The Topic of This Month Vol.20 No.3(No.229)

Shigellosis, Japan, 1996-1998

The incidence of shigellosis in Japan is compiled independently from the following three reports:

(1) Patients and carriers in compliance with the Communicable Diseases Prevention Law (Statistics on Communicable Diseases).
(2) Isolation of Shigella at the prefectural and municipal public health institutes and health centers (Infectious Agents Surveillance Report).
(3) From individual cards of dysentery patients admitted to infectious diseases hospitals in Tokyo and designated cities (the Research Group for Infectious Enteric Diseases).

The following is the summary of incidents of shigellosis in the past three years on the basis of the above reports.

According to the Statistics on Communicable Diseases, Ministry of Health and Welfare, patients of shigellosis (including suspected cases and carriers) numbered 1,063, 1,112, and 1,597 in 1996, 1997, and 1998, respectively (the disease reported as dysentery includes shigellosis and amebic dysentery; the amebic dysentery patients during 1996 through 1998 were 155, 189, and 172, respectively). A significant increase is seen in 1998. If the cases are reviewed by the estimated place of infection (Fig. 1), most cases of shigellosis in 1996 and 1997 may have been infected overseas (imported cases). Those estimated to have been infected in Asian countries (India, Indonesia, Thailand, etc.) occupied greater percentages, 56% in 1996 and 63% in 1997, respectively. Those infected within the country (domestic cases) accounted for 27 and 15%, respectively. In 1998, however, the ratio was reversed and the domestic cases overcame the imported cases, accounting for 61%. The domestic cases numbered 282 in 1996 and 171 in 1997, but markedly increased to 971 in 1998.

According to Infectious Agents Surveillance Report, the reports of isolation of Shigella at prefectural and municipal public health institutes and health centers numbered 408, 326 and 589 in 1996, 1997, and 1998, respectively. Those on isolation from imported cases accounted for 72, 75 and 19% in each of the three years, respectively. These data highlighted the increase in number of domestic cases in 1998. The most prevalent species isolated was S. sonnei, accounting for more than 2/3 during 1996 through 1998 or 76, 75 and 71%, respectively, in each year. The next prevalent species isolated was S. flexneri, accounting for 20, 19, and 28%, respectively. Other species were S. dysenteriae and S. boydii, which were isolated mostly from imported cases.

In the status of isolation by month (Fig. 2), most frequent isolation was seen in April in 1996 and in May and September-November in 1998, coinciding with the outbreaks seen in Table 1. The increased isolation in 1998 can be ascribed to the increased outbreaks within the country (Table 1). Two outbreaks were reported in 1996, involving 116 patients, of which 111 were Shigella-positive ones. One outbreak was reported in 1997, involving three patients, all of which were Shigella positive. Six outbreaks were reported in 1998, involving more than 974 patients (exact numbers of patients of two outbreaks are unknown), of which 290 were Shigella positive. The increase in shigellosis patients in 1998 is thus apparent.

S. sonnei was implicated in the large majority of the outbreaks, in eight of nine outbreaks, during 1996 through 1998 (Table 1). In this connection, 24 outbreaks of shigellosis were reported during the period of five years from 1991 through 1995 and 18 of these outbreaks were caused by S. sonnei (see IASR, Vol. 15, No. 1 and Vol. 17, No. 6). In an outbreak occurring in Hiroshima City in 1997, three travelers to Egypt were diagnosed as shigellosis after returning home and the overseas group infection was suspected. In all other outbreaks, patients have no history of overseas travel and domestic infection was suspected. In another outbreak occurring in Nagasaki Prefecture, Shigella bacilli, having the serotype and genotype identical to those of the isolates from patients, were isolated from water well at the school. The contaminated well water was incriminated as the source of infection for the outbreak, but the route of contamination has not been clarified (see p. 60 of this issue). The rest outbreaks occurred at nursery schools, a hotel, a home for the handicapped, and unknown facilities. The source of infection was not clarified in any of these outbreaks, indicating the difficulty of identification of the source of infection.

According to the data from the infectious diseases hospitals, shigellosis patients were most frequently at the age of 20s, 90% of which were imported cases (Fig. 3). This may be correlated with the age of current overseas travelers. Of domestic cases, on the other hand, those aged 0 to 9 years were predominant in 1996 and 1997, but in 1998, a marked increase was seen in younger groups and also in the age groups of 50s and 60s. The increase in the age groups of 50s and 60s was ascribable to the outbreak occurring in Osaka City from an unknown source (Table 1). The main clinical symptoms of shigellosis patients were predominantly watery diarrhea in 1996 and 1997 with infrequent bloody or puruloid mucous stools. Such stool characters were considered to have been due to the infection with S. sonnei, which is regarded as causing generally mild symptoms. In 1998, however, more of domestic patients showed bloody and mucous stools than did imported cases. This was ascribable to the above-described outbreak occurring in Osaka City due to S. flexneri 2a.

According to the drug sensitivity tests on the isolates performed at infectious diseases hospitals in 1998 (Table 2), more than 69% of those from either domestic or imported cases were resistant to sulfamethoxazole-trimethoprim (ST) and tetracycline (TC). Those resistant to ampicillin (ABPC) from domestic cases accounted for 84%, being apparently higher than 30%, the ratio of those from imported cases. Furthermore, those showing resistance to fosfomycin (FOM) or ofloxacin (OFLX) of a drug of the pyridonecarboxylic acid group were found in the isolates from both domestic and imported cases.

Addendum: From April 1999, the new Infectious Disease Control Law (the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections) is being enforced. According to the law, shigellosis will be one of the category II infectious diseases. All symptomatic patients, suspected cases, and carriers must be reported as before. However, being different from the old law, hospitalization is at ones disposal depending upon the condition of each individual case, i.e., carriers are exempted from legal hospitalization and the patients whose symptoms have disappeared are generally not advised to be hospitalized. Instead of such actions, surveillance is being intensified.

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