The genus Yersinia is now assigned to the family Enterobacteriaceae. Earlier, the plague bacillus and many other Gram-negative, short bacilli that are primary pathogens of rodents were included in the genus Pasteurella. Now, this group has been divided into three genera: Yersinia, Pasteurella, and Francisella. The genus Yersinia is now included in the tribe Yersinieae in the family Enterobacteriaceae and includes 11 species, three of which are pathogenic in humans. Human infections caused by Yersinia species are summarized in Table 34-1.
· Yersinia pestis is the most important species that causes plague, an acute and contagious, febrile illness transmitted to humans by the bite of an infected rat flea. Yersinia entero-colitica is other species, which causes mesenteric lymphad-enitis especially in children.
· Yersinia pseudotuberculosis causes mesenteric adenitis.