W. Rastawicki, S. Kaluzewski, M. Jagielski, R. Gierczyski
Mycoplasma pneumoniae is a common cause of lower respiratory tract disease in humans, particularly among older children, adolescents, and young adults. Infections are endemic in cities and epidemic increases are observed at intervals of 4 to 7 years. M. pneumoniae grows slowly and its culture is difficult and time consuming. The diagnosis of M. pneumoniae infections, therefore, is usually based on serology – mainly complement fixation tests (CFT) (1,2).
Epidemiological data on M. pneumoniae infections have been collected in Poland since 1970; initially in the district of Warsaw and since 1985 throughout the country.
This study analyses the records of serologically confirmed M. pneumoniae infections in the region of Warsaw from 1970 to 1997. Warsaw is a highly industrialised urban region of Poland, where electronic and machinery industries predominate. The population has grown from 1.3 m in 1970 to 1.6 m 1997.
Material and methods
Data were obtained from the Mycoplasma Laboratory of the National Institute of Hygiene and Sanitary and Epidemiological Station in Warsaw. M. pneumoniae infections were confirmed by these laboratories, using the same complement fixation technique (CFT) and the same sonicated antigen (4,5). Complement fixation tests were accepted as positive when the titre was ³ 40, or a fourfold rise in the titre occurred during the illness. Paired specimens were obtained from about 10% of all tested patients.
Epidemiology. From 1970 to 1997, diagnostic serological tests for infection with M. pneumoniae in Warsaw were performed on specimens from 25 932 people (5824 by the end of 1984 and 20 108 between 1985 and 1997). Most of the patients were children at preschool and school age with clinical symptoms of respiratory tract infection, – 87% of whom were admitted to hospital. Six epidemics of mycoplasmosis were noted in Warsaw during these studies during the autumn-winter season in 1970/71, 1975/76, 1980/81, 1985/86, 1991/92, and 1995/96.
Figure 1 shows the total number of clinical cases investigated and the percentage confirmed by the CFT. The incidence of mycoplasmosis varied considerably over the years. At the peak of an epidemic, depending on the year, 20% to 38% of patients with respiratory tract infection had serologically confirmed mycoplasmosis. During the interepidemic period of 1987 to 1990, however, the cases of mycoplasmosis never accounted for more than 1% of all respiratory tract infections investigated. The next wave of M. pneumoniae infections after this interepidemic period, began in 1991 and continued in 1992 when respectively, 9.4% and 20% of patients investigated for respiratory tract infection had serological evidence of mycoplasmosis. In 1993, 1994, and 1995 the incidence was relatively high (respectively 15.7%, 14.7%, and 14.3%). In 1996, five years after the last epidemic began, M. pneumoniae infections rose to 18.5% in Warsaw and then fell to 14% in 1997.
Age and sex distribution. The incidence of M. pneumoniae during epidemics varied with age groups (figure 2). The highest percentage of mycoplasma pneumonia was in children aged 10 to 16 years, who accounted for over 25% of tested cases. Infections became less common with advancing age. The lowest percentage (0.8%) of confirmed cases was in the babies up to 6 months of age. Males and females were equally affected.
Clinical features. The range of disease associated with M. pneumoniae infection extends from mild upper respiratory tract symptoms to severe pneumonia. About 80% of the cases reported in our investigation had lower respiratory tract infection, including pneumonia, as the main clinical manifestation. Patients with symptoms of respiratory tract infections suspected to be due to M. pneumoniae were tested directly for mycoplasmosis.
Seasonal distribution. As a rule, epidemics of M. pneumoniae infections began in Warsaw in the third quarter of the year, peaked in the fourth quarter, and ended in the second quarter of the following year.
Various serological tests are used to diagnose M. pneumoniae infections, of which the CFT is most commonly used in Poland. The value of this test in detecting mycoplasmal antibodies has been confirmed in routine diagnostic studies carried out over an extended period (6).
Often only one serum specimen could be obtained from patients with clinical symptoms of respiratory tract infection and, based on the level of mycoplasmal antibodies in serum from healthy subjects, we accepted a titre ³ 40 as diagnostically significant in the CFT. It should be stressed, however, that it is often impossible to confirm mycoplasmal infection in serum specimens from patients with acute M. pneumoniae disease. Only a fourfold or greater increase in titre in paired sera from patients can provide confirmation of M. pneumoniae infection. In our previous study we estimated that performing a serological test only once during the first week of symptoms reduced the chances of making the diagnosis by 22.7% (7). On the other hand, however, specimens are often taken at such a late point that the first CFT titre is already high and a fourfold rise is not detected.
The present study demonstrated six epidemic waves, which occurred every four to six years in the autumn-winter. A big difference was seen, however, between interepidemic periods of 1987 to 1990, when cases of mycoplasmosis never exceeded 1% of all respiratory tract infections and 1993 to 1995, when they accounted for over 14%. The reasons for the observed change from epidemic to endemic occurrence of M. pneumoniae infections in Poland in recent years are not known. The high level of seropositive cases may be due to an increasing number, since 1992, of people being investigated for mycoplasmosis. The interest among physicians in the diagnosis of M. pneumoniae infection has increased since 1992 in response to knowledge acquired during over 20 years of investigation about the importance of M. pneumoniae pathogens in the aetiology of epidemic pneumonia in children.
The rates at which M. pneumoniae causes pneumonia differ with age. The first evident rise in the incidence of M. pneumoniae infections is at 3 to 6 years, following at the ages of beginning and continuation of school education. The low incidence of mycoplasmosis in infants under 6 months of age is likely to be due to the protective effect of persistent maternal antibodies.
It must be emphasised that this study considers only serologically confirmed M. pneumoniae infections and not the true incidence of M. pneumoniae infections. In spite of limitations in terms of specificity and sensitivity, the complement fixation test remains the commonest serological test in the diagnosis of M. pneumoniae infection.
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