Once considered to be a virus, Chlamydia is in fact a bacteria, although distantly related to other familiar bacteria. The genus is composed of three species:
C. psittaci is a common pathogen on a variety of mammals and birds. It causes psittacosis, or parrot fever, an opportunistic respiratory infection that can be life-threatening when transmitted to humans. However, this disease has been largely controlled by regulation of the trade in exotic birds.
C. pneumoniae is a new and emerging disease. It is a frequent cause of respiratory infections and associated with pneumonia, especially among young adults in northern Europe. It is also associated with arteriosclerosis, although the relation is not yet known.
C. trachomatis can be subdivided into three biovariants that cause very different kinds of disease: (a) the mouse pneumositis biovariant is not a pathogen of humans; (b) the lymphogranuloma venereum (LGV) biovariant, which consists of three serotypes that proliferate in the lymph nodes; and (c) the trachoma biovariant, which consists of 15 different serotypes that cause epithelial infections of the eye or of the genital tract.
Incidence and Burden. As an ocular infection, C. trachomatis causes trachoma, which remains the leading cause of preventable blindness in the world, affecting some 200 million people. Trachoma is no longer encountered in the United States, but exactly the same strains cause genital infections that have made chlamydia the fastest growing sexually transmitted disease (STD) in this country, with over 5 million new cases per year. (By comparison, there are only about 235 cases of LGV, although it remains an important STD in developing countries.) Most of these 5 million cases present as either nongonococcal urethritis or cervicitis. Significantly, many of these cases are largely asymptomatic, which is a problem both for treatment and for identifying individuals at risk for more serious sequelae.
Among men, chlamydial urethritis is a common cause of epididymitis, but the real focus of attention is on women. In addition to the complications of urethritis and cervicitis, infants born to infected women are also at risk for conjunctivitis and chlamydial pneumonia. Because so many infections are asymptomatic, they often go undiagnosed and develop into more serious problems. One of the most serious is pelvic inflammatory disease, which can lead to chronic pelvic pain, involuntary sterility, or ectopic pregnancy.
Diagnosis, Treatment, and Prevention. Over the past 10 years, researchers have improved substantially the ability of clinicians to diagnose chlamydial