CHLAMYDIAL INFECTIONS

The genus Chlamydia contains three species that infect humans: Chlamydia psittaci, Chlamydia trachomatis, and Chlamydia pneumoniae (formerly the TWAR agent). C. psittaci is widely distributed in nature, producing genital, conjunctival, intestinal, or respiratory infections in many mammalian and avian species. Genital infections withC. psittaci have been well characterized in several species and cause complications such as abortion and infertility. Although mammalian strains of C. psittaci are not known to infect humans, avian strains occasionally do so, causing pneumonia and the systemic illness known as psittacosis.
C. pneumoniae is a fastidious chlamydial species that appears to be a frequent cause of upper respiratory tract infection and pneumonia, primarily in children and young adults, and is a cause of recurrent respiratory infections in older adults. No animal reservoir has been identified for C. pneumoniae; it appears to be a human pathogen spread via the respiratory route through close personal contact. To date, all strains of C. pneumoniae studied have been serologically homologous.
C. trachomatis is exclusively a human pathogen and was identified as the cause of trachoma in the 1940s. Since then, C. trachomatis has been recognized as a major cause of sexually transmitted and perinatal infection.
Chlamydiae are obligate intracellular parasites. They possess both DNA and RNA, have a cell wall and ribosomes similar to those of gram-negative bacteria, and are inhibited by antibiotics such as tetracycline. Chlamydiae are classified as bacteria belonging to their own order (Chlamydiales).
Recent studies using monoclonal antibodies to and nucleotide sequencing of the major outer-membrane protein have delineated at least 20 serotypes of C. trachomatis. According to the serovar classification system of Wang and Grayston, strains associated with trachoma have generally been those of the A, B, Ba, and C serovars, while serovars D through K have largely been associated with sexually transmitted and perinatally acquired infections. Serovars L1, L2, and L3 produce lymphogranuloma venereum (LGV) and hemorrhagic proctocolitis. The LGV strains demonstrate unique biologic behavior in that they are more invasive than the other serovars, produce disease in lymphatic tissue, grow readily in cell culture systems and macrophages, and are fatal when inoculated intracerebrally into mice and monkeys. Non-LGV strains of C. trachomatis characteristically produce superficial infections involving the columnar epithelium of the eye, genitalia, and respiratory tract.
C. trachomatis has been reported as an infrequent cause of endocarditis, peritonitis, pleuritis, and possibly periappendicitis and may occasionally cause respiratory infections in older children and adults. Immunosuppressed patients with pneumonia have had, in some cases, either serologic or cultural evidence of C. Trachomatis infection, but more data are necessary to define the role of Chlamydia in these patients.