Intraspecific Classification
The pathogen belongs to the species Francisella tularensis. Within the species, four subspecies are distinguished based on geographic distribution, biochemical properties, and virulence. Two of these are clinically most significant:
- Subsp. tularensis (neararctic subspecies): Circulates in North America. It is highly aggressive, causing severe forms of infection in humans and is lethal to rabbits. A characteristic biochemical feature is the ability to ferment glycerol.
- Subsp. holarctica (palearctic subspecies): Found in Europe and Asia. It is characterized by moderate virulence: the disease runs a moderately severe course, and the microbe is not lethal to rabbits.
Morphology and Cultural Properties
The bacteria are small, non-motile, Gram-negative rods that tend to form a capsule and possess type IV pili. They do not form spores.
Microorganisms are extremely fastidious and strictly aerobic. They grow at 35–37 °C in an atmosphere containing 5% CO₂ for 2–5 days. Growth requires complex media containing cysteine, hemin, and egg yolk, or the use of embryonated chicken egg yolk sacs. They form small, milky-white colonies. Saccharolytic activity is weak, and proteolytic properties are completely absent.
Virulence Factors and Pathogenesis
A key feature of Francisella tularensis is its ability to survive inside macrophages (evading complete phagocytosis). The bacterium blocks lysis within the phagolysosome, escapes into the cytoplasm of the host cell, and actively replicates there. This process is facilitated by the capsule and lipopolysaccharide (LPS), which possess anti-phagocytic activity. The endotoxin (LPS) itself has an anomalous structure and reduced toxicity compared to other Gram-negative microbes.
The incubation period lasts 3–7 days. Pathogenesis begins with the formation of a primary inflammatory lesion at the site of bacterial entry. The pathogen then reaches regional lymph nodes via the lymphatic system, causing inflammation—forming a bubo. The localization of lesions (ulceroglandular, oculoglandular, oropharyngeal, or typhoidal/glandular form) depends entirely on the portal of entry. Without etiotropic therapy, mortality reaches 60%.
Epidemiology
Tularemia is widely distributed. Natural reservoirs include lagomorphs and rodents (especially water voles). Human infection occurs via several routes:
- Vector-borne: through bites of infected mosquitoes, deer flies, and ticks.
- Contact: when the pathogen contacts mucous membranes or broken skin.
- Alimentary and waterborne: through food and water contaminated by rodent excreta.
- Inhalation: by breathing in infectious aerosols or dust.
Due to its potential for airborne dissemination, extremely low infectious dose, and high virulence, the neararctic subspecies is considered a potential bioweapon agent.
Principles of Microbiological Diagnostics
Direct culture of clinical material is rarely successful. The primary method for isolating the culture is animal inoculation (bioassay) — infecting laboratory mice or guinea pigs with patient material (bubo aspirate, blood) followed by isolating the bacteria from animal organs. Note: Such studies are restricted to specialized high-containment biosafety laboratories.
In routine practice, diagnosis is confirmed by serological methods (FAT, ELISA, Indirect Hemagglutination Assay, Agglutination Test), taking into account potential cross-reactivity with Brucella antigens. Allergic skin testing with tularin is also used to evaluate the development of delayed-type hypersensitivity (DTH).