Shigella sonnei Strains Isolated fromU.S.SummerStudents in Guadalajara, Mexico, from1986to1992

Plasmid DNA analysis andantibiotic susceptibilities were usedtostudystrains ofShigeUla sonneiisolated fromU.S. travelers toGuadalajara, Mexico, overaperiod ofseven years (1986 to1992). Onehundredsixty-one isolates wereanalyzed. Bytheuseofcluster analysis, eight diferent plasmid profiles were identified duringthis interval. Atany point intime, three toseven different plasmid profiles were presentinthispopulation. The introduction ofstrains thatcarried a new plasmid witha molecular mass of5.1MDa was coincidental withan increase inisolation ofS.sonnei in1988. Thisnew plasmid was presentin87.5%oftheisolates thatwere resistant tochloramphenicol. Shigellosis inGuadalajara follows a patternofhyperendemic transmission with transient peaksofhigh-frequency isolation ofS.sonnei. Thispattern results fromtheconcurrent presence of a heterogeneous group ofstrains asopposed tothewidespread transmission ofone or a fewclones. Itisestimated that asmany as1,000,000 people traveling to Mexicodevelop diarrhea everyyear.Shigella species account foras many as 20%ofthecasesoftraveler's diarrhea in Mexico, andShigella sonnei isthemostcommon species isolated (3). S.sonnei isalso themostcommon species found intheUnited States, accounting forabout 60%ofall casesof shigellosis reported totheCenters forDisease Control and Prevention everyyear(2). Twoepidemiologic patterns forshigellosis havebeende- scribed. Cyclic epidemics withmajorshifts inserogroups, Shigella dysenteriae 01strains initially predominating andthen being subsequently replaced byShigella flexneri, whichisthen replaced byS.sonnei, eachcycle lasting 20to40years(7), have beenobserved since theturnofthecentury. Thesecycles are considered toreflect changes inherdimmunity toonespecies that ispresent for alongperiod oftimeuntil immunity inthe population reaches a critical level. Morerecently a second pattern, characterized bycontinuous lowlevels oftransmission alternated withperiodic, transient nationwide increases in isolation ofShigella organisms (9), hasbeenrecognized. The explanation forthesetransient peaksinthefrequency of Shigella infection remains unclear. Themostrecent peakinthe United States was seenin1988, andS.sonnei accounted for mostoftheincrease inisolates (9). Theexistence ofonly one serotype ofS.sonnei hascompli- cated thestudy oftheepidemiology andtransmission ofthis organism. Plasmid DNA analysis hasbeenemployed todiffer- entiate strains (1,10), tocharacterize antibiotic resistance (5), andtoinvestigate outbreaks causedbythisorganism (8,16). However, veryfewstudies ofshigellosis haveextended their observations overprolonged periods oftime.Through our studies oftraveler's diarrhea inMexicowe havehadthe opportunity toexamine over thecourseofseveral yearsa well-defined population ofU.S.summer students inan area whereS.sonnei ishyperendemic. Inthis study, we usedplasmid DNA analysis andantibiotic susceptibilities fordifferentiation