A Shigellosis Outbreak Caused by S. flexneri X Variant at a Psychiatric Hospital in Miaoli County

Ying-Shu Liao

2009 Vol.25 NO.11

Correspondence Author: Chien-Shun Chiou


In March 2008, there was an outbreak of acute diarrhea among patients of a psychiatric hospital located in Miaoli County, Taiwan. The outbreak was caused by Shigella flexneri. A total of 15 cases were identified in the outbreak, all of them were inpatients of the hospital, with an attack rate of approximately 5% (15/307). Among those cases, 9 were symptomatic (6 of the specimens collected were cultured positive) and the other 6 were apparently asymptomatic (contacts tested positive). Of 10 S. flexneri isolates obtained, one was subserotype 2b and 9 were the extremely rare X variant type. This was the first shigellosis outbreak caused by X variant type ever reported in Taiwan. The isolates were subjected to antimicrobial susceptibility testing and pulsed-field gel electrophoresis (PFGE). The isolates were resistant to first-line antimicrobials, including ampicillin, streptomycin, tetracycline, chloramphenicol, and trimethoprim/sulfamethoxazole. DNA fingerprint comparison with those in the Shigella DNA fingerprinting database, established by Taiwan Centers for Disease Control, revealed that the outbreak isolates shared more closely pattern similarity than those in the database and suggested that the isolates of subserotypes 2b and serotype X should be derived from a common recent ancestor. In order to search the infection source, the Second Branch Office of Taiwan CDC carried out an epidemiological investigation. Unfortunately, the investigation failed to identify the source of infection. However, reviewing the fact that the X variant was rare in Taiwan and its DNA fingerprints shares high level of pattern similarity to some imported 2b isolates in the database, inferring that the outbreak strain most likely came from abroad.
Keywords: outbreak, shigellosis, pulsed-field gel electrophoresis (PFGE), DNA fingerprinting