Review of techniques for the diagnosis of Chlamydia psittaci infection in psittacine birds.

Abstract

Avian chlamydiosis in psittacine birds is characterized by clinical signs such as anorexia, depression, respiratory distress, or diarrhea. The disease is caused by widespread infection of mononuclear phagocytes by the obligate intracellular bacterium, Chlamydia psittaci. Chlamydial organism can be transmitted to humans via dried feces and nasal discharge, causing the clinical disease of psittacosis. Clinical signs of psittacosis include influenza-like symptoms, and infection can lead to severe pneumonia or other nonrespiratory ailments. 2 Because of the zoonotic potential of C. psittaci ,i t is important to make a rapid, definitive diagnosis in affected birds. Diagnosis of chlamydiosis can be problematic. Isolation and identification is the gold standard for most infectious diseases, but these techniques may be difficult to use for chlamydiosis. 3,7 First, many birds can be carriers and may harbor subclinical infections. Second, culturing requires the use of specialized cell lines or embryonated eggs and inoculation techniques that are not available in all laboratories. 3,7 Third, because chlamydial organisms are shed intermittently in the feces, a single culture can lead to a false-negative result. 7 There are a variety of serologic tests available for diagnosis of chlamydiosis. Unfortunately, serologic responses in infection and disease vary widely in psittacine birds, and early stages of infection can have negative results. 7 Attempts at direct identification using fluorescent antibody testing on fresh tissue or characteristic inclusions seen histopathologically with hematoxylin and eosin (HE), Giemsa, or Gimenez staining have become more widely used. 3,6 Fluorescent antibody testing, although reliable, requires the submission of fresh tissue, and often specimens are submitted to the diagnostic laboratory already in formalin. Diagnosis in formalin-fixed, paraffin-embedded (FFPE) tissues relies on the visual methods of finding organisms morphologically compatible with C. psittaci with HE or histochemical stains. Often this option is the only one available in a diagnostic laboratory setting, and it is fraught with potentials for falsenegative or false-positive interpretations based on insufficient or spurious histochemical reactions. Immunohistochemistry (IHC) can help provide positive identification of a specific antigen within FFPE tissue. The aim of this study was to examine the usefulness of IHC and to compare this method with other methods based on pathology for detection of C. psittaci in FFPE tissues. Thirteen cases of chlamydiosis in psittacine birds were selected from the biopsy and necropsy archives of the Department of Pathology and the Athens Diagnostic Laboratory