Chlamydia are obligate intracellular, gram-negative pathogens. They cannot replicate on their own, but need the enzyme activity of a suitable host cell. Chlamydia relevant in veterinary medicine belong to the family Chlamydiaceae. Some years ago, this family was split into the two genera Chlamydia and Chlamydophila. However, based on recent genetic analyses, this classification is no longer considered justified. Because of this, the single term Chlamydia is used here.

In birds, infections with Chlamydia are of particular significance. Infection rates of 10 to 40% may be prevalent in aviculture. As many birds have a carrier status, the disease can “suddenly” become clinically apparent under stress. Symptoms in birds are manifold and extremely non-specific. Ruffled feathers, apathy and lack of appetite must be mentioned. Basically, every “sick bird” could have a Chlamydia infection. Respiratory symptoms with or without conjunctivitis are often seen, but central nervous disorders are also possible. The extent of clinical signs largely depends on the animals’ condition; the type of symptoms also varies from one bird species to another. Sudden deaths without prior illness might happen. It is therefore not possible to make a diagnosis based on clinical signs. To make a reliable diagnosis, the identification of the pathogen is always necessary. C. psittaci is a zoonotic agent. Infections in humans are normally airborne, resulting in a flu-like disease.