The legionellosis incidence: Legionellosis has been classified as a member of the category IV notifiable infectious diseases 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. After the enactment of this law until the end of December 2002, 465 legionellosis cases were diagnosed (reported cases as of January 15, 2003) (Table 1). Death cases by the time of notification numbered at 34 (7.3%). Among 53 cases during 1997-1999 reported by Y. Niki, a member of the Working Party on Legionellosis in Japan organized by the Ministry of Health and Welfare (headed by Prof. A. Saito), and 87 cases during 1992-1999 reported by K. Yamaguchi, also a member of the Party, the case fatality rate of Legionella pneumonia stood at about 15%. Pontiac fever can be found in outbreaks; it is difficult to diagnose sporadic cases. Most notified cases may have been of the Legionella pneumonia type.
The legionellosis incidence after the enactment of the Infectious Diseases Control Law (the dates of the first medical consultation are shown by month), excluding the peaks reflecting the outbreaks linked to circulating bath water at public bath houses in 2000 and 2002, does not show seasonal characteristics (Fig. 1). This is apparently different from the accepted notion that legionellosis patients increase every August in association with operation of cooling air conditioners. The incidence by prefecture (Fig. 2) does not indicate a clustering of patients in particular districts. Prefectures with high incidence include those where outbreaks have occurred.
The mean age of cases was 60.8 years. There were few child cases; five cases each of 0 and 1 year and one case each of 3, 13, and 16 years. The age of adult cases ranged widely from 20 to 95 years, with a peak at the age of 60 years (Fig. 3). Male cases numbered at 386, accounting for 83% of the total cases, being higher than 65% in the US (1980-1998; Benin, A. L. et al., CID, 35: 1039-1046, 2002) or Europe (1999; WER, Vol. 75, No. 43: 347-352, 2000). The symptoms of the patients given in the reports were mainly fever and pneumonia accompanying respiratory difficulty.
Methods of diagnosis: Of 448 cases, for which the methods for diagnosis were recorded, 230 (51%) were diagnosed by antigen detection in urine, followed by titration of serum antibody in 132 cases (29%) and culture in 88 cases (20%) (Table 1). Diagnosis by antigen detection increased gradually, accounting for 25% in 1999, 43% in 2000, 56% in 2001, and 65% in 2002. By titration of serum antibody only, it took more days for diagnosis than antigen detection or culture. Although PCR is a highly sensitive method, only 28 cases (6.3%) have been diagnosed by this method, thus it has not become a general diagnostic method for legionellosis as yet (see p.29 of this issue).
Species of detected Legionella : Of 88 of above-mentioned culture-positive cases, the etiological agent was recorded in 52; Legionella pneumophila in 42 (of which 17 were serogroup 1 and one to three were each of serogroups 2-6), L. micdadei in two, L. gormanii and L. longbeachae in one each, and unknown Legionella spp. in six cases. All the outbreaks described below were caused by L. pneumophila serogroup 1.
On the other hand, according to the reports of detection of Legionella from environmental sources from 17 prefectural and municipal public health institutes (PHIs) in the whole country to the Infectious Disease Surveillance Center, the National Institute of Infectious Diseases, L. pneumophila serogroup 1 was predominant from cooling tower water, while from hot spring water and circulating bath water, serogroups other than 1 have often been detected (Table 2).
Outbreaks: The followings are notable incidents after the preceding topic of legionellosis (see IASR, Vol. 21, No. 9). The outbreak occurring in July 2002 at a spa resort with circulating bath water as the source of infection in Miyazaki Prefecture involved 295 patients (34 confirmed cases and 7 deaths) as of October 27, 2002 has been the largest in scale in Japan (see p. 29 of this issue). In addition, another outbreak occurred in August 2002 in Kagoshima with hot spring water as the estimated source of infection, involving nine confirmed patients and one death (see p. 31 of this issue). Both occurred from circulating bath water of newly opened spa resorts, and the viable Legionella organisms in 100 ml bath water counted at 1.5x106 and 1.3x105 cfu, respectively. These outbreaks resemble in many respects to the previous two outbreaks occurring at spa resorts making use of circulating bath water in Shizuoka and Ibaraki Prefectures (see IASR, Vol. 21, No. 9), indicating that the previous lesson was not useful. In two incidents in Yamagata Prefecture involving three cases during July-August, 2002, not so many L. pneumophila organisms were detected in spa water, nevertheless there were pneumonia cases (see p. 32 of this issue). Besides, multiple students of a junior high school developed fever with bath water of a youth hotel facility in Ehime Prefecture as an estimated source of infection in September 2002 and increased antibody titer was seen in at least one of the cases. Although three infants at an age of one were infected in an infant home in Fukushima Prefecture during June-July 2002, the source of infection was not found.
Control strategy: It is inevitable that Legionella bacteria together with soil dust contaminate such man-made aquatic environments as cooling-tower, circulating baths, hot water tanks, and humidifiers. Survey of spas, public baths, and other hot water environments detected ameba surviving as the host of Legionella in 64% of specimens (see p. 34 of this issue). In those water environments with a favorable temperature, Legionella multiplies rapidly in coexistence of their host ameba. Therefore, adequate drainage, cleaning, and disinfection of artificial water equipment in compliance with the maintenance manual are essential (see p. 33 of this issue). Based on the above outbreaks, strengthening of regulation of the use of circulating bath water of bath houses and restriction of circulating bath water for bubble spa, jet spa, and shower baths have been incorporated in the guideline (see p. 35 of this issue). The standard of bath water indicated in this guideline shows that no Legionella organisms should be detected upon concentration by such methods as refrigerated centrifugation and filtration (less than 10 cfu per 100 ml). In drowning accidents, pneumonia developed even from bath water satisfying the standard. As in the Miyazaki episode, people from other prefectures may also be affected; nationwide dissemination of information to local governments, medical institutions, and residents is important.