The Topic of This Month Vol. 30, No. 12 (No. 358)

Shigellosis, Japan, 2006-2009
(IASR 30: 311-313, December 2009)

It is estimated that in Asia, 91 million people have shigellosis every year, and 414,000 among them, mostly malnourished children, die of this infection (WHO, WER 80: 94-99, 2005).  Shigella spp. is classified into four serogroups, S. dysenteriae , S. flexneri , S. boydii , and S. sonnei . S. dysenteriae serovar 1 (Sd1) is particularly pathogenic as it produces the neurotoxigenic and cytotoxigenic Shiga toxin closely related to the toxin produced by enterohemorrhagic Escherichia coli .  It has been experimentally shown that as few as tens to hundreds Shigella bacteria can cause infection (Morris, 1986).

The amendment of the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) in December 2006 brought shigellosis together with cholera, typhoid and paratyphoid fever from category II to category III infectious disease from April 2007 (IASR 28: 185-188, 2007).  Consequently physicians no longer need to report suspected cases.  Admission of the patients to hospitals based on advice was repealed (

The amendment in 1999 of Food Sanitation Law Enforcement Regulation added Shigella to the list of etiological agents of food poisoning.  Number of food poisoning incidents caused by Shigella reported in compliance with the law was eight in 2000-2005 (182 patients) (IASR 27: 61-63, 2006), one in 2006 (10 patients) (IASR 27: 340-341, 2006), zero in 2007, and four in 2008 (140 patients) (IASR 29: 342-343, 2008).  All the incidents involved restaurants.

The Infectious Diseases Control Law was amended in November 2003 (IASR 24: 328-329, 2003) to the effect that, since October 2004, veterinarians are under obligation to report Shigella -infected monkeys immediately to the nearby health center when they find them.  From 2005 to 2009, 30-50 infected monkeys were reported every year and 193 monkeys in total (see p. 317 of this issue).

Trends in notified cases: According to the National Epidemiological Surveillance of Infectious Diseases (NESID), the number of reported cases of shigellosis was 477 in 2006, 452 in 2007, 318 in 2008, 166 in 2009 (as of November 18), and 1,413 in total from 2006 to now (excluding 11 and 2 suspected cases reported in 2006 and January-March 2007, respectively).

As reported previously (IASR 27: 63, 2006), most of the suspected places of infection are abroad, particularly Asian countries, which are India, Indonesia (see p. 314 of this issue), China (IASR 28: 326-327, 2007), Viet Nam, Cambodia and Thailand, in the order of frequency (Table 1).  Infections in August-October had previously been the majority.  However, since 2008, the number of infection abroad decreased throughout the year, and the seasonal curve became flat (Fig. 1a).

The domestic infections remained in rather low level throughout the year (Fig. 1b) though there were outbreaks in September-October 2006 (involving a restaurant in Ishikawa Prefecture, IASR 27: 340-341, 2006, and a nursery school in Osaka Prefecture, IASR 28: 45-46, 2007), in June-August 2007 (involving a welfare facility for the retarded in Saitama Prefecture, IASR 30: 99-100, 2009, a college in Tokyo Metropolis and nursery schools in Hiroshima and Shizuoka Prefectures), and in July-August 2008 (involving restaurants in Fukuoka Prefecture, IASR 29: 342-343, 2008).  In 2009, no domestic outbreaks have been reported so far (as of December 10, 2009).

During 2006-2009, among those infected abroad, young adults aged 20-29 were the large majority (Fig. 2a), and in the age group of 20-34 years, significantly more females were affected than the males.  Among those infected domestically, the incidence among 5-9 years of age tended to be high, which was due to outbreaks in the nursery schools in 2006 and 2007 (Fig. 2b).  Total 691 males and 722 females had shigellosis in 2006-2009.

Isolation of Shigella : Frequency distribution of serogroups reported by prefectural and municipal public health institutes (PHIs) in 2006-2008 remained unchanged, 68-90% for S. sonnei and 9-26% for S. flexneri (Table 2).  Among isolates of S. flexneri , serovar 2a was a majority (34/87).  S. dysenteriae was isolated from five cases, but no Sd1 was isolated.  S. boydii was isolated from 13 cases, eight of which were serovar 4.

Until 2006, Shigella reported from the quarantine stations was similar in number as from the PHIs. After cessation of laboratory diagnosis of diarrhea at quarantine stations since June 2007 (as a consequence of removal of cholera from the list of quarantine infectious diseases), there were no reports of Shigella from the quarantine stations since 2008 (Table 2).

Drug resistance: Strains resistant to tetracycline, ampicillin, sulfamethoxazole-trimethoprim, or nalidixic acid emerged in many countries, but ciprofloxacin (CPFX) and norfloxacin of fluoroquinolones are still effective.  Japan Medical Association's guidelines recommend administration for 5 days of either of the fluoroquinolones and fosfomycin.

In recent years, S. dysenteriae and S. flexneri resistant to CPFX are increasing in India, Bangladesh and other East Asian countries (Taneja, 2007).  The epidemiological trend of CPFX-resistant Sd1 should be closely watched.  Since 2006, S. sonnei producing extended-spectrum β-lactamase (ESBL) has been isolated from imported cases (IASR 27: 264-265, 2006 and see p. 316 of this issue) and from outbreaks among those who never traveled abroad (IASR 28: 45-46, 2007).

Control of imported foods: A national food inspection plan is made every year based on the inspection data of imported foods and past cases of Food Sanitation Law breaches.  Depending upon the size of the risk, monitoring by the quarantines is intensified or the importers are ordered to examine all the suspected foods before importation.

In October 2007, on information from abroad, monitoring for Shigella of young corns produced in Thailand was intensified (till August 2008).  In response to the outbreak of food poisoning presumably caused by Shigella -contaminated frozen squids in Fukuoka City in July 2008 (IASR 29: 342-343, 2008), the importers were ordered in August 2008 to conduct microbiological examination of the Vietnamese marine products exported by the implicated exporter, and the quarantine stations intensified monitoring of all Vietnamese marine products.

Though Shigella has not been detected so far through the inspection (Table 3), the continued enforcement of the inspection capacity in the quarantine stations is necessary because not a few food poisonings in Japan were suspected to be caused by imported foods.

Problems and required measures: Most cases of shigellosis in Japan in recent years are infections abroad, secondary infections from the patients primarily infected abroad, or infections from imported foods.  In an outbreak of college students infected abroad, many of them continued food handling job (IASR 28: 326-327, 2007).  It is important to promote public education on imported infectious diseases.  Travelers coming back from abroad should realize the importance of consulting quarantine stations or health centers when they have suspicious symptoms.

The number of Shigella bacteria isolated by PHIs and health centers is decreasing year by year when it is compared with the number of notified cases from clinics (only 150 isolations in contrast to 318 clinical cases in 2008).  From September 2004, according to the amended Infectious Disease Control Law Enforcement Regulation, the health centers have power to request bacterial isolates from clinical institutions or from commercial laboratories when they receive reports of shigellosis.  In the investigation of infectious diseases and food poisonings, it is important to obtain and analyze information on the genetic characteristics and drug sensitivity of the bacteria isolated from the patients (see p. 319 of this issue).  It helps planning of the medical services, prediction of spreading pattern of the infection (e.g. wide-ranged or sporadic), identification of possible infection sources, and prevention of further spread.  The health centers are encouraged to collect the isolates from clinics and commercial laboratories and send them to PHIs and the National Institute of Infectious Diseases (IASR 29: 314-315, 2008).

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