Abstract
Background:
Chlamydia psittaci, the zoonotic pathogen that causes avian chlamydiosis, is well known to infect domestic pigeons (Columba livia domestica), serving as a primary natural reservoir.

Aim:
The purpose of this study was to investigate the gross and histopathological effects of the C. psittaci isolate on hepatic and splenic organ tissues in experimentally infected pigeons.

Methods:
A detailed survey of histopathological abnormalities induced by the genotype B isolate of C. psittaci outer membrane protein A (ompA) in eight experimentally infected (oronasal) healthy pigeons was performed. The strain was isolated from infected domestic pigeons in Baghdad, Iraq. The isolate was registered and obtained an accession number (PV243292.1) in GenBank-NCBI.

Results:
The results showed that the isolate induced hepatitis, splenitis, airsacculitis, conjunctivitis, bronchitis, sinusitis, and enteritis (within 28 days post-infection) in euthanized pigeons. Severe gross and histopathological lesions were noted in the major internal organs of the experimental pigeons, comprising tiny necrosis areas and an infiltration of leukocytes in the liver and spleen, as well as multifocal hepatitis. The birds with hepatomegaly and splenomegaly had increased numbers of mononuclear phagocytic system cells and loss of normal architecture. Intracytoplasmic inclusions were detected in all tissues of those organs.

Conclusion:
The study concluded that C. psittaci is a major pathogen that causes noticeable gross clinical and histological abnormalities in domestic pigeons. This zoonotic pathogen requires increased attention in the context of domestic pigeons in Iraq. It was considered the first study on the histopathological effects of the C. psittaci isolate on experimentally infected pigeons in Baghdad.

Key words: Chlamydiosis; Experimental pigeons; Hepatomegaly; Intracytoplasmic inclusions; Splenomegaly.