in the beginning of october 2014 , six children who were vomiting and experiencing abdominal cramping and diarrhea were admitted to the local hospital in gyeongsangnam - do , republic of korea ( south korea ) . all patients attended the same daycare center that provided care and food to children from low - income families . fecal specimens from 6 patients were submitted to the local public health laboratory and were processed according to a standard bacterial culture method . on october 6 , korea centers for disease control and prevention was notified that s. sonnei phase ii were identified from all fecal samples . an epidemiologic investigation was conducted to determine the extent of the outbreak and to identify the mode of transmission . a confirmed case was identified by passive and active case - finding on the basis of laboratory - identified s. sonnei isolates in the fecal specimens of center attendees and staff members , families of the children , and persons in the community . a probable case was defined as a person with any shigellosis symptoms and an epidemiologic link to infected patients whose cultures were negative . the children s guardians were interviewed by using a standardized questionnaire that requested information on symptoms , food consumption , recent travel history , and contact persons . this investigation was part of a public health emergency response and was accordingly exempt from institutional review board approval . the investigation revealed that an 8-year - old boy ( the index case - patient in this outbreak ) had recently returned after visiting family in vietnam , where s. sonnei infection is highly endemic . he had experienced sustained diarrheal episodes since his return , and after returning to the daycare center , children in the daycare center began having similar symptoms . cases of shigellosis were also identified among the grandparents of the index case - patient and a person the family visited in a geographically distant location on september 27 . no isolates were obtained from the environmental samples collected , including foods , drinking water , and surface swab specimens of the daycare facility . the median age of the patients in the daycare center was 7.8 ( range 413 ) years . overall , the reported symptoms were diarrhea ( 3 loose stools during 24 hours ) and abdominal cramping ; 4 patients were asymptomatic but their stool samples were culture - positive . of the 15 persons who became ill ( figure 1 ) , 10 were treated with cefotaxime or ciprofloxacin , after which their stool samples were culture - negative . for 5 patients with continuing positive fecal culture , antibiotic drug treatment was later changed to cabapenems ( meropenem or imipenem ) . according to local infection control guidelines , symptomatic patients were isolated in single - bed rooms until 2 consecutive fecal cultures tested negative for s. sonnei . to prevent the further spread of the disease , public health interventions were encouraged during the outbreak period : enforced handwashing at predetermined times at the daycare facility , strict hygiene measures in affected households , education about shigellosis , and environmental disinfection of the facility . epidemic curve of the outbreak of illness caused by shigella sonnei infection , by symptom onset date , south korea , 2014 . black bar sections indicate laboratory - confirmed cases ; white bar sections indicate probable cases ; stars indicate cases found in daycare center . arrows indicate dates of the events for an index case - patient with travel history to vietnam and of public health notification of the outbreak . laboratory - confirmed strains of s. sonnei were sent to korea national institute of health for further characterization . all 15 isolates had identical or highly similar pulsed - field gel electrophoresis ( pfge ) patterns after the xbai digestion of chromosomal dna . the main pfge pattern of this outbreak ( sznx01.183 ; pfge pattern number assigned by korea national institute of health ) had not been previously reported in domestic cases , and the isolate was genetically indistinguishable from a ciprofloxacin - resistant s. sonnei strain isolated from a traveler returning from vietnam during 2012 ( figure 2 ) . xbai pulsed - field gel electrophoresis patterns of shigella sonnei strains identified during a 2014 outbreak in south korea and 2 isolated from samples from persons in vietnam . the dendrogram was constructed by using dice coefficient and upgma clustering , with 1.5% optimization and 1.5% position tolerance . antibiotic resistance profiles and resistance determinants to extended - spectrum cephalosporins and fluoroquinolones are plotted next to the dendrogram . all strains had qrdr mutations gyra(s83l , d87 g ) and parc(s80i ) . * strains 14 - 5222 and 12 - 3580 originated in vietnam . amp , ampicillin ; cip , ciprofloxacin ; ctx , cefotaxime ; nal , nalidixic acid ; tet , tetracycline ; sxt , trimethoprim / sulfamethoxazole ; qrdr , quinolone resistance  determining region . scale bar indicates percentage relatedness . on the basis of mics of antimicrobial agents determined by using a broth microdilution method ( 5 ) , the outbreak strains were found to be resistant to both extended - spectrum cephalosporins ( cefotaxime , mic > 32 g / ml ) and fluoroquinolones ( ciprofloxacin , mic > 8 g / ml ) . the strains were also resistant to tetracycline and trimethoprim / sulfamethoxazole but were susceptible to chloramphenicol , gentamicin , amikacin , and carbapenem . for azithromycin , an alternative oral agent for shigellosis , extended - spectrum -lactamase ( esbl ) typing by using pcr and further sequencing ( 6 ) showed that all isolates carried the blactx - m-15 and blatem-1 genes . esbl plasmid of s. sonnei isolate from the index case - patient were successfully transferred to the recipient e. coli j53 azi strain . pcr - based inc / rep typing and pmlst analysis of a transconjugant strain ( 7,8 ) showed that this esbl plasmid was of the st16/inci1 type , which was previously identified in strain pkhsb1 from vietnam ( 9 ) . the genetic environment of the blactx - m-15 gene was analyzed by pcr and sequencing with specific primers for the insertion sequences isecp1 and orf477 ( 6 ) . an intact isecp1 and truncated orf477 were identified at 48 bp upstream and downstream of the bla gene , which has also been found in ctx - m-15-encoding plasmids from enterobacteriaceae ( 6,9,10 ) . the outbreak strains had 2 mutations in the quinolone resistance - determining region of gyra ( ser83leu and asp87gly ) and 1 mutation in parc ( ser80ile [ figure 2 ] ) , which have been reported to be responsible for ciprofloxacin resistance in s. sonnei ( 11 ) . however , gyrb and pare mutations and plasmid - mediated quinolone resistance genes were not detected ( 12 ) . we describe a shigellosis outbreak affecting children attending a daycare center , their family members , and residents of the surrounding community . to limit the extent of the outbreak , laboratory investigations of outbreak strains and infection - control measures including contact isolation and hand hygiene were immediately implemented , which may have contributed to preventing the further spread of this multidrug - resistant s. sonnei strain . the outbreak strain was resistant to extended - spectrum cephalosporins and fluoroquinoloness and was introduced by a daycare center attendee who had returned from travel to vietnam . the blactx - m-15 gene in s. sonnei was first described in 2005 ( 13 ) and since then has been reported worldwide ; we described an outbreak of ctx - m-15producing s. sonnei in korea in 2008 ( 6 ) . the pfge pattern of the 2008 outbreak strain ( sznx01.176 ) showed only 82.8% genetic similarity with that of the outbreak strains of the current study but was observed in several traveler - associated cases originating from china . these findings suggest that , despite the lack of direct evidence , various antimicrobial drug resistant s. sonnei clones have been imported across geographic regions and may eventually spread globally and lead to increased illness and death rates . in summary , we report a shigellosis outbreak in south korea caused by a ciprofloxacin - resistant ctx - m-15producing s. sonnei strain that originated from vietnam . because international travel can contribute to the spread of multidrug - resistant pathogens , enhanced surveillance is necessary to control the dissemination of antimicrobial drug resistance . improved hygiene , infection control plans , and better education for travelers are also required .