Pathogenesis ofFrancisella tularensis
- Francisella tularensisis carried by many species of wild rodents, rabbits, beavers.
- Humans are infected by fewer than 50 organisms by either aerosol or cutaneous routes.
- F. tularensis亚种。tularensisis the most virulent for humans, with an infectious dose of less than 10 colony forming units.
- The pathogens invade the host either through microtrauma in the skin or through the mucosa.
- An ulcerous lesion develops at the portal of entry that also affects the local lymph nodes (ulceroglandular, glandular, or oculoglandular form).
- Through lymphogenous and hematogenous dissemination, the pathogens then spread to parenchymatous organs, in particular reticuloendothelial system (RES) such as the spleen and liver.
- F. tularensisis an intracellular pathogen that can survive in the cells of the reticuloendothelial system, where it resides after a bacteremic phase because the organism inhibits phagosome–lysosome fusion through secretion of proteins that facilitate bacterial escape from the phagosome and subsequent replication in the macrophage cytosol.
- The polysaccharide rich capsule ofF. tularensisis a necessary component for expression of full virulence, allowing the organism to avoid immediate destruction by polymorphonuclear neutrophils and is antiphagocytic in nature.
- The capsule protects the bacteria from complement-mediated killing during the bacteremia phase of disease.
- 一个strong, innate immune response with production of interferon-γ and tumor necrosis factor (TNF) is important for controlling bacterial replication in macrophages in the early phase of infection.
- 与之相关的疾病F.tularensis, is称为tUlaremia.
- Disease caused byF.tularensisis subdivided into several forms based on the clinical presentation given as:
1. Ulceroglandular tularemia
- Ulceroglandular tularemia is the most common manifestation.
- The skin lesion, which starts as a painful papule at the site of the tick bite or direct inoculation of the organism into the skin.
- The papule then ulcerates and has a necrotic center and raised border.
2. Glandular tularemia
- It involves primarily swollen lymph nodes with no other localized symptoms
3. Oculoglandular tularemia
- .The organism can be introduced into the eyes, for example, by contaminated fingers or through exposure to water or aerosols.
- Affected patients have a painful conjunctivitis and regional lymphadenopathy (swollen cervical lymph nodes).
- Patients present with atypical pneumonia and typically has systemic signs of sepsis.
5. Oropharyngeal tularemia
- It occurs following ingestion of contaminated undercooked meat.
- It is characterized by membranous pharyngitis with cervical lymphadenopathy
6. Typhoidal tularemia
- There is no ulcer or lymphadenopathy.
- 它发生在摄取F.tularensisand is not very common clinical form.