Incidence: Pertussis is a category V infectious disease to be reported by pediatric sentinel clinics under the National Epidemiological Surveillance of Infectious Diseases (NESID) and cases are reported every week from approximately 3,000 sentinels all over the country. The annual cases per sentinel were <1.00 during 2005-2007 as was the case during 2001-2004, approximately 1/10 that during 1982-1983 (Fig. 4). The traces of periodic epidemics can still be seen (Fig. 1). Pertussis epidemics have occurred every 3-5 years, and 1999-2000, 2004 and 2007 correspond to epidemic years. Since the increasing tendency of cases in 2007 beyond those in 2004 (Fig. 4) is still continuing after the year-end (Fig. 1), care must be taken for the trend of incidence in 2008.
In incidence by prefecture, more than 2.00 cases per sentinel were reported only in Tochigi and Chiba Prefectures (Fig. 2). Small epidemics occurred in 2004 and 2007, and prefectures reporting more than 1.00 case per year numbered 13 in 2004 and 2007. In 2003, 2005 and 2006, more than 1.00 cases per sentinel was seen in only 5, 3 and 4 prefectures, respectively, therefore, periodical pertussis epidemics may occur nationwide.
Outbreaks: In Japan, small outbreaks (nosocomial infections) in maternity and children's wards have sporadically occurred (IASR 26: 64-66, 2005), however no large-scale outbreak has been reported. In 2007, outbreaks occurred in universities/colleges and developed into such large scale ones as involving more than 200 cases (see p. 68&70 of this issue). For their control, such measures as antibiotic administration (including prophylaxis) or cancellation of lectures were undertaken. The outbreaks in 2007 occurred in such places as those sharing a small space for a long time (see p. 68, 70&71 of this issue), and it was pointed out that pertussis would easily transmit in such places. It was found that epidemic strains differed in each incident, and B. pertussis strain latent in each locality may have prevailed in respective localities (see p. 67 of this issue).
Adult pertussis: In the United States, patients have increased from latter half of 1980's among adolescents and adults in which vaccine efficacy has been worn out (IASR 26: 69-70, 2005) and adult cases in 2004 accounted for 27% of all cases. A similar tendency was seen also in Japan and adult cases in 2007 more than doubled from those in the preceding year (Fig. 3). Even so, no marked increase in reported cases among 0 to 3-year young children was seen, indicating that immunity attained by vaccination is enough. The age distribution of cases during 1982-2007 indicates that the proportion of adults apparently increased after 2002, and they accounted for 31% of all cases in 2007 (Fig. 4). Pertussis cases in Japan are reported by pediatric sentinels, therefore a considerable number of adult cases may not have been reported.
Clinical features of adult cases: Diagnosis of pertussis in children is dependent upon prolonged cough, staccato, reprise, and increase in the number of peripheral blood lymphocytes as markers. However, adult cases may show only prolonged or paroxysmal cough, which make it difficult to differentiate from other diseases. This causes delay in detection of outbreaks among adults. The study group of the Ministry of Health, Labour and Welfare (MHLW) is advancing analysis of clinical features of adults (see p. 75 of this issue).
Laboratory diagnosis of adult pertussis: For laboratory diagnosis of pertussis, B. pertussis isolation, serological test, and gene detection are applicable. Since adult cases contain a small number of organisms, gene detection is effective. However, the generally practiced method is a serological test and the bacterial agglutination titer is principally determined. In children, diagnosis depends on the bacterial agglutination titer. It is not known, however, whether this method is applicable to adults and future investigation seems necessary. The National Institute of Infectious Diseases distributes gene detection kits utilizing loop-mediated isothermal amplification (LAMP) to Pertussis Reference Centers in the whole country (IASR 29: 42, 2008), planning to enhance the laboratory network (see p. 73 of this issue).
Future problems: The immunity on account of pertussis vaccine is estimated to last for about 5-10 years, and even vaccinated adults are susceptible to pertussis. If adults are infected, symptoms are mild and such a severe case as encephalopathy or death is extremely rare. If adults are carrying B. pertussis , however, it must be considered that they will become source of infection to children unconsciously. When unvaccinated children, especially young infants, are infected with B. pertussis , they tend toward seriously ill and fatal cases are still seen in Japan. In the US, as a new pertussis-control strategy, introduction of adult booster tetanus, diphtheria and pertussis vaccine (Tdap) was approved in 2005, while in Japan there has been no program for adult booster immunization. At present, the study group of MHLW is accumulating scientific evidences for the propriety of its introduction.
Since pertussis cases are reported from pediatric sentinels, it seems difficult to find the accurate number of adult cases (see p. 74 of this issue). In order to find the accurate pertussis incidence including adult cases, re-categorization toward notification of all cases from sentinel reporting seems necessary. The study group of MHLW is preparing a database management system for pertussis in order to connect to a quick response, compensating the incomplete data from sentinel reporting under the NESID.
Conclusion: The first outbreak of adult pertussis in Japan occurring in 2007 allowed recognition of necessity of a new strategy to control adult pertussis. The direct factor of these outbreaks among adults is unknown and it can not be denied that such would occur in future. For prevention of pertussis outbreaks, early detection of the index case and quick response are effective and enforcement of pertussis surveillance and laboratory examination are important. For drastic measures, it seems necessary to re-evaluate current pertussis immunization program and to study booster immunization of adolescents and adults.