D. L. WOODWARD*¹, R. CALDEIRA², R. AHMED1, G. SOULE¹, C. G. CLARK¹ , L. BRYDEN¹, P. MELITO¹, H. TABOR¹, R. FOSTER¹, J. WALSH¹, G. B. MALCOLM³ and N. STROCKBINE³

¹National Laboratory for Enteric Pathogens (NLEP), National Microbiology Laboratory, Winnipeg, Manitoba, R3E 3R2, ²Bureau of Microbial Hazards, Tunneys Pasture, Ottawa On. K1A OL2, ³Center for Infectious Diseases, Centers for Disease Control (CDC) and Prevention, Atlanta, Georgia 30333.

Objective: The NLEP and CDC have detected a new, emerging Shigella serotype, which has been designated as Shigella boydii 20. This new Shigella serotype was further characterized using phage and molecular typing.

Methods: Shigella isolates belonging to this new serotype did not react with O antisera belonging to recognized Shigella serotypes. An O antiserum was prepared from a representative Shigella strain belonging to the group. Sixteen bacteriophages were selected and used for sub-typing on the basis of formation of distinct lytic patterns. Isolates were further characterized using Pulsed-Field Gel Electrophoresis (PFGE).

Results: There have been 48 cases of the newly described S. boydii 20 serotype identified in Canada. In 1999 there were 8 cases, 11 cases in 2000, 25 cases in 2001, and 4 cases up to June, 2002. In 2001, the CDC reported an outbreak of 45 cases in California, 12 of which were associated with travel to Mexico. All isolates investigated agglutinated in an antiserum prepared from a representative Shigella boydii 20 strain (99-4528). All isolates fermented mannitol and did not possess any S. flexneri antigens. Forty-three Canadian isolates were differentiated into 10 different phage types. 16S rRNA gene sequencing, detected a close resemblance of serotype 20 strains to Shigella boydii ATCC 9207. Positive results were obtained for the detection of the plasmid-encoded ipaH and ial genes responsible for invasiveness. Shiga toxin was not detected. PFGE patterns using Xba I were unique and all isolates investigated showed greater than 90% similarity.

Conclusion: This newly described emerging serotype has been designated Shigella boydii 20.