Based on genetic analysis, the genus Chlamydia has been reclassified into 2 genera. Organisms previously called Chlamydia psittaci have been renamed as a separate species in the genus Chlamydophila. More than 60 species of Chlamydophila have been identified, including C. psittaci, C. felis, C. abortus and C. pneumonia. The latter species is primarily a human pathogen. Chlamydophila psittaci, one of the possible etiologic agents of chlamydiosis in birds, is a highly infectious, obligate intracellular bacterium that can induce disease in most species of free-ranging and domestic birds as well as in humans. Birds may prove susceptible to a variety of chlamydial species, and it is important to identify which organism is causing disease in order to understand the sources of infection and management required to prevent further cases. Strains of Chlamydophila can also infect numerous mammals and people.

Chlamydophila spp. are excreted in the feces and nasal discharges of infected birds. The organisms are resistant to drying and can remain infectious for several months. Some infected birds can appear healthy and shed organisms intermittently. Shedding can be activated by stress, including shipping, crowding, chilling, and breeding. The usual time between exposure to Chlamydophila spp. and onset of illness ranges from 3 days to several weeks. Whether the bird exhibits acute or chronic signs of illness or dies depends on the species of the bird, virulence of the strain, infectious dose, stress factors, age, and extent of treatment or prophylaxis.

Non-specific clinical signs in birds with chlamydiosis include lethargy, anorexia, and ruffled feathers. Other clinical signs include serous or mucopurulent ocular or nasal discharge, diarrhea, and excretion of green to yellow-green urates. Anorectic birds can produce sparse, dark green droppings, followed by emaciation, dehydration and death.

Diagnosis of avian chlamydiosis can be difficult, especially in the absence of clinical signs. As a single testing method might not be adequate, use of a combination of tests is recommended, particularly when only a single bird is tested. Live birds being screened for Chlamydophila spp. may only shed the organism intermittently. Therefore, serial specimens should be collected for 3–5 consecutive days and tested separately or pooled before being submitted.

a. Culture
Chlamydophila spp. are obligate intracellular bacteria that must be isolated in tissue culture, mice or chick embryos. Culture is a sensitive tool, but can take 3–4 weeks which limits its clinical utility. Tissue samples from the bird’s liver and spleen are the preferred necropsy specimens. In live birds, combined choanal and cloacal swab or liver biopsy samples can be used.

b. Antibody Tests
Antibody tests, although useful adjuncts to diagnosing avian chlamydiosis, have several important limitations. A positive serologic test is evidence that a bird was infected by Chlamydophila spp. at some point, but does not necessarily indicate an active infection. Serological tests also do not differentiate amongst the different chlamydial species to which birds may be susceptible. False-negative results can occur in birds with acute infections that are sampled before serocon-version. Treatment with an antimicrobial agent may also diminish the antibody response. A greater than four-fold increase in acute and convalescent titers, or a combination of positive titer, antigen or PCR test is needed to confirm the diagnosis of chlamydiosis.

Serological test results are best interpreted in con-junction with clinical signs of disease, the history of the flock or aviary, and other laboratory data such as white blood cell counts, liver enzyme activities, and other chlamydia tests.

i. Elementary-Body Agglutination (EBA). The EBA test detects primarily IgM antibody and thus can detect early infection. Titers > 1:10 in budgerigars, cockatiels and lovebirds; and titers > 1:20 in larger birds are frequently seen in cases of recent infection. However, elevated titers can persist after treatment is completed. A negative titer does not guarantee that a bird is free of infection.

ii. Immunofluorescence antibody (IFA). The IFA test detects primarily IgG antibody. A titer of < 1:5 is considered negative, whereas a titer of 1:5 is borderline. As with the EBA test, elevated titers can persist after treatment is completed, and a negative titer does not guarantee that a bird is free of infection.

c. Antigen Tests (ELISA)
ELISA tests were originally developed for identification of Chlamydia trachomatis in people, and so their precise sensitivity and specificity for detecting Chlamydophila spp. in birds is uncertain. These tests give rapid results, although false-positive results can occur from cross-reacting antigens. False-negative results can occur if insufficient antigen is present or if shedding is intermittent.

d. Polymerase Chain Reaction (PCR)
PCR-based tests are sensitive and specific for detection of Chlamydophila DNA sequences in collected specimens (e.g., choanal and cloacal swabs, blood). The PCR test that Antech offers can be used to differentiate amongst different species of Chlamydophila and so is very important in confirming the precise species of organism involved in chlamydiosis cases.

e. Necropsy Findings
Findings often present in birds with chlamydiosis include slightly thickened and/or cloudy air sacs, hepatomegaly, and splenomegaly. Histologically, the most frequent findings include Kuppfer cell proliferation in the liver, portal lymphocytic and plasmacytic hepatitis, and single cell necrosis of hepatocytes. Occasional foci of hepatocyte necrosis may be observed. There may be air saccultitis and splenic plasmacytosis. Impression smears of the liver or spleen, or formalin-fixed tissue may be stained with a routine special stain (Gimenez) or with immunohistochemical stain for Chlamydophila antigen. DNA testing (in-situ hybridization) can also be performed on tissue sections.