Proventricular dilation disease in Psittaciformes: a retrospective study of cases received in four diagnostic laboratories in California, USA, 2014–2024

Introduction Proventricular dilation disease (PDD), caused by parrot bornaviruses (PaBVs), is a major cause of morbidity and mortality in captive psittacines worldwide. However, comprehensive retrospective studies evaluating its clinicopathologic spectrum remain scarce. This study aimed to characterize the epidemiologic, clinical, gross, microscopic, and concurrent disease findings of naturally occurring PDD in psittacine birds. Methods A retrospective study was conducted using necropsy records of Psittaciformes submitted to four veterinary diagnostic laboratories in California, USA, between 2014 and 2024. Cases were included based on compatible lesions and immunohistochemical detection of PaBV antigen. Clinical signs, gross and microscopic lesions, concurrent diseases, and differences between psittacine superfamilies were evaluated. Results Of 1,466 Psittaciformes necropsies, 51 cases met the inclusion criteria. Gastrointestinal signs alone predominated (59%), followed by combined gastrointestinal and neurologic signs (14%) and neurologic signs alone (12%). Death without previously reported clinical signs occurred in 12% of birds. Gross gastrointestinal dilation was observed in 70% of cases, whereas 30% lacked specific gross lesions. Histologically, lymphoplasmacytic infiltrates most frequently involved the gizzard (86%), proventriculus (73%), adrenal glands (72%), brain (71%), and heart (49%). Concurrent diseases were identified in 57% of cases. PDD was diagnosed significantly more frequently in Cacatuoidea than in Psittacoidea (9/98 [9.2%] versus 42/1,304 [3.2%]; p = 0.007), although clinical signs and lesions did not differ significantly between superfamilies. Discussion These findings highlight the broad clinicopathologic spectrum of naturally occurring PDD, including cases without characteristic gross lesions or previously reported clinical signs. The frequent involvement of gastrointestinal and nervous tissues underscores the importance of comprehensive tissue sampling and histologic examination for an accurate diagnosis.