The Topic of This Month Vol.21 No.9(No.247)

Legionellosis is an infectious disease caused by a bacterium of the genus Legionella. The disease has two distinct forms: Legionella pneumonia (Legionnaires' disease) and Pontiac fever, a milder illness. It is difficult to distinguish Legionnaires' disease from other types of pneumonia by symptoms alone; diagnosis must rely on laboratory-based tests. Attention must be paid to the fact that some cases are accompanied with such neuromuscular symptoms as weakness of the limbs or disturbance of consciousness and that sudden aggravation of general conditions occurs in other cases. Legionella can often be found in aquatic bodies, including rivers, lakes and polluted waters, and moist soils. They multiply at 20-50��, best conditions being at about 36��. The bacteria multiply abundantly within the cells of protozoa (ameba), inhabiting such man-made hot/warm aquatic environments as cooling towers, circulating bath systems, and hot water tanks. Legionella is transmitted to man through inhalation of aerosol mist arising from such water sources and multiples in the mononuclear phagocytes. Older persons, newborns and persons whose immune system is suppressed by certain underlying diseases are considered as high-risk groups.

Legionellosis has been classified as a member of the category IV notifiable infectious diseases by the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) enacted in April 1999. From the enactment of this law until July 31, 2000, 145 legionellosis cases were reported, from which 10 dead cases (6.9%) occurred by the time of the reporting. The Working Party on Legionellosis in Japan organized by the Ministry of Health and Welfare (headed by Prof. Ueda, Y.) found only 86 cases, during the 14 years from 1979 through 1992, showing rather a rarity of the disease, nevertheless the case fatality rate stood at 32%.

The legionellosis incidence (the dates of the first medical examination are shown by month) showed no seasonal changes after the enforcement of the Law (Fig. 1). Cases are standing out in March and June 2000, reflecting the outbreaks from public bath houses mentioned below. This is apparently different from the accepted notion that legionellosis patients could increase in association with operation of cooling air conditioners every August. As seen in Table 1, cases reported with estimated sources of infection numbered at as small as 69, of which 59 (86%) were ascribed to bathing facilities.

In an outbreak occurring in March 2000 linked to circulating bath water at a spa resort in Shizuoka Prefecture, 23 patients involving two deaths were reported (see p.188 of this issue). Another outbreak occurring in June 2000 from a welfare facility's bathing house in Ibaraki Prefecture was in a large scale involving 43 patients, of which three died (see p. 188 of this issue). Besides, seven patients were reported during April-August of this year in Yamagata Prefecture, but the source(s) of infection has not been identified (see p. 189 of this issue). Legionella pneumonia occurred in two newborns in a hospital presumably from a humidifier in Hiroshima Prefecture in January 2000.

The ages of reported cases were distributed widely from neonate to 91 years (none of the cases aged 2-20 years) with an average age of 60.1 years and a peak at 50s to 70s (Fig. 2). Male patients accounted for 78% of all, in agreement with the previous view that male patients of legionellosis are approximately three times as many as female patients. The principal symptoms of the patients were fever and respiratory symptoms as shown in Table 2.

The methods used for confirmatory diagnoses in 135 cases were titration of serum antibody and antigen detection in urine in 57 cases each (42%) and culture in 28 cases (21%). By titration of serum antibody only, it took a longer time for diagnosis than antigen detection in urine or culture (Table 3). PCR, giving a high detection rate, was applied in six cases only (4.4%) and is not yet routinely applicable for legionellosis diagnosis.

Of the etiological agents recorded, L. pneumophila was found in as many as 17 cases (of those, seven cases were ascribed to serogroup 1 and other three to serogroup 6), and L. micdadei and L. gormanii in one case each. All the outbreaks described above were caused by L. pneumophila serogroup 1. The titration of serum antibody done at commercial diagnostic laboratories is against L. pneumophila serogroup 1, and it is known that the commercial kits for antigen detection in urine have a low sensitivity to L. pneumophila serogroups other than 1. It is inevitable, therefore, that the etiological agents of reported cases tend to be L. pneumophila serogroup 1.

According to the reports of Legionella detection from environmental sources by 12 prefectural and municipal public health institutes in the whole country to the Infectious Disease Surveillance Center, the National Institute of Infectious Diseases, L. pneumophila serogroup 1 was predominantly detected in cooling towers, while serogroups other than 1 were often detected from samples of hot springs and circulating baths (Table 4). If the culture test of Legionella is more widely practiced and the detection of L. pneumophila serogroups other than 1 and other species of the genus Legionella becomes universal, reports of cases due to these other etiological agents will certainly be in the increase.

Since the genus Legionella is an intracellular pathogen, selection of therapeutic drugs is important. The efficacy of ��-lactam antibiotics generally used for treating pneumonia is not accepted. Erythromycin, rifampicin, and fluoroquinolones, all being permeable to host cells, will be the drugs of the first choice.

It is inevitable that Legionella bacteria together with soil dust contaminate previously mentioned man-made aquatic environments. In those environments with a favorable temperature, Legionella multiplies rapidly in coexistence of their host protozoa. Therefore, adequate drainage, cleaning, and disinfection of artificial water equipment in compliance with the maintenance manual are essential, and for commercial and public facilities, making and keeping the records of operation and maintenance are indispensable. Legionella bacilli and their host ameba have been detected from about 70% of water samples of hot bathtubs with circulating systems (Kuroki, T., et al.: Kansenshogaku Zasshi, 72, 1050-1063, 1998). Unless hygienic maintenance of bathing equipment is widely known to everybody, large-scale outbreaks could recur in the future. Special care is necessary for hygienic maintenance of air conditioners, hot water systems, bathing equipment, and humidifiers at various facilities and hospitals particularly for older persons and newborns that form risk groups of this disease (see the Guideline for Prevention and Control of Legionellosis, the Ministry of Health and Welfare, p. 192 of this issue).

During the 16 months since the enforcement of the new Infectious Diseases Control Law, there have been far more patients than those during the 14 years reported by the Working Party on Legionellosis in Japan. This reflects the enriched knowledge of this disease owing to the enforcement of the new Law and the increase in cases subjected to confirmatory diagnosis due to the development of the test methods. If urinary antigen detection, the most rapid and simple diagnostic method of legionellosis, becomes more popular, patients could receive more rapid and adequate treatments, leading to a further decrease in the case fatality rate.

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