Prevalence of antibodies to Chlamydia pneumoniae in a Belgian population

To evaluate the prevalence of antibodies to Chlamydia pneumoniae in a healthy adult Belgian population a study group of 150 medical students was chosen. Sera were collected in the period between March and October 1990 and assessed by the microimmunofluorescence test. Sixty-one per cent were found to have IgG antibodies to C. pneumoniae in a titre greater than or equal to 16, which showed evidence of past infection. Twenty-one per cent had IgA in a titre greater than or equal to 8. In none were antibodies of the IgM fraction detected. The same sera were tested for the presence of antibodies to Chlamydia trachomatis. One hundred and thirty-one sera with no or low titres of antibodies to C. pneumoniae tended to have low or no detectable antibodies to C. trachomatis. Nineteen sera with high (greater than 128) titres of antibodies to C. pneumoniae had IgG antibodies in a titre of greater than or equal to 32 to C. trachomatis. This prevalence (13%) is much higher than one would expect in a population at low risk for C. trachomatis infection. The problem of possible cross-reactions between the three species in the micro-immunofluorescence test is discussed.