PLAGUE

OVERVIEW

• infection of rodents caused by Yersinia pestis
• accidentially transmitted to humans by the bite of infected fleas.
• possible agents that could be used by terrorists
• gram-negative coccobacillus belonging to the Enterobacteriaceae

• Human to human transmission: By droplets (pneumonic) or by fleas.
• Epizootic plague. Sensitive or moderately-resistant hosts are infected by fleas or by ingestion, resulting in a highly visible die-off, e.g., rats, prairie dogs, rock squirrels.

• Bubonic
• Pneumonic

• Yersinia pestis is primarily a rodent pathogen
• While growing in the flea, Y. pestis loses its capsular layer
• Most of the organisms are phagocytosed and killed by the polymorphonuclear leukocytes in the human host.
• A few bacilli are taken up by tissue macrophages.
• The macrophages are unable to kill Y. pestis and provide a protected environment for the organisms to synthesize their virulence factors.
• The organisms then kill the macrophage and are released into the extracellular environment, where they resist phagocytosis (YopH and YopE; Yersinia outer membrane protein) by the polymorphs. The Y. pestis quickly spread to the draining lymph nodes, which become hot, swollen, tender, and hemorrhagic. This gives rise to the characteristic black Buboes responsible for the name of this disease.

• Incubation period of 1-3 days (pneumonic) or 2-6 days (bubonic).
• Septicemia with regional lymph node involvement (bubonic plague, 85-90% of the cases)
• Septicemia without lymph node involvement (primary septicemic plague, 10-15% of the cases); depends on level of lymph node inflammatory response.

Category: Medicine Notes , Microbiology Notes