Approach
Pneumonia due to Chlamydia psittaci cannot be clinically differentiated from other community-acquired, atypical pneumonias, particularly pneumonia caused by Chlamydophila pneumoniae or Mycoplasma pneumoniae. The clinical presentation varies from a mild influenza-like illness to fulminating pneumonia complicated by multiorgan involvement. Molecular testing and/or serology is required to confirm the diagnosis. A chest x-ray is also recommended. Some patients may present with acute or chronic follicular conjunctivitis only. Although gestational psittacosis is rare, delays in diagnosis can lead to significant maternal and fetal morbidity and mortality.[30]
Psittacosis is a notifiable condition in the US. Clinicians are encouraged to use a case report form to gather comprehensive case information.
History
A history of exposure to birds and mammals is paramount to the diagnosis. Contact with birds, for example among pet bird owners, people who work in zoos or pet shops, veterinarians, poultry and wildlife workers, and diagnostic laboratorians, appears to be the primary risk factor.[12] Illness in other family members may also prompt suspicion, as spread often occurs from pet birds to many members of a family. Approximately 25% of affected patients deny exposure to birds or mammals; therefore, infection should be considered in all patients with clinically compatible symptoms even if there is no history of exposure.[12]
Presentation is usually nonspecific. The most common presentation is a respiratory tract infection with constitutional symptoms. Patients may have a history of gradual onset of fever, malaise, headache, and sore throat, with later onset of a nonproductive cough.[1][31] Less commonly, the onset may be more abrupt.
C psittaci infection has also been associated with acute and chronic follicular conjunctivitis. Patients with ocular infection often complain of a foreign body sensation.[3][4][32]
Physical exam
Patients are usually febrile, and may have pharyngeal erythema and diffuse rales with or without tachypnea. There may also be hepatomegaly. Occasionally patients present with confusion, tachycardia, and splenomegaly.[1]
When patients have infection confined to the eye, the only finding may be unilateral or bilateral diffuse erythema of the sclera, with or without a discharge.[32]
Investigations
Diagnosis can be difficult. Molecular testing and/or serology are required to confirm the diagnosis. Several methods are used to detect C psittaci infection, but some tests are only available in specialized laboratories. Tests should always be interpreted in the context of the history, clinical presentation, and response to treatment. Physicians are encouraged to contact their local health department early to discuss laboratory testing.
Laboratory investigations
General: white blood cell count with differential may be normal or elevated with a shift to the left; eosinophilia is occasionally present. Liver function tests may be normal or slightly elevated.
Polymerase chain reaction (PCR): send a respiratory specimen for PCR, if available. PCR is more readily available than it was in the past, and real-time PCR assays are now available in some specialized laboratories. It is a highly sensitive and specific test for C psittaci.[12] It offers rapid detection and results can be obtained in time to guide treatment decisions.
Serology: microimmunofluorescence on paired serum samples (taken 2 to 4 weeks apart) is the preferred serologic test and can be performed as a supportive test when PCR is available, or as an initial test when PCR is not available. Tests are available in many laboratories. Consider collecting a third specimen 4 to 6 weeks after the acute specimen in patients started on antimicrobial therapy. Cross reaction between other Chlamydia species can occur; therefore, results should be interpreted with caution, especially if the titer is <1:128.[12]
Culture: cultures of sputum, pleural fluid, or clotted blood can be performed; however, culturing C psittaci is not recommended unless an experienced reference laboratory is available. Testing can be hazardous to laboratory personnel and it is not as sensitive as PCR. Also, detection of the organism in tissue culture is not standardized.[1]
Imaging
Chest x-ray: may reveal the presence of pneumonia, and shows single lobar consolidation in approximately 90% of cases, usually in the lower lung. Approximately 50% of patients may have a small pleural effusion.[1]
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