tularemia
- Also called:
- rabbit fever or deerfly fever
tularemia, acute infectious disease caused by the gram-negative bacterium Francisella tularensis known for its different clinical forms, which are characterized by varying signs and symptoms that range from mild to severe. F. tularensis is extremely infectious, with as few as 10 organisms capable of causing disease in humans; the organism also has different subspecies, which are associated with variations in disease severity.
Tularemia was described in 1911, having been discovered in ground squirrels in Tulare county, California (from which the name is derived). It was first reported in humans in the United States in 1914. Today approximately 200 cases are reported each year in the United States, and the disease has been encountered in all parts of the country except Hawaii, although it is most common in the south-central and western states.
Transmission
Tularemia is a disease primarily of animals; human infections are rare. It occurs naturally in many types of wildlife but especially in hares, rabbits, and rodents. In the United States the cottontail (Sylvilagus) is an important source of human infection, but other mammals, birds, and insects also spread the disease. Human cases in Sweden, Norway, and Russia have been transmitted by hares, field mice, and muskrats. F. tularensis has been found in some natural water sources, causing incidences of the disease in humans and animals.
Tularemia can also be spread to humans by the bite of an infected animal, by contact with blood or fine dust from the animal’s body during skinning or similar operations, by the ingestion of infected animal products that have not been properly cooked, or by the bite of an insect, most commonly a deerfly (Chrysops discalis). Various ticks of the genera Dermacentor, Haemaphysalis, Rhipicephalus, Amblyomma, and Ixodes may be largely responsible for maintenance of animal infection. In addition, the infection is transmitted from the adult tick to the egg, and both larvae and nymphs are infectious and form an insect reservoir of infection. No case of human-to-human contamination has been reported.
Forms of tularemia
The different forms of tularemia are associated with mode of transmission. The most common form in humans is ulceroglandular tularemia, in which F. tularensis is transmitted through the skin via a bite from an infected tick or deerfly or by handling infected animal tissue. Ulceroglandular tularemia is characterized by a painful sore at the site of the infection and a swelling of the lymph node that drains the area; the sore is often on the finger and the swelling, or bubo, in the armpit. The bubo can break down and discharge pus, but it sometimes remains hard and tender for weeks. Along with these local signs, the infected person has a fever that may persist for two or three weeks, with headache, body pains, and general weakness.
Other forms of the disease include glandular tularemia, which is associated with skin transmission and produces symptoms similar to the ulceroglandular form but without skin ulcers. Oculoglandular tularemia affects the eyes, sometimes causing the formation of an ulcer on the inside surface of the eyelid; it is primarily transmitted through direct contact with infected materials or by the bacteria entering the eye after handling infectious material. Oropharyngeal tularemia, which is caused by ingestion of contaminated food or water, affects the mouth and is associated with abdominal pain, vomiting, and diarrhea. Pneumonic tularemia resembles pneumonia and is caused by inhalation of dust contaminated by F. tularensis. Typhoidal tularemia, which also is thought to be caused by inhalation of infectious aerosols, is rare and associated with vomiting, diarrhea, pneumonia, and enlargement of the spleen and liver.
Treatment and prevention
With prompt treatment, the fatality rate of tularemia is very low, with death occurring in fewer than 2 percent of cases. Treatment typically is with antibiotics, such as ciprofloxacin, doxycycline, gentamicin, and streptomycin, all of which are effective against F. tularensis. Treatment may last 10 to 21 days, and most patients recover.
Infection can be prevented by wearing long-sleeved shirts and long pants when outdoors in seasons when ticks and deerflies are prevalent, by using insect repellent, by wearing gloves when handling animals, and by thoroughly cooking meat prior to consumption. A live attenuated vaccine has been generally successful in conferring immunity on susceptible hosts, although its use is usually limited to persons at high risk.