Diseases > Horse and Deer Flies > Tularemia

Tularemia

Tularemia is a disease that was first recognized as a plague-like disease of rodents in 1911 in Tulare, California. It is caused by a bacterium that is widespread in nature, occurring in a variety of wild mammals and birds, in water, and even in soil. Several strains of the bacterium are known, with the strain associated with rabbits, hares, and rodents more being the most virulent. Approximately 200 human cases are diagnosed yearly in rural areas of the U.S., with a fatality rate of 5-7% in untreated patients. Initial symptoms of infection vary depending on the route of inoculation (see below).

Causative agent

• A bacterium with the scientific name Francisella tularensis.
• It is a highly infectious, invasive, and potentially dangerous pathogen.

Geographical distribution of cases

• Worldwide, but primarily in the northern hemisphere, including Asia and North America.
• In the U.S., most cases occur on the Ozark Plateau of Missouri and in the western states.

Symptoms of infection

• General symptoms include fever, headache, chills, nausea, and muscle and joint pain.
• An ulcerated lesion at the site of bacteria inoculation occurs in about 80% of patients, including at the site of a tick bite or following other contact with F. tularensis such as in a cut on a finger or inside the mouth.
• Complications can include swollen lymph nodes and pneumonia with cough and bloody sputum.

Reservoir of Francisella tularensis

• Various wild animals, especially rabbits, hares, and rodents, plus several species of hard ticks.
• NOTE: This bacterium also exists in water and in soil.

Vectors of Francisella tularensis

• One species of deer fly, Chrysops discalis, is a vector in the western U.S.
• Hard ticks, especially the lone star tick, Amblyomma americanum, in the south central U.S.

Modes of Transmission

• Most infections are acquired when hunters handle infected small mammals, especially rabbits.
• Some infections via ingestion of under cooked meat of infected small mammals, especially rabbits.
• Some infections via handling infected aquatic mammals such as muskrats.
• Some infections via ingestion of water that is contaminated with bacteria.
• Via bites of infected deer flies in the western U.S. and infected ticks in the south central U.S.
• NOTE: There is no known human to human transmission.

Diagnosis of infection

• A small, ulcerated lesion at the site of bacterial introduction (e.g., in the bite wound of a deer fly or tick, or in a cut on a hand).
• The lesion usually has a pinkish pit in the center and a raised, ridge-like wheal around the perimeter.
• General symptoms together with a history of activity in habitats known to support tularemia.
• Confirmation is based on laboratory test of blood or bloody sputum.

Treatment of infection

• Antibiotic prescribed by a physician as soon as possible after initial diagnosis.

Prevention of infection

• There is no vaccine available for the public.
• Wear clothing consisting of a long sleeved shirt, long pants, boots, and a hat.
• Use a repellent recommended by CDC and treat clothing with permethrin (limited effect on deer flies).
• Handle game animals such as rabbits with care, especially when skinning and dressing.
• Cook the meat of reservoir animals thoroughly.