Tick species that transmit tularemia: Lone star tick (Amblyomma americanum), Rocky mountain wood tick (Dermacentor andersoni), and American dog tick (Dermacentor variabilis).
Tularemia (also known as rabbit fever) is caused by oval-shaped bacteria (coccobacilli) called Francisella tularensis. F. tularensis is transmitted to humans by the bite of infected ticks, deer flies, contact with infected animals or infected carcasses, inhalation of air-borne bacteria, and ingestion of infected food or water. In the summer, most cases come from infected tick bites. In the winter, cases are reported by hunters who trap and skin infected animals. Person-to-person transmission of tularemia does not occur.
The symptoms an infected person experiences depends on how F. tularensis gains entry into the body. A common type of tularemia is ulceroglandular tularemia, which normally results from the bite of infected ticks, or contact between broken skin and F. tularensis. A more severe form of the disease, pneumonic tularemia, is caused by inhalation of the bacteria. Immediate treatment with the appropriate antibiotics is recommended by the Centers for Disease Control (CDC). Tularemia can be fatal if left untreated.
Tularemia is a reportable disease. CDC Morbidity and Mortality Weekly Report (MMWR) case report data can be found here.
Cases of tularemia have been reported across the United States with a concentration of reports in Missouri, Arkansas, and Oklahoma. From 1990 to 2000, there were 1,368 cases reported in the United States. On average, 124 cases per year were reported during this period of time. A cluster of cases were reported at Martha’s Vineyard (Massachusetts) in 2000. A study published by the journal Emerging Infectious Diseases showed that these cases were associated with landscaping work. Landscape workers may be at a greater risk for tularemia because they are exposed to aerosolized bacteria originating from infected rabbit nests.
Reported Cases of Tularemia (1990 to 2000)
Map Source: CDC MMWR Report
Symptoms often appear abruptly three to five days after infection, but can take as long as two weeks to appear. A fever of 38°C to 40°C (100.4°F to 104°F) is the most common symptom. Other symptoms include joint pain, swelling of lymph nodes, headache, chills, dry cough, sore throat, ulcers at the site of infection, sore eyes, weakness, and diarrhea.
There are several forms of tularemia, each specific to a particular route of entry by F. tularensis into the body. Ulceroglandular tularemia is the most common form of the disease and is accompanied by flu-like symptoms, ulcers at the site of infection, and swollen lymph nodes.
Inhalation of the bacteria leads to pneumonic tularemia, the most severe form of the disease. Pneumonic tularemia is characterized by non-specific respiratory symptoms including hemorrhagic inflammation of the lungs and bronchopneumonia. This, in addition to low suspicion of tularemia (due to its relatively low occurrence), makes it challenging for physicians to correctly diagnose isolated cases.
According to the CDC, a clinical diagnosis of F. tularensis can be confirmed by examining stained secretions, exudates, and biopsy specimens. The definitive confirmation of F. tularensis infection is growth of the bacteria in culture.
The CDC recommends intramuscular and intravenous antibiotic therapy for treatment of tularemia. A more detailed description of dosages recommended by the CDC can be found here. In summary, streptomycin is the recommended drug, with gentamicin serving as an alternative. The CDC recommends that these drugs be administered for 10 days. Both can be used in children and pregnant women.