Erysipelothrix rhusiopathiae: genetic characterization of midwest US isolates and live commercial vaccines using pulsed-field gel electrophoresis

Department of Veterinary Diagnostic and Production Animal Medicine, Iowa State University, Ames, IA 50011, USA.

This is the first report of molecular characterization of US erysipelas field isolates and vaccine strains of Erysipelothrix rhusiopathiae by pulsed-field gel electrophoresis (PFGE). Erysipelas in pigs is mainly caused by E. rhusiopathiae serotypes 1a, 1b, and 2. In 2001, erysipelas reemerged as a clinical problem in pigs in the midwestern United States. In this work 90 erysipelas isolates (58 recent and 28 archived field isolates as well as 4 live-vaccine strains) were genetically characterized. Because of the limited availability of antiserum, 74/90 isolates (44/58 recent isolates) were serotyped. The serotype of the majority (79.6%) of the 44 recent isolates tested was determined to be 1a, 13.6% were serotype 1b, and 6.8% of recent isolates were serologically untypeable. Among all 90 isolates, 23 different PFGE patterns were identified. There were 43 isolates identified as serotype 1a with 4 genetic patterns: 38/43, 1A(I); 3/43, 1A(III); 1/43, 1B(V); and 1/43, 3B. Sixteen serotype 1b isolates had 11 unique genetic patterns: 4/16 were genotype 1B(III), 2/16 were genotype 3A(I), and 1/16 was in genotype groups 1A(V), 1A(VI), 1A(VII), 1B(I), 1B(IV), 1B(VII), 2, 4, and 5. Six genetic patterns were distinguished among the 10 serotype 2 isolates: 1A(IV) (1/10), 1A(V) (1/10), 1B(VI) (1/10), 2 (4/10), 7 (1/10), and 8 (2/8). Erysipelas vaccine strains (modified live) were similar to each other but different from current field strains, sharing 78.6% identity with the most prevalent genotype 1A(I) based on the PFGE-SmaI pattern. Compared with serotyping, PFGE genotyping is a more distinguishing technique, easy to perform and not dependent on the limited availability of antiserum.

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