Molecular epidemiology of a Shigella flexneri outbreak in a mountainous township in Taiwan, Republic of China

Abstract

An outbreak of shigellosis occurred in a township of Nantou Conuty in central Taiwan from August to October in 1996. The infections extended to two neighboring townships and continued to the end of 1996. Forty cases were confirmed during the period, in contrast to only one confirmed case in Nantou County in 1996 before the outbreak. All of these 41 cases in 1996 were identified as infections with Shigella flexneri serotype 2a. In order to trace the source of the infections, the 41 isolates recovered were analyzed by plasmid profile and pulsed-field gel electrophoresis (PFGE). There was no correlation between the plasmid profile results and the PFGE results, and the latter were used for subtyping of the 41 isolates. Twenty-two isolates (53%) had the same NotI and XbaI PFGE patterns, and 4 isolates (10%) had an additional unstable plasmid band in their NotI patterns but otherwise had the same NotI and XbaI patterns as the 22 isolates. These 26 isolates were designated the outbreak strain, and of these, 24 appeared in eight villages in one township and 2 appeared in a neighboring township. Fourteen of the remaining 15 isolates, including the isolate recovered 7 months before the outbreak, had both NotI and XbaI PFGE patterns closely related to those of the outbreak strain, indicating that Shigella infections were endemic in the area. By tracing the first isolation dates of the outbreak strain in individual villages and the neighboring township, it was found that the strain spread along the major arterial road and its branch road as time passed. Our molecular typing results and epidemiological data demonstrated the endemic nature of the outbreak strain as well as a person-to-person mode of transmission for the widespread infections the strain caused.

FIG. 1

Map of 14 villages in Renai Township and the neighboring townships depicting the distribution of the S. flexneri 2a outbreak strain. The dates when an isolate of the outbreak strain was recovered from a specific village or township are indicated next to the villages or township, with the first date being in boldface and underlined. Dates are shown as month/day. Continuous lines represent the road systems, thick lines represent the main road, and thin lines represent branch roads.

FIG. 2

Plasmid profiles of the 45 S. flexneri 2a isolates. Only plasmid profiles of representative isolates are shown, and lanes are labeled I to V, corresponding to the plasmid profile. The leftmost lane contains supercoiled DNA size standards, and numbers at left indicate the sizes of the corresponding plasmids in kilobases. Chr., chromosomal DNA.

FIG. 3

PFGE patterns of NotI-digested genomic DNAs of the 45 S. flexneri 2a isolates. Only NotI patterns from representative isolates are shown. Lane 1, ATCC 29903 (the N5 pattern); lane 2, SH1105 (the N1₃ pattern); lane 3, SH2276 (the N2 pattern); lane 4, SH2683 (the N1₃ pattern); lane 5, SH3151 (the N1₄ pattern); lane 6, SH4232 (the N1 pattern); lane 7, SH4799 (the N1 pattern); lane 8, SH46949 (the N3 pattern); lane 9, SH46959 (the N4 pattern); lane 10, SH46993 (the N2₃ pattern); lane 11, SH2343 (the N1₍₁₎ pattern); lane 12, SH2182 (the N1 pattern); lane 13, SH2558 (the N1₂ pattern); lane 14, SH2576 (the N1₄ pattern); lane 15, SH2590 (the N1₁ pattern); lane 16, SH2953 (the N1₃ pattern); lane 17, SH2594 (the N1₂ pattern); lane 18, SH3162 (the N1₄ pattern). Lanes M contain yeast chromosomes that were used as molecular size standards, with sizes of selected chromosomes indicated in kilobases. The arrow indicates the 225-kb band in lane 11.

FIG. 4

PFGE patterns of XbaI-digested genomic DNAs of the 45 S. flexneri 2a isolates. Patterns from the representative isolates with their NotI patterns shown in Fig. 3 are shown in the same order here. Lane 1, ATCC 29903 (the X6 pattern); lane 2, SH1105 (the X1 pattern); lane 3, SH2276 (the X2 pattern); lane 4, SH2683 (the X1₂ pattern); lane 5, SH3151 (the X1₃ pattern); lane 6, SH4232 (the X1 pattern); lane 7, SH4799 (the X1₃ pattern); lane 8, SH46949 (the X4 pattern); lane 9, SH46959 (the X5 pattern); lane 10, SH46993 (the X3 pattern); lane 11, SH2343 (the X1 pattern); lane 12, SH2182 (the X1 pattern); lane 13, SH2558 (the X1 pattern); lane 14, SH2576 (the X1₂ pattern); lane 15, SH2590 (the X1₁ pattern); lane 16, SH2953 (the X1₂ pattern); lane 17, SH2594 (the X1 pattern); lane 18, SH3162 (the X1₂ pattern). Lanes M contain concatemers of bacteriophage λ DNA that were used as molecular size standards, with sizes of selected λ DNA concatemers indicated in kilobases.

FIG. 5

PFGE of NotI-digested DNAs of isolates of the N1 and the N1₍₁₎ patterns and undigested DNA of an isolate of the N1₍₁₎ pattern. Lane 1, NotI-digested DNA of SH4798 (the N1₍₁₎ pattern); lane 2, NotI-digested DNA of SH2308 (the N1 pattern); lane 3, NotI-digested DNA of SH2343 (the N1₍₁₎ pattern); lane 4, undigested DNA of SH2343 (the N1₍₁₎ pattern). Lane M contains yeast chromosomes that were used as molecular size standards, with sizes of selected chromosomes indicated in kilobases.

FIG. 6

Dendrogram of the 45 S. flexneri 2a isolates based on their NotI and XbaI PFGE patterns. The NotI and XbaI patterns of the isolates are indicated in parentheses.