In Taiwan, S. flexneri and S. sonnei are the most prevalent species [5, 14, 31]; S. dysenteriae and S. boydii are infrequently observed, and all are found in imported cases in the most recent 2 decades . S. flexneri, the predominant subserotype 2a, circulated for a long period of time in several mountainous townships in which most of the population includes Taiwanese aborigines [5, 6]. S. sonnei often causes outbreaks in populated institutes, including elementary schools, military camps, jails, and daycare centers in the industrialized western region of Taiwan [26, 27, 31]. S. sonnei is not prevalent in aboriginal tribes and typically circulates among communities for only a short time. Since 2001, a 4-year shigellosis control program had been implemented in mountainous townships in which endemic shigellosis has been identified . Currently, shigellosis is infrequently observed in aboriginal communities; it was not been detected in the Zhuoxi Township from March 2, 2007 to December 31, 2010.
S. flexneri and S. sonnei were the causes of shigellosis in the Zhuoxi Township in 2000–2007. Most infections occurred in V2 and V5. S. sonnei infections in each village lasted for only 1–2 years, whereas S. flexneri infections persisted for much longer as it circulated in tribe V2-1 for at least 8 years (Table 1). PFGE analysis of 38 S. flexneri 2a isolates revealed that nearly all villages or tribes had unique circulating strains (Table 2, Figure 1). Although tribe V2-1 had 6 genotypes, all but 1 belonged to the same clone. Two genotypes (P08 and P09) identified in isolates from V5 differed in only 1 DNA band, indicating that they were derived from a common ancestor and caused infections in the village for 3 years (from 2004–2006). Of the 11 genotypes, only P01, the most prevalent type in the V2-1 tribe, was detected in an isolate from V3 (Figure 2). These data indicate that cross-transmission of shigellosis occurred infrequently among the villages/tribes of the Zhuoxi Township during 2000–2007, which was likely due to the inconvenient transport system between villages and different ethnic groups. This reduced interaction and lowered disease transmission between geographic locations.
The OB2 outbreak occurred in tribes V2-1 and V2-2 in March 2004. The 2 tribes are composed of aborigines from different ethnic groups (Figure 1). The 10 infected children attended 2 day care centers and 1 elementary school. Two children from tribe V2-1 and 3 from tribe V2-2 attended the same elementary school (Table 2). Epidemiological investigation suggested a common infection source. However, genotyping data indicated that cases from the tribe V2-1 were infected by 2 strains (P01 and P02) of clone 1 and those from tribe V2-2 were infected by a strain (P07) of clone 2 (Table 2). Thus, OB2 was not a single source outbreak. Genotyping results and epidemiological evidence suggested that infections were likely to occur via person-to-person transmission among family members and playmates in day care center in their respective tribes rather than school where the students came from V2-1 and V2-2. For example, cases 8, 9, 10, and 13 were playmates in day care center A1 in tribe V2-2; they were infected by the same strain (genotype P07) with different onset days (Table 2), suggesting that they were transmitted via personal contact.
The 38 S. flexneri 2a isolates were discriminated by PFGE into 11 genotypes that fell into 2 genetic clusters (clones), suggesting that S. flexneri 2a had been circulating in the Zhuoxi Township for many years. Most isolates of clone 1 displayed ACSSuX resistance and were distributed in tribe V2-1 and villages V3 and V6, circulating from 2000 to 2007. All isolates of clone 2 tested belonged to the ACSSuTX resistance type, which was distributed in tribe V2-2 and the villages V5 and V6, circulating from 2004 –2005, except for 1 case identified in 2002. Although each of the 2 clones was widespread in 3 villages, disease transmission between villages is likely very rare. Briefly, in this study, clones 1 and 2 were associated with antibiogram types ACSSuX and ACSSuTX, respectively, and circulated in different geographical locations inhabited by aborigines from different ethnic groups.
Shigellosis outbreaks in schools can persist for months and can be exacerbated by the poor hygienic practices of young children, making it difficult to control . Since the first case of the OB2 outbreak was reported on March 12, 2004, several intervention measures were implemented in V2 to control the disease. These measures included the following: (1) enforcement of screening of all contacts and school children by stool microbiological examination, (2) enhanced hand-washing practices and environmental disinfection in the schools and community, (3) separation of suspected cases from classrooms, dining tables, and toilets, (4) closing of day care centers for 6 days, (5) confirming shigellosis cases using cultures, (6) chlorination of the community water tower, and (7) implementation of health education of infectious diseases to the community residents. After these efforts, no shigellosis cases were reported from March 2, 2007 to December 31, 2010.