The "atypical" pathogens are important causes of pneumonia, causing illness ranging from mild to severe and life threatening. The most important atypical pathogens are Mycoplasma pneumoniae (M. pneumoniae) and Chlamydia pneumoniae (C. pneumoniae). Others include Legionella spp., Chlamydia psittaci (C. psittaci), Coxiella burnetti (C. burnetti) and viruses (influenza, parainfluenza, adenoviruses, respiratory syncycial virus-RSV). Variations in etiology have been detected in different times of life and different geographical regions. The season of the year may also influence etiology and some infections follow a cyclical pattern. Direct diagnostics is sometimes difficult because most of these agents are intracellular and cannot be easily cultured. Thus, diagnosis relies on serology. The complement fixation (CF) test is the most widely used serologic test in diagnosis of atypical pneumonia. We performed two epidemiological studies (in 1992 and 2002) to determine the frequency of the most common agents of atypical pneumonia in Croatia in the same diagnostic laboratory using the same serologic method. The studies included 581 and 630 patients, respectively from nearly all regions of Croatia with clinical diagnosis of atypical pneumonia based on clinical symptoms, laboratory findings and chest X-rays. In all of them, paired sera were collected at an interval of two weeks or more and tested for CF-specific antibodies against the most common causative agents of the atypical pneumonia syndrome. An etiological diagnosis of pneumonia was established in 27% (156/581), respectively 25% (158/630) of the patients. M. pneumoniae was the most frequently demonstrated pathogen in Croatia in 1992 (22%). It was followed by RSV (21%), adenoviruses (14%), parainfluenza viruses (12%), C. burnetti (12%), C. psittaci (10%) and influenza viruses (9%. Most cases of atypical pneumonia in 2002 were due to adenoviruses (47%), C. psittaci (19%), parainfluenza viruses (14%), influenza viruses (9%), M. pneumoniae (6%), C. burnetti (3%) and RSV (2%). Whereas adenoviral, mycoplasmal and psittacosal pneumonias occured throughout the year, influenza and most parainfluenza and RSV pneumonias occured in winter months. In March 2002, small epidemics of psittacosis (11 patients) were registered in the Split area. Q-fever pneumonias occurred in winter and spring. It was noted that RSV pneumonias appeared only in children (except for one man older than 60 years). While M. pneumoniae-caused pneumonia occurred mainly in children and adolescents, the parainfluenza and adenoviral pneumonias were distributed in all age groups. Influenza pneumonia was distributed in all age groups in 1992, but only in adults in 2002. Psittacosis occured in only two children but was more common in adolescents and especially adults. Q-fever pneumonia occurred only in adults.

Kliničke medicinske znanosti