The bacillus has been isolated from a variety of wild and domestic animals, which presumably serve as the reservoirs for human infection. Although the organism has often been isolated from freshwater sites, only a few human cases have been linked to this source. Indeed, one study demonstrated spread to 47 percent of family members of an index case. Y. enterocolitica has been estimated to be responsible for two to five percent of cases of acute enterocolitis, a percentage that is comparable to that observed for shigellas and salmonellas. Most clinically overt infections occur in children who are younger than 10 years.
Most human cases are sporadic, and males are predominantly affected.
Y. pseudotuberculosis has been isolated at a much greater frequency in Europe, particularly Scandinavia, than in North America. The major clinical effect of Y. enterocolitica infection is enteritis, whereas Y. pseudotuberculosis produces purulent mesenteric adenitis, with little if any diarrhea. Y. enterocolitica produces an enterotoxin similar to the one produced by E. coli . However, Y. enterocolitica manufactures enterotoxin in significant quantities only at 25 C, and it is uncertain how much of a role this enterotoxin plays in disease. The critical factor in pathogenesis appears to be invasion of the ileal mucosa.