The Topic of This Month Vol.23 No.9(No.271)
V. cholerae O139 was first found along the coast of the Bay of Bengal, India, in 1992 and now it is isolated principally in Indian Subcontinent and Southeast Asia. In Japan, it was first detected from returnees from India in April 1993, followed by reports of isolation from 12 cases (see IASR, Vol. 19, No. 5). Nevertheless, there has been no report of its isolation since October 1997. There are some exceptional V. cholerae strains of serotypes other than O1 or O139, producing CT and causing cholera-like symptoms, however such organisms are not included in the cholera-causing pathogens (see p. 226 of this issue).
1. Cholera control in Japan
Cholera, since the Japanese first epidemic occurring in 1822, has been feared for its high case-fatality rate. Under the Communicable Diseases Prevention Law established in 1897, prevention of epidemics has been dependent upon mandatory isolation of cholera patients and carriers caused by V. cholerae O1. Since CT-non-producing V. cholerae O1 does not develop cholera symptoms, only those CT-producing V. cholerae O1 isolated have been the target of cholera control since October 1988 (see IASR, Vol. 9, No. 11). 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, cholera is listed under the category II notifiable infectious diseases. Cholera caused by newly emerged CT-producing V. cholerae O139 has also been included. Cholera control no longer depends on mandatory isolation of patients and carries. Although the seventh amendment of the Quarantine Law was published, cholera is still defined as a quarantine infectious disease. Since cholera infection due to food contamination has occurred, the Enforcement of Regulation of the Food Sanitation Law has been amended in December 1999 and cholera pathogens have been added to the list of etiological agents of foodborne diseases.
2. Cholera incidence in Japan
Cholera incidence in Japan during 1989-August 2002 is shown in Table 1. The notified cholera cases (the total of laboratory-confirmed cases and carriers) used to be accounted at 40 to a slightly less than 100 before the enactment of the Infectious Diseases Control Law, except in 1995. However, after the enactment, the cases have decreased to fewer than 40. Many of the infected cases had a history of overseas traveling. In 1995, many cholera cases broke out among returnees from Bali Island, Indonesia (IASR, Vol. 16, No. 4). Recently, domestic cases without a history of overseas traveling have been detected; such outbreaks were found in Nagoya in 1989 (see IASR, Vol. 11, No.1) and in the metropolis of Tokyo in 1991 (see IASR, Vol. 12, No. 10). In 1994, 1995, and 1997, 19-28 sporadic, domestic cases occurred. The V. cholerae O1 strains originating from sporadic patients in 1997 shared identical or very similar pulsed-field gel electrophoresis (PFGE) patterns, indicating an identical source of infection, although it was left unidentified (IASR, Vol. 19, No. 5).
National Epidemiological Surveillance of Infectious Diseases after enactment of the Infectious Diseases Control Law:
During April 1999 to August 2002, 185 cholera cases were notified, of which 110 cases and 15 carriers were laboratory confirmed (as of August 28, 2002; see on p. 221 of this issue). The suspected region of infection was in principal Asia; the Philippines, India, Indonesia, and Thailand, the frequency being in this order (Table 2). Imported cases presumably infected overseas are seen in all months year round (Fig. 1); domestic cases without history of overseas traveling were frequently seen in July, August, and September as was the case in 1997 (see IASR, Vol. 19, No. 5). Imported cases are seen in a wide range of age groups, being peaked at 20s, while domestic cases peaked over 45 years (Fig. 2). There were much more male cases than female ones (imported cases 58:29, domestic cases 25:13).
The most predominant serotype of CT-producing V. cholerae O1 confirmed by local health departments was serotype Ogawa, being 12/13 among domestic cases until 2000. Since 2001, however, serotype Inaba took the lead, being 24/25. In imported cases, similarly from the end of 2000, serotype Inaba has been predominant among returnees from Thailand (see p. 221 of this issue), although still after 2001, serotype Ogawa has been predominant being 25/38.
Domestic outbreaks with serotype Inaba occurred in Tokyo in 1978 and in Nagoya in 1989. When the PFGE patterns of the strains derived from these two episodes were compared with those of the Inaba type strains isolated from domestic cases in1997 and later than 2001, the latter was apparently different from the former. Attention must be paid to the tendency of serotype Inaba, which is apparently on the increase. Recently, an increase in drug-resistant strains has been reported (see p. 226 of this issue).
3. Future problems
Yearly incidents of cholera after enactment of the Infectious Diseases Control Law have counted as few as 40 or less, about half of that before the enactment. The reduction by half of the imported cases has largely contributed to the reduced incidents (Table 1). Since domestic cases have not been on the decrease, the relative rate of the domestic cases has increased. From now on, surveillance of returnees from overseas and domestic cases must be intensified.
At the time of the enactment of the Infectious Diseases Control Law, the Ministry of Health, Labour, and Welfare issued a notice on March 30, 1999, to conduct bacteriological tests according to the guidelines of Vibrio cholerae isolation and identification (issued on September 28, 1988; see IASR, Vol. 9, No. 11) as previously. However, pathogenic agents of cholera reported from public health institutes and quarantine stations after the enactment of the Infectious Diseases Control Law numbered at about half of the confirmed case number (see Table 1 and p. 224 of this issue). In cholera surveillance and investigation of the source of contamination, isolation of the pathogenic agents from cases and their analysis are all essential (see p. 225 of this issue). Therefore, in order to maintain the systems of bacteriological examinations and of collection of information of pathogenic agents, we must overlook at the present status.
According to the notification under the Food Sanitation Law, food poisoning incidents with cholera pathogens as the etiological agent were reported once in 2000 (two cases) and the second in 2001 (seven cases) both in August. In incidents of cholera cases without history of overseas traveling and suspected of food intervention, isolation of cholera pathogen from the incriminated food and such reports of isolation are preeminent and such epidemiological investigations as food-specific attack rate tables are also important.
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