The Topic of This Month Vol.19 No.2(No.216)
Atypical pneumonia is a term contrary in meaning to typical bacterial pneumonia and implicated in the concept of nonbacterial pneumonia, showing a feature of transient pulmonary infiltration in an X-ray photograph of the chest. Although the proportion differs from report to report, usually 30-40% and about 60% of atypical pneumonia cases during an epidemic period are caused by Mycoplasma pneumoniae. Adenoviruses or chlamydiae besides M. pneumoniae are also implicated.
Mycoplasma is the smallest self-replicating microorganism. The organism shows polymorphism because of the lack of the cell wall and is insensitive to such inhibitors of cell wall synthesis as penicillin and cephem antibiotics. Seven species of Mycoplasma are frequently isolated from human sources, but only one of these species, M. pneumoniae, has proved to be pathogenic, causing upper respiratory tract infection, bronchitis, and pneumonia. Pneumonia is evoked in 3-5% of cases of M. pneumoniae infection, not showing puruloid expectoration, which is often seen in bacterial pneumonia and characterized by considerably prolonged symptoms such as stubborn nonproductive cough. In addition to such common symptoms of respiratory tract infection as fever, sore throat, headache, chill, generalized malaise, such gastrointestinal symptoms as diarrhea and emesis are often recognized. The incubation period of M. pneumoniae infection is fairly long, being 10-15 days. For instance, the Kanagawa Prefectural Public Health Laboratory, investigating familial infections with M. pneumoniae, reported that the intervals between the first attack of the disease and the following one ranged from less than seven to 28 days, mostly 15-21 days (21 of 43 cases), and the average interval was 14 days (see IASR, Vol. 18, No. 12). Varieties of complications have been reported; above all, central nervous system disorders including meningitis, encephalitis, and Guillain-Barre syndrome and such dermal disorders as exanthema are often seen among children, and dysfunction of the liver is relatively common in adults.
According to an epidemiological investigation based on laboratory diagnosis carried out during the period from 1968 to 1978 (T. Niitsu et al., Kosankinbyo Kenkyu Zasshi, 30: 57-64, 1978), epidemics of mycoplasmal pneumonia in Japan broke out every four years during this period, being synchronized with the Olympic Games, and the disease has been called "Olympic disease." In recent years, bacterial pneumonia cases have markedly decreased in number and the ratio of mycoplasmal pneumonia to all pneumonia cases has increased; among pediatric patients, it is regarded as one of the infections occurring in high frequency. Bacterial pneumonia frequently affects infants, young children, and the aged older than 65 years, whereas mycoplasmal pneumonia frequently affects young children, school children, and adolescence. There is no sex difference in the incidence of mycoplasmal pneumonia.
The isolation of M. pneumoniae from pharyngeal specimens for etiological diagnosis is not well accepted, because it requires special culture media, a long incubation period (2-4 weeks), and somewhat complicated procedures, and involves invalid testing results in 5-10% of specimens due to the growth of contaminants. At present, serodiagnosis is commonly used. Of varieties of antibody-determining methods, several kinds of kits for indirect hemagglutination (IHA) are available commercially. By using one of such kits, antibody assay can be made simply and rapidly, but such specimens that are taken within a week after onset of the disease may often give negative results. Recently, detection of M. pneumoniae by the PCR method has become available and more laboratories have been using this method (see p. 29 of this issue).
Since mycoplasmal pneumonia resembles clinically chlamydial pneumonia, tetracyclines and macrolides effective against both are generally used, but for children, tetracyclines are not the antibiotics of the first choice because of the possible toxic effects. Until now, none of M. pneumoniae strains has been recognized to be resistant to these drugs.
As epidemiological data on mycoplasmal pneumonia in this country, the incidence of atypical pneumonia compiled by the National Epidemiological Surveillance of Infectious Diseases (NESID) during the 16 years from 1982 through 1997 and the results of isolation of M. pneumoniae reported to IASR are given below:
1) NESID collects information on cases of primary atypical pneumonia (PAP) with a purpose of surveillance for mycoplasmal pneumonia. The trend of weekly patients reported by sentinel clinics (mainly pediatrics) for surveillance is shown in Fig. 1. High peaks are seen in 1984 and 1988, but after 1992, this periodicity was given way and the term "Olympic disease" has seldom been used in recent years. Since 1991, small peaks have been seen regularly in late autumn through early spring. No large-scale epidemics have occurred probably because of the decreased incidents of familial infection and mass infection at schools owing to early diagnosis and early treatment.
PAP cases were often reported among children of the age group corresponding to school children, and in 1984, when there were the largest number of patients, those aged 5-9 years accounted for the highest proportion (Fig. 2).
2) Prefectural and municipal public health institutes taking part in isolation of M. pneumoniae are limited and there are a small number of reports. The patients from which M. pneumoniae was isolated from the pharyngeal specimens by the culturing method numbered 615 during 1982-1997. The clinical symptoms of M. pneumoniae-isolated cases were such lower respiratory tract inflammation as bronchitis and pneumonia in 337 patients (55%) accounting for more than half and upper respiratory tract inflammation in 180 patients (29%). This may have been due to the fact that the attention of clinicians in isolation of the pathogen was focused on such patients that were having severe symptoms. The trend of reports on isolation is shown in Fig. 3. Peaks are seen in 1984 and 1988, being synchronized with the periods in which PAP patients largely increased as shown in Fig. 1, and reports of isolation since 1991 have markedly decreased. This may have been due to the early diagnosis by PCR and other simple serodiagnostic methods and to the difficulty of isolation of M. pneumoniae resulting from early administration of effective antibiotics based on early diagnosis.
The age distribution of cases from which M. pneumoniae was isolated is shown in Fig. 4. Such cases increased at ages of 1-4 years, became the largest number at ages of 5-8 years, and decreased at ages over 15 years.