The Topic of This Month Vol.27 No.3(No.313)

Shigellosis, Japan, 2003-2005

(IASR 27: 61-63; Mar, 2006)

It has been estimated that annually 91,000,000 people are infected with shigellosis, of which 410,000, mostly malnourished children, die in Asia (WHO, WER 80: 94-99, 2005). Shigella spp. is classified into four serogroups, S. dysenteriae , S. flexneri , S. boydii , and S. sonnei . S. dysenteriae type 1 (Sd1) is highly pathogenic as it produces Shiga toxin, reportedly possessing neurotoxicity and cytotoxicity similar to those of enterohemorragic Escherichia coli . Shigella can cause infection experimentally with such a small number of organisms as tens to hundreds (Morris, 1986).

In compliance with the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law), physicians who have diagnosed confirmed cases, suspected cases, or asymptomatic carriers of shigellosis must immediately report to the nearby health center. Since human infection from a pet monkey has been reported (see IASR 15:3-4, 1994), the veterinarian who has diagnosed a bacillary dysentery-infected monkey must immediately report to the nearby health center since October 2004. Thirty-seven cases of monkey infection were reported in 2005 and six cases in 2006 (as of February 21).

The Food Sanitation Law Enforcement Regulation was amended in 1999 and Shigella has been added to the category of etiological agents of food poisoning. When shigellosis is ascribed to consumption of contaminated food and food poisoning is reported by a physician or the director of a health center accepts food poisoning, investigation by the health center in the prefecture and reporting to the national government are undertaken. Reported incidents of food poisoning by Shigella were one in 2000 (involving 103 cases), three in 2001 (19 cases), two in 2002 (36 cases) (see IASR 24:187-188, 2003), one in 2003 (10 cases) (see IASR 25:153-154, 2004), one in 2004 (14 cases) (see IASR 25:337-338, 2004), and zero in 2005 (provisional data). These incidents were mostly outbreaks occurring at restaurants.

Trend of notified cases: According to the National Epidemiolodical Surveillance of Infectious Diseases (NESID), notification of shigellosis cases counted at 471 in 2003 (including 13 suspected cases), 597 in 2004 (including 12 suspected cases), 560 in 2005 (including 14 suspected cases), totaling 1,628 cases (as of February 6, 2006). Annual numbers in 2003-2005 were smaller than those in 2001 and 2002, when diffuse outbreaks due to oysters occurred (IASR 24:1-2, 2003). The estimated places of infection were mostly outside of Japan as usual (see Table 1 in p.63, excluding 39 suspected cases). The foreign countries were mostly Asian countries. During 2003-2005, the proportions of India (8.6%19%), Indonesia (7.9%12%), and the Philippines (2.3%4.7%) increased from those in 2001-2002 and those of China (7%6.9%) and Viet Nam (4.5%4.9%) were about the same, and that of Thailand (6.5%3.6%) decreased slightly (Table 1). Cases outside of Asia were few, but it is noteworthy that the estimated places of infection were diversified. In the 35-36th week of 2004, outbreaks among returnees from Hawaii due to in-flight meal were reported (see p. 64 of this issue).

Monthly reports during 2003-2005 by estimated place of infection are shown in Figure 1. Imported cases increased during August-October; in 2004, cases suddenly increased in August and September and in both 2003 and 2005, the peaks were in September (Fig. 1a). Although domestic outbreaks were reported in Aichi prefecture in March 2005 (see p. 64 of this issue), no further epidemics occurred (Fig. 1b).

During 2003-2005, there were a little more female cases (853) than male ones (736). Age distribution by gender and estimated place of infection show peaks of cases in young adults of both sexes among imported cases. Most cases were at the age of 20-34 years and there were more female cases than male ones (Fig. 2a). No gender difference can be seen in domestic cases; cases occur in wide age group from infants to the aged (Fig. 2b).

Reports of Shigella isolation: Isolation of Shigella by serogroup reported by prefectural and municipal public health institutes (PHIs) in each year of three years during 2003-2005 resembled as usual. Isolation of S. sonnei was kept at high rates, 67% in 2003, 75% in 2004, and 62% in 2005 (Table 2). S. flexneri accounted for 16-20% of all, and serovar S. flexneri 2a for a large part (44%). S. dysenteriae was isolated from six cases during 3 years, of which Sd1 was from two cases. S. boydii was seldom isolated; no particular serovar was piled. Five of six S. dysenteriae -isolated cases and seven of 12 S. boydii -isolated cases were imported ones. The serogroup proportion of Shigella isolated at quarantine stations was in the same tendency; S. boydii and S. flexneri were often isolated from returnees from India.

Current problems and countermeasures: Since there are many resistant strains of Shigella , information on drug-susceptibility of isolates from patients is indispensable for therapy (see p. 69 of this issue). Strains resistant to tetracycline, ampicillin, sulfamethoxazole-trimethoprim, or nalidixic acid are seen in many countries. At present, ciprofloxacin and norfloxacin of fluoroquinolones are effective against Shigella and administration of these fluoroquinolones and fosfomycin for 5 days is recommended in the infectious disease treatment guidelines of the Japan Medical Association. Recently in Africa and Asia, Sd1 resistant to ciprofloxacin has been reported (WHO, WER 80:94-99, 2005). Importation of such Sd1 will become a subject of discussion.

Since Shigella , as is the case with enterohemorrhagic Escherichia coli , causes infection with a minute quantity of the organisms, infection is liable to spread in ones community and to injure community health. To prevent secondary infection of shigellosis, early detection and treatment of cases and carriers as well and confirmation of no excretion of the bacilli are important. Mere disappearance of symptoms does not mean the end of treatment. Since food contaminated with a minute quantity of the bacilli may cause human food poisoning, incrimination of particular dish is often difficult. The control of shigellosis, therefore, is still important in not only individual countermeasure but also from the public health viewpoint. Many of shigellosis occurring recently in Japan is estimated to be imported infection, secondary infection derived from such patients, or domestic infection from contaminated imported food. Propagating knowledge of shigellosis as an imported infectious disease to overseas travelers and understanding the importance of medical consultation at quarantine stations and health centers when infection is suspected at the time of coming home is important. It is also important to understand that thorough cooking and hand washing with soap are effective as a means of prevention of infection on a personal level, and that there are many chances of familial infection (see p. 65 of this issue). As administrative countermeasure, active epidemiological survey such as interview and investigation into attack rates and to identify quickly the route of infection are important.

Reports of isolation of Shigella from PHIs are decreasing year by year as compared with the cases reported by NESID. At present, such information as serotyping, genetic analysis (see p. 66 of this issue), and drug susceptibility indispensable for the control of shigellosis is difficult to ensure. After enactment of the Infectious Diseases Control Law in 1999, outpatients have increased and inpatients largely decreased, which caused decrease in tests at infectious disease hospitals and increase in tests at general hospitals and subcontract examination at commercial laboratories, and problems in Shigella identification are pointed (see IASR 24:208-214, 2003 & 26:94-96, 2005). For this reason, the Infectious Diseases Control Law Enforcement Regulation has been amended and, since September 2004, health centers can request medical institutes and commercial laboratories for isolated strains when patients are notified. As an active surveillance, health centers are desired to collect the strains isolated at general hospitals and commercial laboratories and submit them to PHIs.

2006 prompt report: From six high school students traveling to Malaysia and Singapore late in January 2006 and their family member, S. sonnei was isolated (see p. 68 of this issue).

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