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Identification Pandemic outbreaks of the Plague are responsible for more human deaths than any other infectious disease, other than malaria. During the Middle Ages, the Black Death killed between one quarter and one third of Europe's population. Y. pestis is a gram-negative, facultatively aerobic rod. The bacilli usually have a characteristic bipolar or "safety-pin" shape. Laboratory indicators:
Pathogenesis Y. pestis is primarily a rodent pathogen. Humans and other animals are generally accidental hosts infected via the rat flea, Xenopsylla cheopis. The flea ingests viable cells which then multiply in the intestinal tract. Some of these cells are regurgitated when the flea prepares for its next meal, thereby infecting the next host. Most cells are phagocytosed or killed by the polymorphonuclear leukocytes of the immune system. However, a few are enveloped by macrophages, which are unable to kill the cells and, in fact, provide protection while Y. pestis re-grows its capsular layer and other virulence antigens. When transformation is complete, the re-encapsulated organisms break out of the macrophages into the extracellular environment where they are more virulent. Y. pestis travels quickly to the lymph nodes where they become inflamed and hemorrhagic, giving rise to the black buboes of "Bubonic" Plague. Within hours of initial infection, cells spread to the liver, spleen, and lungs via the bloodstream. Severe bacterial pneumonia develops leading 50-60% of untreated individuals to death. During periods of coughing, scores of viable cells are scattered in the air. Virulence factors: the V
& W antigens of Y. pestis cell walls are protein-lipoprotein
complexes that prevent phagocytosis. Manifestations Onset is marked by a high fever. Many victims complain of a general feeling of malaise accompanied by pain or tenderness in the lymph nodes, which may swell forming buboes, a characteristic of Bubonic Plague. Symptoms progress to include convulsions, shock, and hemorrhagic changes in the skin. Cyanosis from the necrotizing pneumonia produce the dark skin at the extremities lending to the term "black death." Treatment Streptomycin, tetracyclines, and chloramphenicol are effective antibiotics, as ß-lactams are not effective. Without treatment, fatality rates are 100% for septicemic or Pneumonic Plague and 90% for Bubonic Plague. With treatment, fatality rates drop to 5-20%. Individuals with Pneumonic Plague must, by law, be isolated. Severe necrosis or dry gangrene of the extremities may appear on those who have survived septic shock. Prevention Threat of Plague outbreaks could be virtually eliminated by taking appropriate sanitary precautions. In the United States, a majority of cases occur in the desert southwest where the disease is endemic among wild rodents. A formalin-killed vaccine is available, but it is only recommended for individuals at high risk for exposure. |
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