Tularemia remains an important zoonosis as well as primary pathogen of wild
and domestic animals. It is caused by a small, aerobic, facultatively intracellular,
Gram-negative coccobacillus, Francisella tularensis. The species consists of
four recognized subspecies: F. tularensis subsp. tularensis (formerly type A),
F. tularensis subsp. holarctica (formerly type B), F. tularensis subsp. mediasiatica
and F. tularensis subsp. novicida. Although the organism is pathogenic to
190 animal species, clinical cases are associated mainly with lagomorphs and
rodents. Tularemia can take multiple clinical forms depending on the portal
of entry. It is plague-like and for humans the infectious dose is very low. The
natural reservoirs are mice, muskrats, water rats, ground squirrels, voles and
rabbits. In humans infection by F. tularensis occurs primarily after inadvertent
exposure to infected wildlife species, most frequently rodents, hares, and
rabbits. Transmission to animals and humans occurs through arthropod or
insect vectors, via direct contact, by ingestion or by inhalation of aerosolized
organisms. F. tularensis invades macrophages and spreads to regional lymph
nodes causing inflammatory lesions resembling buboes encountered in
plague. The organism disseminates throughout the body to the internal
organs, reaching lungs, spleen, liver and kidneys. At the final, sepsis stage, the
secretory changes develop, taking ulceroglandular form of tularemia. In sheep
clinical signs include fever, rigid gait, diarrhea, frequent urination, weight loss,
and difficulty breathing. In humans the ulceroglandular tularemia is the most
common form of disease and usually occurs following a tick or deer fly bite or
after handling of an infected animal. The most serious form is pulmonary and
typhoid form of tularemia. Oropharyngeal form results from eating or drinking
contaminated food or water. PCR, RT-PCR, a specific fluorescent antibody test
or immunohistochemistry are safe and convenient methods for detection and
identification of F. tularensis in clinical specimens. Serological tests (slide
agglutination test, tube agglutination test, microagglutination test and ELISA),
are useful for diagnosing human infection, but are of limited value in the more
susceptible animal species that usually die before developing antibodies. The
organism is considered as a serious potential bioterrorist threat and is classified
as Category A select agent.