At present, HBV is classified into eight genotypes (types A-H). It is known that these genotypes exhibit geographical preference and are different in such clinical courses as the ratio of becoming chronic. The genotype A is a Western type, and it is a noticeable tendency that persons infected with genotype A, which may tend to take more chronic courses than the other types, are on the increase in Japan (see p. 219 & 223 of this issue).
In compliance with the amendment of the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) in November 2003, acute hepatitis B has been classified into gviral hepatitis (excluding hepatitis E and A)h of the category V notifiable infectious diseases under the National Epidemiological Surveillance of Infectious Diseases (NESID). Physicians who have diagnosed hepatitis B are obliged to notify within 7 days. The guideline of reporting and the reporting forms were amended in April 2006 (see http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou11/01-05-02.html). For a primary screening method of HBV infection, detection of HBs antigen has widely been used. For detection of HBs antigen, particle agglutination, immunochromatography, enzyme immunoassay (EIA), chemiluminescent immunoassay (CLIA), and chemiluminescent enzyme immunoassay (CLEIA) are being used. Since they are different in the sensitivity depending on the detection principle, it is necessary to select the method and the reagent depending upon the purpose of detection (see p. 221 of this issue).
Annual and monthly incidence: Since 2002, cases diagnosed and reported as acute hepatitis B under the NESID have totaled 333 in 2002, 244 in 2003, 243 in 2004, 208 in 2005, and 107 from January through July in 2006. The number of cases, which was on the decrease until 2003 (approximately 500 in 1999, 400 in 2000, and 300 in 2001, see IASR 23:163-164, 2002), has remained on the same level since then. No particular tendency has been seen in the number of reports by month; the seasonal fluctuation has been small (Fig. 1).
Suspected areas of infection: Of 1,135 cases diagnosed during 2002-July 2006, 983 (87%) acquired infection domestically; the majority of cases in each year have been domestic cases. Eighty-five imported cases (7%) have also been reported (Fig. 1). Domestic areas of infection were not reported in the reporting form before the amendment in April 2006; domestic cases by the reporting prefecture are shown in Fig. 2. Many cases tended to be within areas of such large cities as Tokyo and Osaka; however reports have come from all 47 prefectures.
Age distribution by gender: Distribution of cases by gender and age is shown in Fig. 3. In teens, male and female cases are almost the same in number (31 male cases and 29 female cases). In other age groups, there were more male cases. Male cases accounted for 72% (820 cases) of all reported cases. In male cases, twenties (246 cases) and thirties (230 cases) formed peaks and so did twenties of female cases (137 cases). There were few reports of children under 14 years and of cases aged over 70 years.
Suspected transmission routes: Cases with unknown transmission route account for 40%, and for most cases (55%), sexual contact is suspected (male 56% and female 50%). For males aged 20-60 and females aged 15-45, sexual contact is a more likely suspected transmission route than others. It has been shown that hepatitis B as well as HIV/AIDS has become an important sexually transmitted infection in Japan. For other transmission routes (6%), rare instances of blood transfusion, dental treatment, tattoo, piercing, and accidental injecting have been recorded.
Countermeasures: Since 1985, a project toward prevention of mother-to-child infection of hepatitis B has started. With this project, the occurrence of asymptomatic virus carrier due to mother-to-child vertical HBV infection has dramatically been reduced. The positive rate in HBV screening of donated blood is on the yearly decrease (Fig. 4). For further countermeasures taken against transfusion hepatitis, nucleic acid amplification tests (NAT) for HBV, hepatitis C virus (HCV) and human immunodeficiency virus (HIV) have been conducted since 1999; reports of these virus infections following transfusion have markedly decreased (see p. 223 of this issue).
For general countermeasures against hepatitis including chronic hepatitis, for not only hepatitis C but also hepatitis B, the following policies are enforced based on the report of the authorized committee on the control of hepatitis C established by the Ministry of Health, Labour, and Welfare (MHLW) in March 2005: 1) Conducting hepatitis virus tests and reinforcing the test system, 2) Upgrading the therapy (improving the diagnostic system, research and development of therapeutic method), 3) Thorough prevention of infection, and 4) Promotion and education, and counseling projects. For HBV testing, being enforced are 1) tests for hepatitis viruses in the regular health examination for people aged over 40 years under the Health and Medical Service Law for the elderly, 2) tests for hepatitis viruses in the government-managed health insurance, and 3) tests for hepatitis viruses at health centers. The Q & A section about hepatitis B for the public made by MHLW was revised in March 2006 and is shown on the web site (http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou09/01.html).
Conclusion: For prevention of acute hepatitis B transmitted mostly by sexual contact, such preventive education as the use of condoms is important continuously. Prevention is possible for such high-risk individuals as sexual partners of infected persons, hemodialysis patients, medical staffs, and ambulance persons by voluntary immunization with hepatitis B vaccine.