SUMMARY
Cryptosporidiosis is a sporadic protozoal disease of chicken and turkeys accompanied by respiratory and/or gastrointestinal disease, and it may cause considerable morbidity and mortality. In October 2022, 12 (nine live and three dead) 46-day-old Ross 708 broiler chickens were received for necropsy at the Poultry Diagnostic and Research Center of the University of Georgia. The flock of 99,200 birds was experiencing an abrupt increase in respiratory signs and mortality associated with bacterial septicemia. At clinical examination, five out of nine live birds showed respiratory noise and marked bilateral swelling of eyelids. At macroscopic examination, birds had abundant muco-catarrhal exudate in the trachea (9/12) and moderate white-yellow fibrinous exudate deposition within the air sacs (7/12), pericardium (6/12), and liver capsule (4/12). Cloacal bursas were atrophic (5/12), and femoral head necrosis was observed in more than half of the birds (7/12). Escherichia coli was isolated in pure culture from (1/2) pericardial sacs. Histology revealed moderate to severe heterophilic conjunctivitis (2/6 tissue sections), tracheitis (2/17 tissue sections), and bursitis (3/5 tissue sections) associated with marked protozoal colonization of the epithelial apical surface, marked epithelial hyperplasia, and severe lymphoid depletion in a subset of sections. Protozoa were morphologically compatible with Cryptosporidium spp. The protozoa colonizing the bursa were identified as Cryptosporidium baileyi by PCR and sequencing. Concurrent moderate to severe lymphoplasmacytic conjunctivitis (6/6 tissue sections) and tracheitis (17/17 tissue sections) were compatible with infectious bronchitis virus infection, which was confirmed by real-time reverse-transcriptase polymerase chain reaction (RT-PCR) and characterized as a Massachusetts-type strain. Sections of tracheas tested negative for infectious laryngotracheitis by RT-PCR. The presence of C. baileyi in association with respiratory and bursal infections in broiler chickens highlights the importance of histopathology in the comprehensive diagnosis of respiratory diseases in poultry.