The Topic of This Month Vol.22 No.4(No.254)
Shigellosis used to be one of the legally defined communicable diseases notifiable under the former Communicable Diseases Prevention Law. Under the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the new Infectious Diseases Control Law) enacted in April 1999, the disease is placed under the category II notifiable infectious diseases. All physicians who have diagnosed confirmed cases, suspected cases, or asymptomatic carriers of the pathogen must promptly report to the prefectural governor through the nearby health centers. Yet, amebic dysentery is listed under the category IV notifiable infectious diseases (see IASR Vol. 20, No. 4).
The following is a summary of the incidence of shigellosis in Japan for the period of two years from 1999 to 2000. The information is based upon; (1) The notification of patients and carriers under the Infectious Diseases Control Law [National Epidemiological Surveillance of Infectious Diseases (NESID)]. (2) The Shigella isolation report by prefectural and municipal public health institutes (PHIs) [Infectious Agents Surveillance Report (IASR)].
According to NESID, the notified patients of shigellosis (including suspected cases and carriers) numbered 581 during April through December 1999 and 821 during January through December 2000, totaling at 1,402 (677 males and 725 females). By estimated regions of acquiring infection, overseas-infected cases (imported cases) counted at 968 (69%), infected within Japan (domestic cases) at 376 (27%) and unknown stood at 58 (4.1%). With regards to the reports of monthly incidence, imported cases showed peaks in August-September and March-April, while domestic cases increased in September-October 1999 and October-November 2000 (Fig. 1). The notification of shigellosis under the former law during January-March 1999 totaled at 218 (not including suspected cases) (see Statistics on Communicable Diseases in Japan).
A large number of imported cases were aged at 20s; peaks were seen at ages of 20-24 years in males and of 25-29 years in females. Female cases outnumbered male ones in any of the age groups between 20-34 years (Fig. 2). Most cases visited various Asian countries, especially those who visited India or Indonesia accounted for 31% (Fig. 3). The infection-acquiring regions have been almost the same for the past several years (see IASR, Vol. 20, No. 3). Out of 240 cases who visited India, 122 were females, and out of 201 cases who visited Indonesia, 114 were females. An obvious outnumbering of the female cases who visited those countries was seen.
Reports of Shigella isolation during the 16 years from 1985 through 2000 based on IASR are shown in Table 1. Similar tendencies could be seen yearly in the isolation by serogroup; S. sonnei was most frequently isolated followed by S. flexneri. S. boydii and S. dysenteriae were infrequently isolated principally from imported cases. Isolation of S. flexneri 2a from domestic cases increased in 1998 (see p. 84 of this issue), and repeated itself in 1999. The Shigella-isolation reports confirmed by PHIs and health centers in 1999-2000 were less than case reports, particularly in imported cases. This might be explained by the decreased stool examinations at PHIs, resulting from voluntary submission of stool specimens by overseas travelers, following enactment of the Infectious Diseases Control Law (see Monthly Epidemiological Record, Tokyo, p.1, Vol. 22, No. 1).
The reported outbreaks of shigellosis in IASR in 1999 and 2000 are shown in Table 2. Four outbreaks occurred in 1999; in outbreak #3, infection was acquired in a foreign country and in the other three, infection occurred within the country. Mode of transmission (source and route of infection) were unknown in all outbreaks. The etiological agent was S. sonnei for both #1 and #2 outbreaks and S. flexneri 2a for #3 and #4. Two groups staying on the same day at the same hotel were infected with the same genotype Shigella (#4 outbreaks) revealed by pulsed-field gel electrophoretic (PFGE) analysis in addition to the conventional serogrouping and serotyping and drug-susceptibility tests of the isolates in the epidemiological survey. Identical DNA patterns were obtained with the isolates from the two groups (see IASR, Vol. 21, No. 4).
Three outbreaks occurred in 2000, acquiring infection within the country, all caused by S. sonnei. Outbreak #5, affecting a large number of persons, was due to the consumption of hand-shaped sushi prepared by an infected sushi-bar staff. Subsequent investigations found out cases in not only Ehime but also in Aichi, Osaka, Hyogo, Shimane, Okayama, Hiroshima, Kochi, and Oita Prefectures. All the isolates shared the identical drug-susceptibilities and DNA patterns in PFGE analysis (see IASR, Vol. 22, No. 2).
The results of drug susceptibility tests of Shigella isolates performed at the infectious diseases hospitals in Tokyo and 12 designated cities in 1999 and 2000 are shown in Table 3. The isolates from more than 82% of both domestic and imported cases were resistant to sulfamethoxazole/trimethoprim and tetracycline. The ratio of ampicillin-resistant strains was 75% of total isolates from domestic cases, compared with 35% of those from imported cases. Four fluoroquinolone (ofloxacin)-resistant strains, being paid attention nowadays, were reported during 1996 through 1998 (see IASR, Vol. 20, No. 3), while no such strain was detected among isolates during the period of 1999-2000.
Under the present survey system, quarantine stations, general hospitals and commercial diagnostic laboratories start isolation of Shigella from the stool samples of shigellosis-suspected cases among travelers returning from foreign countries and among domestic sporadic or outbreak cases before notification. Health centers and PHIs undertake etiological examinations in line with the epidemiological investigation after finding out the cases following notification. For rapid identification of the source and route of infection and for the prevention of spreading, efficient cooperation is desired between the diagnostic laboratories, having detected Shigella, and PHIs undertaking serotyping, drug-susceptibility tests, and PFGE analyses.
On December 28, 1999, the enforcement regulations of the Food Sanitation Law were partially amended and Shigella has been added to the list of etiological agents of foodborne diseases. In shigellosis incidents suspected of foodborne, Shigella isolation from the incriminated foodstuff is also important.