Chlamydia pneumoniae strain TWAR, the new third species of Chlamydia, is a common cause of pneumonia and other acute respiratory tract infections. About 10% of hospitalized and outpatient pneumonia cases have been associated with TWAR infection. TWAR is among the four or five most commonly identified causes of all pneumonia. Most TWAR infections are mild or asymptomatic, but occasionally severe pneumonia with death has been observed. Laboratory diagnosis is not generally available. Vigorous treatment with tetracycline or erythromycin is recommended. Both epidemic and endemic infections have been described in North America and the Nordic Countries. Population prevalence antibody studies suggest that TWAR infection is wide-spread throughout the world, that nearly everyone is infected and reinfected during their life-time, and that infection is common in all ages except those less than 5 years in temperate zone countries. The infection is transmitted from person to person, apparently with a long incubation period.

During a 2 1/2-year period, we studied 386 University of Washington students with acute respiratory disease, to determine whether a Chlamydia psittaci strain, here designated TWAR, is an important respiratory pathogen. Serologic evidence of recent TWAR infection was found in 13 students, and the organism was isolated from 8 of these. TWAR infection occurred in 12 percent of the students who had pneumonia (9 of 76), 5 percent of those with bronchitis (3 of 63), and 1 percent of those with pharyngitis (1 of 150). The TWAR infections occurred throughout the study period. Pharyngitis, often accompanied by laryngitis, was a common first symptom. Clinically, the infections resembled those with Myco-plasma pneumoniae; therefore, the patients were given courses of erythromycin used for the treatment of M. pneumoniae infections. This therapy proved to be inadequate. The limited data available suggest that the TWAR strain is a "human" C. psittaci that is spread from human to human, without a bird or animal host.

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Chlamydia pneumoniae TWAR has been associated with coronary heart disease by seroepidemiologic studies and direct detection of the organism in atheromatous lesions of coronary arteries and aorta. In this study, 38 fresh tissue specimens from patients with coronary artery lesions that were treated by directional coronary atherectomy were tested for C. pneumoniae. Twenty-three specimens were from patients with primary lesions and 15 were from patients with restenoses. C. pneumoniae was detected by polymerase chain reaction (PCR), immunocytochemical stain (ICC), or both in 20 of 38 specimens. Using cell identity markers, the organism was localized to macrophages. Ultrastructural evidence of the organism was found in the 2 specimens examined by transmission electron microscopy, which were also positive by both ICC and PCR. C. pneumoniae was found more frequently in tissues from restenoses than in primary lesions (P = .17). There was no relation between the frequency of detection of the organism and C. pneumoniae-specific antibody titers.

A third species, Chlamydia pneumoniae, is proposed for the genus chlamydia. This bacterium is a human respiratory pathogen, which has been referred to as the TWAR strain of Chlamydia. Species identification is based on ultrastructural differences in the elementary bodies, deoxyribonucleic acid analysis, and serology.