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Pathology of Psittacosis (Ornithosis, Parrot Fever)

Dr Sampurna Roy MD




Psittacosis, also known as "parrot fever" and "ornithosis", is a bacterial infection of humans that can cause severe pneumonia and other serious health problems.

It is caused by Chlamydophila psittaci (obligate intracellular bacterium), formerly known as Chlamydia psittaci.

Once thought to infect only psittacine birds, Chlamydia psittaci is harboured by many other birds, including chickens, turkeys, pigeons and sea gulls and by many mammals. Many usually acquires the disease by contacting infected birds.

Chlamydia psittaci causes systemic disease in man, but pulmonary involvement is most prominent.

The organisms are inhaled with dust-borne contaminated excreta or aerosolized droplets.

The organisms are carried to the reticuloendothelial cells of the liver and spleen, proliferate, and disseminate to the lungs and other organs.

Clinical presentation:

The disease ranges in severity from inapparent subclinical to nonspecific respiratory and influenza-like symptoms to severe systemic illness and pneumonia.

After an incubation period of 7 to 21 days, an intense headache and fever herald the disease.

A faint macular rash (Horder's spots), resembling the rose spots of typhoid fever, may be present.

Pharyngitis, malaise, anorexia and painful myalgias and arthralgias are common, as is hepatosplenomegaly.

A persistent dry, hacking cough, fine crepitant rales, and tachypnea are typical.

Morphological features:    

Lungs:  Psittacosis begins as an inflammatory process in the lung and progresses to consolidation, primarily lobular but occasionally lobar.

It progresses through a sequence of congestion, edema  and red and gray hepatization.

Histopathologically, fibrin, erythrocytes and neutrophils appear early in the alveolar exudate.

Later the alveoli contain large mononuclear cells and epithelial cells. 

Interstitial infiltration is not present in early stages, but as the disease progresses lymphocytes and monocytes invade the alveolar walls.

Hyperplasia of alveolar type 2 pneumocytes is typical.

In severe disease abscesses form in alveolar septa and hemorrhage and fibrin may fill the alveoli.

The hilar lymph nodes show lymphadenitis and reticulo-endothelial hyperplasia.

Elementary bodies in the alveolar lining cells appear as clusters of minute, intracytoplasmic, basophilic, coccobacillary inclusions upto 0.8 micrometer in greatest dimension.

The organisms are dark blue with the Van Gieson and Giemsa stains and red against a blue backround with the Gimenez or Machiavello stains.

They are also demonstrated by direct immunofluorescence.

Rarely, severe extrapulmonary manifestations may occur. Dissemination is characterized by foci of necrosis in the liver and spleen and diffuse mononuclear cell infiltrates in the heart, kidneys and brain.

Liver:  In the liver, swelling, vacuolization, and phagocytic activity of Kupffer's cells are prominent, and intracytoplasmic elementary bodies can be seen in the Kupffer cells.

Focal necrosis in the spleen is accompanied by diffuse reticuloendothelial hyperplasia and desquamation of mononuclear cells into the splenic sinuses.

Pericardial and myocardial inflammation have been described.

Heart: Chlamydial infection should be considered in cases of idiopathic dilated cardiomyopathy, myocarditis , pericarditis, and endocarditis of unknown origin.

An early diagnosis has important therapeutic and prognostic implications. C. psittaci myocarditis is probably a rare disease, but the exact prevalence remains obscure.

Brain:  It is rarely involved and shows edema, congestion and focal hemorrhage.

Ocular Adnexal Non-Hodgkin Lymphoma : A recent report suggests that ocular adnexal non-Hodgkin lymphoma (NHL) may be related to Chlamydia psittaci infection.

The distinctive patterns of ocular NHL call for further  studies to identify risk factors and mechanisms, including the potential role of Chlamydia psittaci or other infections.

Studies from Italy showed Chlamydia psittaci infection in 87% of ocular adnexal MALT lymphomas and complete or partial regression of the lymphoma after Chlamydia psittaci eradication in a nuber of cases.

In pregnancy: Chlamydia psittaci is associated with significant morbidity and mortality during pregnancy, and its rarity can delay early diagnosis and treatment.

Vasculitis:  Some authors suggest the role of Chlamydia in the pathogenesis of atherosclerosis and some vasculitis.

There is a possible association between Chlamydia psittacci infection and temporal arteritis.

Diagnosis of the disease:

The clinical signs and symptoms are not diagnostic and routine laboratory findings are not specific ; a history of exposure to birds may be the best clue.

Radiographs of the chest usually reveal a patchy lower lobe infiltrate, but findings vary.

The diagnosis is made either by isolating Chlamydia  psittacci from sputum, blood or tissue specimens or by serologic tests.

The possibilities for diagnostic detection of chlamydiae have considerably improved following the introduction of molecular methods, particularly the polymerase chain reaction (PCR), which permits direct identification from clinical specimens and differentiation of species.

Information about psittacosis is essential for public health officials, physicians, veterinarians, the pet bird industry, and others concerned about controlling these diseases and protecting public health.

Visit: Chlamydial Infection ; Chlamydial Conjunctivitis (Inclusion Conjunctivitis). ; Chlamydial Infection of the Genital Tract  ; Trachoma ;  Lymphogranuloma Venereum  


Further reading

Pathogenicity of low and highly virulent Chlamydia psittaci isolates for specific-pathogen-free chickens.

A new role of the complement system: C3 provides protection in a mouse model of lung infection with intracellular Chlamydia psittaci.

Emerging Chlamydia psittaci infections in the chicken industry and pathology of Chlamydia psittaci genotype B and D strains in specific pathogen free chickens.

Chlamydophila psittaci subclinical infection in chronic polyarthritis.

Chlamydia psittaci Infection in nongastrointestinal extranodal MALT lymphomas and their precursor lesions.





Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)







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