Description and importance of the disease: Avian chlamydiosis (AC) is caused by a Chlamydia species in birds. The taxonomy of the family Chlamydiaceae was recently revisited. The genus Chlamydia currently includes 11 recognised species, and among them C. psittaci, C. avium and C. gallinacea have been isolated from birds. Outbreaks of AC in psittacine birds and domestic poultry farms cause considerable economic damage. The infection can lead to systemic and occasionally fatal disease in birds. The clinical signs are generally nonspecific and vary greatly in severity, depending on the species and age of the bird and the virulence of the Chlamydia strain, but respiratory distress is mostly involved. Many birds, especially older psittacine birds and poultry, may show no clinical signs; nevertheless, they may often shed the agent for extended periods. Special laboratory handling as determined by biological risk analysis (see Chapter 1.1.4 Biosafety and biosecurity: Standard for managing biological risk in the veterinary laboratory and animal facilities) is recommended and even obligatory in many countries because avian chlamydial strains can cause serious illness (pneumonia) and death in humans when left untreated. Identification of the agent: The preferred method for the identification of AC is no longer isolation of the organism. Considering the time involved, the need for high-quality samples, the fact that some strains will never grow in vitro and the hazard to laboratory personnel, nucleic acid amplification tests (NAATs) are currently recommended for quick, sensitive and specific diagnosis. These methods include conventional and real-time polymerase chain reaction (PCR), DNA microarray-based detection and DNA sequencing. Isolation, cytological staining of smears of exudate or faeces, and of impression smears of tissues, immunohistochemical staining of cytological and histological preparations and antigen-capture enzyme-linked immunosorbent assays (ELISA) can be used if NAATs are not available. Serological tests: Serology alone is not particularly useful in diagnosing a current chlamydial infection in birds because of the high prevalence of this infection in birds and the long-term (up to several months) persistence of antichlamydial antibodies. In most bird species, there is a high background rate of antichlamydial antibodies. Thus, to determine if a single bird is infected, serology should always be used in conjunction with gene or antigen detection, or paired sera should be examined. A positive test is evidence that the bird was infected by the bacterium but does not necessarily indicate an active infection. False negative results can occur in birds with acute infections that are sampled before seroconversion. Treatment with antibiotics may also delay or diminish the antibody response. The main serological methods that are used for detecting chlamydial antibodies are: (1) various methods of elementary body agglutination (EBA), (2) the complement fixation test and (3) ELISA. ELISA is highly sensitive and specific when using recombinant proteins/peptides as antigen targets and it detects IgM, IgG and IgA.