Yersinia pseudotuberculosis with limited genetic diversity is a common finding in tonsils of fattening pigs.

A total of 425 pig tonsils, including 210 tonsils from fattening pigs and 215 from sows, from seven different abattoirs in Finland were studied for the occurrence of Yersinia pseudotuberculosis from 1999 to 2000. The mean prevalence of Y. pseudotuberculosis in fattening pig tonsils was 4%, varying from 0 to 10% between slaughterhouses. Y. pseudotuberculosis was not recovered from sow tonsils. All 30 Y. pseudotuberculosis isolates from eight pig tonsils were recovered after cold enrichment. Seventeen isolates from seven tonsils were found after cold enrichment for 14 days, followed by alkali treatment. Y. pseudotuberculosis was not isolated after direct plating, overnight enrichment, or selective enrichment. All 30 isolates belonged to bioserotype 2/0:3 and carried the virF gene in the virulence plasmid. The isolates exhibited calcium dependence and Congo red absorption. The pyrazinamidase test gave variable results. All isolates were characterized with pulsed-field gel electrophoresis (PFGE). Using SpeI, NotI, and XbaI enzymes, seven, five, and two different PFGE patterns were obtained, respectively. A total of 11 genotypes, gI to gXI, identified by a combination of the various SpeI, NotI, and XbaI profiles, were detected. Three pigs were found to carry more than one genotype. Overall, variations between PFGE patterns were small, indicating genetic homogeneity among pig strains of bioserotype 2/0:3.