Chlamydia pneumoniae, a causative agent of middle and lower respiratory infections, is referred to as an “atypical” organism because the clinical presentation is often less dramatic than illness caused by Streptococcus pneumoniae or Haemophilus influenzae. However, the treatment guidelines for community-acquired pneumonia call for the addition of a macrolide antibiotic with a beta-lactam drug because a patient's clinical presentation often does not allow the physician to make an accurate bacteriologic diagnosis. Studies have suggested that C. pneumoniae causes about 6 to 10 percent of cases of community-acquired pneumonia requiring hospitalization. As a way to elucidate the clinical findings in patients infected with C. pneumoniae, File, Jr., and colleagues performed a prospective study of patients admitted to the hospital with a diagnosis of pneumonia.

Patients included were adults who had various clinical symptoms of a respiratory illness plus a new infiltrate on chest radiograph consistent with pneumonia. At the time of admission, patients completed a questionnaire of more than 100 questions about their symptoms and medical history. Cultures of blood and sputum were obtained for all patients. Other cultures for viral and mycobacteria were selectively obtained by the attending physician. A urine specimen to detect Legionella antigen was obtained from all patients. Lastly, all cases had paired serum samples obtained four to six weeks apart for Legionella, Mycoplasma pneumoniae and C. pneumoniae. These serologic tests were performed at the Centers for Disease Control and Prevention. A diagnosis of C. pneumoniae infection was made if there was a fourfold or greater increase in antibody titer, a single IgG titer of greater than 1:512 or an IgM titer of greater than 1:16.

Approximately 1,200 cases of pneumonia were diagnosed during the study. Of these, definite infection with C. pneumoniae was found in 26 (2.4 percent) of the patients and possible C. pneumoniae infection in 6.5 percent. The authors analyzed data only in definite cases. Seven patients who had an early IgM antibody titer were considered to have a primary infection; 18 patients with an initial IgG titer followed by at least a fourfold increase were considered to be reinfected; one patient was excluded because test results were inconclusive.

The average age of patients with definite C. pneumoniae infection was 55 years (range: 21 to 82 years), with an equal distribution of men and women. Forty percent of patients were smokers, and 50 percent had a prior history of community-acquired pneumonia.

The most common radiographic finding was a localized segmental infiltrate. The mean white blood cell count was 12,700 per mm³ (12.7 × 10⁹ per L). The median duration of illness before hospital admission was seven days.

When evaluating further those patients with primary infection versus reinfection, it was noted that the newly infected patients tended to be younger (mean age: 38 versus 63 years), had a higher mean temperature and were less likely to have wheezing. The clinical characteristics of the 26 patients with C. pneumoniae infection are noted in the accompanying table.

The authors note that no specific signs and symptoms are unique to infection with C. pneumoniae. The perception that this organism causes a mild “atypical” pneumonia is not always correct. Some consistent clinical findings are nonproductive cough, low-grade fever, sore throat and hoarseness. Patients with primary infection tend to be clinically “sicker” than those who are reinfected. The most common reason for hospital admission in these patients was the need for supplemental oxygen and the presence of comorbid illnesses, including heart disease, chronic obstructive pulmonary disease and diabetes mellitus.