters, game wardens, trappers, and campers are particularly susceptible. In the Southwest United States, tribal populations are overrepresented among tularemia cases. 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. Prevalence is greatest from June through August (more tick-related infections) and in the fall (during hunting season).

There are 7 major clinical patterns of tularemia: glandular, ulceroglandular (most common), oculoglandular, typhoidal, pneumonic, oropharyngeal, and septicemic. Any form of tularemia can be complicated by hematogenous spread, resulting in secondary pleuropneumonia, sepsis, or meningitis (rare).

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. Tularemia typically has an abrupt onset consisting of fever, headache, chills and rigors, myalgia (especially the low back), coryza, and sore throat. In 42% of patients, pulse-temperature dissociation has been observed. Other manifestations depend more on the type of tularemia.

• In glandular tularemia, no primary inoculation site lesion is seen and adenopathy may be generalized.
• In oculoglandular tularemia, a purulent conjunctivitis is apparent, with pain, edema, and possibly ulceration.
• The pneumonic form presents with acute signs of illness such as pharyngitis, bronchiolitis, pleuropneumonitis, and hilar lymphadenitis along with other signs of systemic illness.
• Typhoidal tularemia presents with nonproductive cough, fever and chills, chest discomfort, sore throat, abdominal pain, and diarrhea.

Additional signs and symptoms may include splenomegaly and generalized lymphadenopathy. Hepatomegaly, pharyngotonsillitis, pneumonia, pleural effusion, and mediastinal node enlargement may occur. Depending on the severity of the illness, dehydration, hypotension, diffuse abdominal tenderness, and meningismus can be demonstrated. Tender regional adenopathy occurs with potential suppuration.

Complications include acute respiratory distress syndrome (ARDS), renal failure, rhabdomyolysis, hepatitis, and disseminated intravascular coagulation. Less common complications include meningitis, osteomyelitis, splenic rupture, encephalitis, and peritonitis. Seeding of internal organs can result in granulomas in the lung, liver, spleen, lymph nodes, and bone marrow.

The Centers for Disease Control and Prevention (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 Soviet Union had 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. 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.

While postexposure prophylaxis is not recommended for natural exposure, it 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).