Disease
F. tularensis causes tularemia.
Important Properties
F. tularensis is a small, pleomorphic, gram-negative rod. It has a single serologic type. There are two biotypes, A and B, which are distinguished primarily on their virulence and epidemiology. Type A is more virulent and found primarily in the United States, whereas type B is less virulent and found primarily in Europe. Type A is associated with rabbits, whereas type B is more commonly found in rodents and water sources.
Pathogenesis & Epidemiology
F. tularensis is remarkable in the wide variety of animals that it infects and in the breadth of its distribution in the United States. It is enzootic (endemic in animals) in every state, but most human cases occur in the rural areas of Arkansas and Missouri. It has been isolated from more than 100 different species of wild animals, the most important of which are rabbits, deer, and a variety of rodents.
The bacteria are transmitted among these animals by vectors such as biting flies and ticks, especially the Dermacentor ticks that feed on the blood of wild rabbits. The tick maintains the chain of transmission by passing the bacteria to its offspring by the transovarian route. In this process, the bacteria are passed through ovum, larva, and nymph stages to adult ticks capable of transmitting the infection.
Humans are accidental “dead-end” hosts who acquire the infection most often by being bitten by the vector or by having skin contact with the animal during removal of the hide. Rarely, the organism is ingested in infected meat, causing gastrointestinal tularemia, or is inhaled, causing pneumonia. Rarely, the person is infected by a penetrating skin lesion in a water environment, such as a fish-hook injury. There is no person-to person spread. The main type of tularemia in the United States is tick-borne tularemia from a rabbit reservoir.
The organism enters through the skin, forming an ulcer at the site in most cases. It then localizes to the cells of the reticuloendothelial system, and granulomas are formed. Caseation necrosis and abscesses can also occur. Symptoms are caused primarily by endotoxin. No exotoxins have been identified.
Clinical Findings
Presentation can vary from sudden onset of an influenza-like syndrome to prolonged onset of a low-grade fever and adenopathy. Approximately 75% of cases are the “ulceroglandular” type, in which the site of entry ulcerates and the regional lymph nodes are swollen and painful. Other, less frequent forms of tularemia include glandular, oculoglandular, typhoidal, gastrointestinal, and pulmonary. Disease usually confers lifelong immunity.
Laboratory Diagnosis
Attempts to culture the organism in the laboratory are rarely undertaken, because there is a high risk to laboratory workers of infection by inhalation, and the special cysteine-containing medium required for growth is not usually available. The most frequently used diagnostic method is the agglutination test with acute- and convalescent-phase serum samples. Fluorescent anti body staining of infected tissue can be used if available.
Treatment & Prevention
Streptomycin is the drug of choice. There is no significant anti biotic resistance. Prevention involves avoiding both being bitten by ticks and handling wild animals.
