Chapter 12: Yersinia, Francisella, Brucella, and Pasteurella

Yersinia pestis, the etiological agent of plague, demonstrates sophisticated immune evasion through its F1 capsular antigen and V and W virulence factors, which protect the organism from phagocytic destruction by macrophages. The bacterium exhibits characteristic bipolar staining morphology and circulates through natural cycles involving wild and commensal rodent populations and flea vectors, with transmission occurring through flea bites during feeding. Francisella tularensis causes tularemia and maintains its reservoir primarily in lagomorphs, particularly rabbits and hares, while arthropod vectors including Dermacentor ticks and Chrysops deerflies facilitate human infection through varied exposure routes. The organism's capacity to cause infection through multiple portals of entry makes it a significant occupational and recreational hazard. Brucella species represent a major public health concern in agricultural and veterinary settings, with cattle serving as the principal animal reservoir and transmission occurring through contact with infected animal products, body fluids, and reproductive tissues. Humans occupationally exposed through veterinary practice, farming, and animal processing face elevated infection risk. Pasteurella multocida typically affects humans following traumatic inoculation through animal bites and scratches, particularly from domestic cats and dogs that harbor the organism as part of their normal oral microbiota. The bacterium maintains ecological niches in diverse animal reservoirs including birds and small mammals. Understanding the epidemiological ecology, vector relationships, and occupational exposure scenarios for these pathogens is essential for clinicians to recognize and manage infections appropriately and to implement effective prevention strategies in at-risk populations.