The Topic of This Month Vol.24 No.1(No.275)

Shigellosis, Japan, 2001-2002

(IASR 2003; 24:1-2)

In the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) enacted in April 1999, shigellosis is placed under the category II notifiable infectious diseases. The physician who has diagnosed confirmed cases, suspected cases, or asymptomatic carriers must notify it promptly to the prefectural governor through the nearby health center. Amebic dysentery, requiring differential diagnosis from shigellosis, is classified under the category IV notifiable infectious diseases.

Trend of notified cases: According to the National Epidemiological Surveillance of Infectious Diseases (NESID), reported dysentery patients including suspected cases and asymptomatic carriers counted at 824 in 2001 and 641 in 2002 (from January through November), totaling at 1,465 cases, which differed only little from 581 in 1999 (from April through December) and 821 in 2000, totaling at 1,402 cases. In 2001, 485 cases (59%) were estimated to have been infected overseas (imported cases), 301 cases (36%) infected within Japan (domestic cases), and 38 cases (5%) infected in unknown regions, and in 2002, 348 cases (54%) were imported cases, 245 (38%) domestic cases, and 48 (8%) of unknown place of infection (Table 1). The tendency that the imported cases accounted for the majority was the same, but the domestic cases during 2001-2002 were larger in number than those during 1999-2000 (27%) (see IASR, Vol. 22, No. 4). Acquiring infection in Asian countries was as frequent as before. As compared with the ratio in 1999-2000, those acquiring infection during 2001-2002 in India (17%8.6%) or Indonesia (14%8.0%) decreased and those in China (4.7%6.7%) or Thailand (4.9%6.5%) increased slightly. There were more male cases among those who had presumably been infected in India or China, while there were more female cases among those who had been infected in Indonesia or Vietnam.

Weekly reports during 2001-2002 by places of infection (Fig. 1) show a rapid increase in domestic cases to 86 cases in the 49th week of 2001, which continued to the first half of 2002. Imported cases were fewer than 20 except in the 11th (22 cases) and 40th weeks (21 cases) of 2001.

The age distribution of cases in 2001 and 2002 by the estimated regions of infection (Fig. 2) shows more cases at ages of 5-9 years in both years among domestic cases, in contrast to more cases at ages of 20s and 30-34 years among imported cases. Children under 14 years were mostly domestic cases, reflecting successive outbreaks occurring at kindergartens, nursery schools (see IASR, Vol. 23, Nos. 3 & 8 and p. 7 of this issue) and elementary schools (see IASR, Vol. 23, Nos. 5 & 6) after occurrence of a diffuse outbreak of Shigella food poisoning described later.

A diffuse outbreak of Shigella sonnei food poisoning presumably caused by imported oysters: A diffuse outbreak of S. sonnei food poisoning occurred mainly in western Japan from the end of November 2001. A summary report of the Ministry of Health, Labour and Welfare (MHLW) tells that 160 shigellosis cases were reported from 30 different prefectures before January 30, 2002 (see p. 5 of this issue). The Department of Food Sanitation, MHLW, issued a note (dated January 9, 2002) giving instruction on Shigella isolation and identification. The Department of Bacteriology I, the National Institute of Infectious Diseases (NIID) conducts gene analysis of the strains submitted and the results are returned. It has been reported that the strains isolated from cases presumably due to ingestion of oysters in various places showed the pulsed-field gel electrophoresis (PFGE) patterns identical to those of the strains isolated from oysters (see p. 3 of this issue and IASR, Vol. 23, Nos. 3 and 5-8).

Reports of Shigella isolation: Reports of Shigella isolation during the 8 years of 1995-2002 from prefectural and municipal public health institutes (PHIs) are shown in Table 2. The reports of isolation by the serogroups show similar tendencies every year; S. sonnei was the most prevalent followed by S. flexneri , S. boydii and S. dysenteriae were very rarely isolated, mostly from imported cases. On the other hand, S. flexneri 5a, a rare serotype in Japan, was isolated from domestic cases in Aomori in November 2002 (see p. 6 of this issue) and S. flexneri 4a from returnees from China in Yamagata in November 2002 (see p. 6 of this issue). Besides, isolation of a new serotype has been reported (see p. 7 of this issue).

Drug-susceptibilities: The results of drug-susceptibility tests conducted at 15 infectious disease hospitals in Tokyo and 12 designated cities in 2001 are shown in Table 3. Both domestic and imported cases tend to show a high rate of resistance to sulfamethoxazole-trimethoprim (ST), tetracycline (TC) and kanamycin (KM), whereas domestic cases did so to chloramphenicol (CP) and ampicillin (ABPC). No strain was shown to be resistant to fluoroquinolones in 2001, which are now the drugs of first choice against shigellosis.

Problems and countermeasures: Shigella spp.: S. dysenteriae , S. flexneri , S. boydii , and S. sonnei are carried by humans and monkeys and the infections are not endemic in Japan. Shigellosis cases occurring recently in Japan are caused mainly by overseas infection, domestic infection from imported food, or secondary infection from these cases, except for the incident occurring in Osaka during 1998-1999 (see IASR, Vol. 22, No.4). Against overseas infection, it is important to propagate to overseas travelers the general knowledge of imported infectious diseases and the importance of stool tests of those who were having symptoms at the time of returning to Japan or those who had had symptoms during the overseas travel. Against domestic infection, it is necessary to analyze attack rates by foods for cases not having overseas travel and to conduct more active epidemiological investigation to rapidly identify the route of infection such as through contaminated food (see IASR, Vol. 22, Nos. 4 & 6).

The Shigella isolation reported from PHIs and health centers is on the yearly decrease compared with the number of shigellosis cases reported by NESID. Laboratory-based information essential for infectious disease control is available only partially in the present situation. The results of drug susceptibility tests described above were obtained by follow-up studies performed by the Research Group for Infectious Enteric Diseases, Japan, on the isolates derived from inpatients. Due to the marked decrease in number of inpatients after amendment of the Infectious Diseases Control Law in 1999, only a small number of strains have been submitted to the test. Therefore, it has become difficult to compare the rates of resistance with the years. To make up for these data, it is necessary to strengthen the pathogen surveillance activities comprising collection and analysis of bacterial strains, now conducted in cooperation of health centers, PHIs and NIID, so as to cover the many other strains isolated by hospitals and commercial diagnostic laboratories.

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