Overview and Clinical Importance
Tularemia (also known as rabbit fever) is a highly infectious zoonotic disease caused by the gram-negative bacterium Francisella tularensis. This disease primarily affects lagomorphs (rabbits and hares) and rodents, often resulting in high mortality rates during outbreaks. Tularemia is considered a Category A bioterrorism agent and is a reportable disease in the United States due to its high infectivity, potential for airborne transmission, and significant public health implications.
| Subspecies |
Geographic Distribution |
Clinical Significance |
| F. t. tularensis (Type A) |
Predominantly North America |
Highly virulent; associated with lagomorphs and ticks; causes severe disease with high mortality if untreated |
| F. t. holarctica (Type B) |
Europe, Asia, and North America |
Moderately virulent; associated with aquatic animals; less severe disease than Type A |
| F. t. mediasiatica |
Central Asia |
Limited information available; rarely causes human disease |
| F. t. novicida |
North America |
Low virulence; causes disease primarily in immunocompromised individuals |
Etiology and Microbiology
Causative Organism
Francisella tularensis is a small (0.2 x 0.2 μm), gram-negative coccobacillus that is nonmotile, nonspore-forming, and facultatively intracellular. The organism requires cysteine-enriched media for growth and is considered highly fastidious.
NAVLE TipF. tularensis requires only 10-50 organisms to cause disease in humans, making it one of the most infectious bacterial pathogens known. Always maintain BSL-3 precautions when tularemia is suspected. This extremely low infectious dose is a favorite NAVLE question topic.
Subspecies Classification
| Diagnostic Method |
Details and Interpretation |
Considerations |
| Bacterial Culture |
Isolation on cysteine-enriched media: chocolate agar, buffered charcoal yeast extract (BCYE), or cysteine heart agar (CHAB). Slow growth, 48-72 hours required. Forms small, gray-white or greenish-white colonies |
Requires BSL-3 laboratory. Notify lab of suspected tularemia before submission. Gold standard for diagnosis |
| PCR Testing |
Rapid molecular detection of F. tularensis DNA from blood, tissue samples, or lymph node aspirates. Results available within 24-48 hours |
Faster than culture. Highly sensitive and specific. Preferred method for rapid diagnosis |
| Serology |
Tube agglutination test with titer ≥1:80 is presumptive. Four-fold increase in titer between acute and convalescent sera (3 weeks apart) confirms infection. ELISA and fluorescent antibody tests also available |
Cross-reactions with Brucella, Yersinia possible. Most animals die before antibody development, limiting utility in acute cases |
| Immunohistochemistry |
Direct or indirect fluorescent antibody testing on formalin-fixed tissue sections to identify F. tularensis antigens |
Useful for retrospective diagnosis on archived tissues. Can be performed on fixed specimens |
| Hematology |
Lymphopenia, thrombocytopenia, elevated ESR, increased ALT, cholesterol, granulocytes, and monocytes |
Nonspecific but supportive. Helps assess severity and systemic involvement |
Epidemiology
Geographic Distribution
Tularemia has been found in every U.S. state except Hawaii. The highest incidence occurs in Arkansas, Missouri, Oklahoma, and Kansas, which together account for more than half of reported cases. The disease is most commonly reported during late spring, summer, and early autumn when tick and deer fly vectors are most active.