Clinical Manifestations and Diagnosis

Once a human is infected with Y. pestis, the organism rapidly replicates at the site of the flea bite. This area can subsequently become necrotic, where dead tissue blackens to produce a carbuncle or necrotic pustule often called a "carbone" in many historical accounts. But in many cases the progress of infection is too rapid for this to happen. The lymphatic system attempts to drain the infection to the regional lymph node, where organisms and infected cells can be phagocytized (ingested by macrophages and white blood cells). That node becomes engorged with blood and cellular debris, creating the grossly swollen bubo. Because infected fleas usually bite an exposed area of the body, often a limb or the face, the location of the subsequent bubo is often visible. Frequent sites are the groin, the axilla, or the cervical lymph nodes.

But drainage can occur to an internal lymph node, or the infection can proceed too rapidly for the lymphatic system to effect a defense. In the latter case, "septicemic" or blood-borne plague has occurred, seeding the organism quickly in many organs. Victims of septicemic plague become rapidly moribund and often develop neither eschar nor bubos, although the more usual clinical course is the formation of a bubo, described in historical accounts as reaching the size of an egg, orange, or even grapefruit. The area is inflamed, boggy or doughy to the touch, and exquisitely painful. Patients often demanded that a physician or surgeon incise and drain the hubo, a process that could have liberated infective organisms.

The bubo often appears within 4 to 6 days after infection, and because of the multiple ways in which Y. pestis is virulent to humans, disease progresses quickly after this point. In about 5 to 15 percent of the cases the lungs are infected, but more often a high fever with headache and mental disorientation are characteristic symptoms. Occasionally circulatory collapse and hemorrhagic sepsis occurs, blackening the surface of the body. Death typically occurs within 4 to 6 days after the onset of symptoms. In the past, a diagnosis of plague might have been made on the basis of this rapid clinical progression from health to death alone, although clearly this could have been caused by countless other conditions. But the acute formation of a bubo, visible in 60 percent of bubonic plague victims, is pathognomonic of plague, meaning that no other disease commonly causes this reaction.

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