Yersinia pestis

Categorization
Cell Wall: Gram Negative	Shape: Rod
Life Cyle: Facultative Intracellular

Transmission

The natural reservoir of Y. pestis are rodents and thus the organism is a zoonotic. Humans can be infected either through the bite of a flea which has taken a blood meal from an infected rodent or through contaminated aerosolized droplets.

Epidemiology

Y. pestis is endemic in several rural areas in the world where it infects wild rodents and can be transmitted to humans by fleas. There are only a handful of sporadic cases in the US every year and are concentrated in the desert southwest.

The epidemic pattern of Plague transmission is largely of historical significance. It occurred in densely-populated areas with poor sanitation where rodents and fleas are common. In epidemic scenarios, person-to-person transmission can occur via aerosolized droplets.

Because of its highly virulent nature and capacity to be aerosolized, use of Y. pestis as a bioterrorist weapon is a concern.

Virulence Factors

Y. pestis synthesizes a wide variety of virulence factors; however, the precise mechanisms by which these factors yield the organisms pathogenesis is not well-understood. Several antigens synthesized by the organism are notable because Y. pestis virulence is dependent on their presence.
F1 Antigen: Is a critical component of the Y. pestis anti-phagocytic capsule
V and W Antigens: Probably play a variety of roles

Pathogenesis

Regurgitation of the blood meal by infected fleas inoculates the skin with Y. pestis which is phagocytosed by macrophages. The facultative intracellular organisms can resist death within the phagosome and multiply. Subsequently, Y. pestis is transported within macrophages to the regional lymph Nodes which rapidly undergo hemorrhagic inflammation. Bacteria released from the lymph nodes disseminate throughout the body via the blood stream and can infect nearly any organ.

Clinical Consequences

Bubonic Plague is characterized by the rapid development of painful lymphadenopathy which manifests as red, swollen, and inflamed lymph nodes termed "Buboes". Because fleas usually bite the legs, the inguinal lymph nodes are the most commonly affected
Bubonic Plague can quickly evolve into Septicemic Plague as organisms gain access to the blood stream.

Septicemic Plague usually follows untreated Bubonic Plague although in some individuals it can develop in the absence of observable buboes. sepsis and bacteremia develop rapidly followed by hemorrhagic necrosis of a wide variety of organs which rapidly leads to death.

Primary Pneumonic Plague is due to inhalation of bacteria-laden aerosolized droplets. It is characterized by a highly hemorrhagic and necrotic pneumonia which is rapidly fatal.

Treatment

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