Abstract. A case of proventricular dilatation disease is described in a scarlet macaw (Ara macao) from clinical presentation to diagnosis with molecular methods. The initial clinical signs were depression progressing to head pressing over several days. A leukocytosis with toxic heterophil changes, hypoalbuminemia, and increased serum activity of aspartate aminotransferase and creatine kinase were present. Lead and zinc assays were within reference ranges, and results from Chlamydophila and polyomavirus testing were negative. Contrast-enhanced fluoroscopy revealed normal gastrointestinal transit times and motility as well as the presence of 2 small metallic foreign bodies in the ventriculus. The macaw was treated with antimicrobials, analgesics, vitamins E and B complex, force-feeding, and fluid administration with little improvement. Euthanasia was elected, and histologic examination of brain tissue revealed a perivascular lymphoplasmacytic infiltration, while the lungs had evidence of a fungal pneumonia. Tissue samples from the brain and proventriculus tested positive for the presence of Avian bornavirus genotype 2, while serology confirmed Avian bornavirus infection.Key words: Ara macao; Avian bornavirus; macaws; proventricular dilatation disease.Neuropathic gastric dilation or proventricular dilatation disease (PDD) has been variably described in the veterinary literature, and was initially referred to as macaw wasting syndrome in the early 1970s. 9 The terms ''neuropathic gastric dilatation'' and ''encephalomyelitis'' are in reference to the most commonly observed clinical and pathologic features of PDD. Historically, affected birds present with varying degrees of gastrointestinal signs, including anorexia, regurgitation, or presence of undigested seeds in feces, with or without neurologic signs (e.g., ataxia or obtundation). Frequently, the clinical signs are nonspecific and reflect disturbance in a number of different organs. 17,20,22 Although PDD appears to have a predilection for psittacines, it has been presumptively identified in toucans, Canada geese, and spoonbills.7 A viral etiology for the disease has been suspected for many years because of the epidemiologic evidence and the characteristic histopathologic changes observed in tissues of affected birds, but no specific virus was identified until the recent discovery of avian bornaviruses. 8,11,12 The present report describes a case of Avian bornavirus (ABV) in a scarlet macaw (Ara macao) exhibiting vague neurologic signs from initial presentation to identification and classification of the virus genotype.A 1.5-year-old male scarlet macaw weighing 0.95 kg was referred to the University of Wisconsin Veterinary Medical Teaching Hospital (UW-VMTH; Madison, Wisconsin) with a 2-week history of depression and loss of appetite. The bird was housed part of the time in a pet store, which sold and boarded birds of many different species. The bird had been acquired by the pet store owner 1 year earlier from a breeder and had been apparently healthy during this ...