Potential hosts include feral birds and domesticated poultry, as well as cattle, pigs, sheep, and horses.

Chlamydia psittaci in birds is often systemic, and infections can be inapparent, severe, acute, or chronic with intermittent shedding.

Septicaemia eventually develops and the bacteria become localized in epithelial cells and macrophages of most organs, conjunctiva, and gastrointestinal tracts.

C. psittaci strains are similar in virulence, grow readily in cell culture, have 16S rRNA genes that differ by <0.8%, and belong to eight known serotypes.

Chlamydia psittaci serovar A is endemic among psittacine birds and has caused sporadic zoonotic disease in humans, other mammals, and tortoises.

Serovar B is endemic among pigeons, has been isolated from turkeys, and has also been identified as the cause of abortion in herds of dairy cattle.

The disease caused by C. psittaci, psittacosis, was first characterized in 1879 when seven individuals in Switzerland were found to experience pneumonia after exposure to tropical pet birds.

The related bacterial species Chlamydia trachomatis was described in 1907, but was assumed to be a virus, as it could not be grown on artificial media.

Though C. psittaci was identified in 1930 as the agent responsible for psittacosis, it was not found to be a bacterium until examination by electron microscopy in the 1960s.

[1] However, this reclassification "was not wholly accepted or adopted"[6] among microbiologists, which "resulted in a reversion to the single, original genus Chlamydia, which now encompasses all 9 species including C.

[10] In addition to symptoms and CHX, complement fixation, microimmunofluorescence, and polymerase chain reaction tests can be used to confirm the diagnosis.