Y. pseudotuberculosis belongs to the genus Yersinia and family Enterobacteriaceae with two other human pathogens: Yersinia enterocolitica and Yersinia pestis. Y. pseudotuberculosis is closely related to Y. pestis (97% to 100% DNA homology) and more distantly related to Y. enterocolitica. Although genetically it is closely related to Y. pestis, clinically it behaves like Y. enterocolitica.

Y. pseudotuberculosis is a non-lactose-fermenting, aerobic, gram-negative coccobacillus. This oxidase-negative and urease-positive organism reduces nitrates and ferments glucose, galactose, maltose, mannose, and xylose. It often is motile when grown at 22°C to 25°C, but nonmotile at 37°C. It grows better at 4°C in buffered saline than most other common bacteria.

Six serogroups (I through VI) have been defined antigenically on the basis of somatic O antigens. Four flagellar (H) antigens (A through D) are recognized. Serotypes are defined by both O and H antigens. Most human infections (approximately 80%) are related to serogroup 1.

The oral infection of guinea pigs with Y. pseudotuberculosis causes necrotic Peyer patches in the terminal ileum and cecum, with caseous necrosis in the mesenteric lymph nodes. The infected lymph nodes are enlarged and inflamed, with hemorrhages on their surface, microabscess formation, and the occasional development of a granulomatous reaction with giant cells. The intestinal tract, particularly the terminal ileum, may have edema, leukocyte infiltration, and superficial ulcerations. The appendix usually is grossly and microscopically normal or is mildly inflamed, but some cases of acute phlegmonous appendicitis associated with this organism have been described.

Human infection occurs via the ingestion of contaminated food or water. After reaching the gastrointestinal tract, the organism is taken up by M cells overlying Peyer patches, and it spreads from there to mesenteric nodes. Infection usually is contained at this point, but systemic spread occurs occasionally.

Pathogenic strains of Y. enterocolitica, Y. pestis, and Y. pseudotuberculosis carry a plasmid for Yersinia virulence (pYV), which codes for several virulence proteins, including the type-III secretion system, the surface-exposed adhesion (YadA), and the effector proteins (Yops, for Yersinia outer proteins). The type-III secretion system enables the bacteria to secrete and inject bacterial proteins into host cells. This system is composed of a needle-like structure, a translocation machinery, and at least six effector proteins. Pathogenic strains also carry chromosomal genes for attachment and invasion.

Y. pseudotuberculosis is found in animals worldwide. The bacteria have been recovered from a wide range of domestic and wild animals and birds. Rodents and fowl, especially turkeys, pigeons, ducks, and canaries, frequently are infected and represent important reservoirs. The organism is excreted in feces and animal-to-animal transmission has been documented.

Human infections by these bacteria are infrequent occurrences and are reported mainly in northern Europe, generally as sporadic cases for which the source of the infection is not found. Direct or indirect contact with infected animals probably represents the major mechanism of infection. Household pets often are the source of infection for children. The organism also has been recovered from foods, milk, water, soil, and other environmental sources. A history of drinking well water or mountain stream water is common. Most infected patients are children older than 5 years or adolescents. Human infections occur more frequently during fall and winter months. The incubation period typically is 4 to 6 days, varying from 1 to 14 days.

Epidemics of Y. pseudotuberculosis are rare events. One outbreak of undetermined source was caused by serogroup III in Finland. The infected cases were scattered over a large geographic area, without clustering in families, thus leading to the conclusion that the level of infectivity was low, at least for this strain. The disease was more severe in children than in adults.