Yersinia Pestis

Other Yersinia are mentioned in enteric gram-negative bacilli section.

Causes plague

Characteristics/Epidemiology

Produces V and W antigen

Virulence factor: V and W antigen, endotoxin, antiphagocytic capsule

Rat, ground squirrel, prairie dogs - common reservoirs

Pathogenesis/Transmission

Transmitted by fleas, or ingestion of contaminated animal tissues, or by inhalation.

Transmission vector - fleas 

Carried from inoculation site to regional lymph node lymphatically

   -> Ingestion by phagocytes

   -> Multiply within phagocytes

   -> Released with new envelope

   -> Increased resistance to phagocytosis

Haemorrhagic necrosis in affected lymph nodes

High concentration of PMN and extracellular bacteria in these nodes

Haematogenous spread to other sites.

Clinical significance

Bubonic plague

Flea ingest blood from infected animal

   -> Y. pestis produce coagulase - blood clot in flea foregut

   -> Multiply within the foregut

   -> Next attempt to feed - regurgitate from foregut

Incubation: 2 - 8 days

Sudden high fever, headache, myalgia, weakness -> prostration.

   -> Then characteristic painful bubo develops

      * Pronouced swelling with one or more infected lymph nodes

      * Typically in the groin, occasionally in neck or axillae

   -> Hypotension

   -> Septic shock, death

May also have pustules/vesicles containing leukocytes and Y. pestis.

Mortality >50% if untreated

Septicaemic plague

Patient overwhelmed by massive bacteremia before buboes develop.

Pneumonic plague

* Highly contagious

Mortality almost 100% if untreated.

Transmission by aerosol inhalation.

Plague meningitis

Haematogenous dissemination to the meninges.

Bacteria demonstrated in CSF

Cutaneous plague, pharyngitis

Cause by direct handling or ingestion of contaminated tissue.

Laboratory identification

Stains bipolarly

Diagnosis can be made clinically, by culture, gram stain.

Culture on MacConkey or blood agar.

Treatment

First line: gentamicin, doxycycline (tetracycline)

For plague meningitis - chloramphenicol

Prevention/immunity

Formalin-killed vaccine available.

Efforts to minimise exposure to rodents and fleas.

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Things to revise/add later:

Bibliography:

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