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*Cause disease in animals and are important zoonoses
*Cause disease in animals and are important zoonoses
*10 species of which ''Yersinia pestis, Yersinia pseudotuberculosis amd Yersinia enterocolitica'' are pathogenic to animals and humnans
*10 species of which ''Y. pestis, Y. pseudotuberculosis and Y. enterocolitica'' are pathogenic to animals and humans; ''Y. pestis'' is the most pathogenic
*Rodents provide a reservoir of ''Yersinia pestis'', which is the cause of human plague; fleas transmit the infection to other animals and humans
*Rodents provide a reservoir of ''Y. pestis'', which is the cause of human plague; fleas transmit the infection to other animals and humans
*''Yersinia pseudotuberculosis and enterocolitica'' reside in the intestine of domestic and wild animals and birds
*''Y. pseudotuberculosis and Y. enterocolitica'' reside in the intestine of domestic and wild animals and birds
*Birds may cause mechanical transfer of the organisms
*Birds may cause mechanical transfer of the organisms
===Pathogenesis===
===Pathogenesis===
*Invasive ''Yersinia'' grow inside macrophages
*''Y. enterocolitica and Y. pseudotuberculosis'' enter the intestinal mucosa via M cells of the Peyer's patches
*Engulfed by macrphages in the mucosa
*All three invasive species are facultative intracellular organisms and grow inside macrophages
*Plasmid and chromosomal-encoded virulence factors required for survival and multiplication in macrophages
*Survive in phagolysosomes and do not interfere with degranulation or lysosomal fusion
*Survive in phagolysosomes and do not interfere with degranulation or lysosomal fusion
*Resistant to macrophage killing mechanisms
*Resistant to macrophage killing mechanisms
*The bacteria destroy macrophages in lymph nodes, liver and spleen, causing septicaemia
*Antiphagocytic proteins secreted by the organisms interfere with host neutrophils
*''Y. pestis is more invasive than the other species and also possesses and antiphagocytic capsule and a plasminogen activator which aids systemic spread; endotoxin also contributes to its pathogenicity
*Transport within macrophages to mesenteric lymph nodes
*Replication in lymph nodes and development of necrotic lesions, with neutrophil invasion
*The bacteria destroy the macrophages causing septicaemia
===Clnical infections===
===Clnical infections===