Abstract
Introduction
Proventricular Dilatation Disease (PDD) is a horrible disease of psittacines, especially the large species such as macaws, African Greys and cockatoos. First recognized in the early 1980s, the disease was originally called Macaw Wasting Disease. In its classical form, the disease results from paralysis of the gastrointestinal tract, especially the ventriculus. Because of this paralysis, the flow of food through the ventriculus is blocked and so begins to accumulate in the proventriculus. The proventriculus enlarges as it fills with food, while the bird slowly starves to death. Subsequent studies traced the cause of the paralysis to a brain infection (encephalitis). It was soon recognized that proventricular dilatation, while common, was not the only symptom of this disease. Some birds, especially African Greys showed other signs of brain damage such as behavioral changes, loss of balance, blindness and paralysis.
From the earliest days, the disease was assumed to be infectious and likely of viral origin. The disease clearly occurred in outbreaks and numerous reports testified to its transmissibility. Unfortunately, it proved very difficult or impossible to isolate the virus responsible. Large psittacines may carry several viruses not responsible for PDD. Thus Dr. Christian Grund in Germany suggested that a paramyxovirus related to Newcastle disease virus was the cause. Unfortunately, although present in some PDD birds, it was also absent in many cases and is almost certainly not the cause. Likewise a coronavirus was isolated by Dr Gough and his colleagues in the United Kingdom from a PDD case but was not a consistent finding. Investigators were frustrated with our inability to determine the cause of the disease.
Discovery of the virus
The impasse was broken in the late summer of 2008 when two groups of investigators used sophisticated modern screening techniques to identify a hitherto unknown virus while a third group succeeded in isolating this virus from the brains of birds with PDD. These investigators, Dr Kistler and her colleagues at UCSF in San Francisco and Dr Honkavuori and her colleagues at Columbia University in New York, had been developing methods for the very rapid diagnosis of new and unknown viral infections. Thus they isolated viral nucleic acids from infected tissues, sequenced them and then ran the results through large computers to determine if there was any match with known viruses. Using POD-derived material, both groups found unique viral genes that were related to a virus that causes encephalitis in mammals. It is called Barna Virus after the town in Germany where the dis ease was first identified. Additional analysis of this new virus from PDD cases showed that it was distinctly different from mammalian Barna Virus so it was called Avian Barna Virus (ABV).
Simply identifying the presence of a virus in the tissues of a bird with PDD does not, of course, prove that that virus actually causes PDD. Nevertheless, testing numbers of birds for the virus can provide suggestive evidence. Thus the UCSF group tested eight birds with confirmed PDD and detected the virus in five of them. They also tested 14 birds not known to have PDD and all were negative for ABV. The Columbia University group tested tissues of three PDD birds and all were positive for PDD while none of four unaffected birds were positive,
Meanwhile, a third research group at the Schubot Center at Texas A&M University approached the problem from a different direction. We reasoned that sick birds would respond by mounting an immune response and making antibodies against the virus. We took blood from affected birds and investigated whether it would react with anything in the tissues of other PDD affected birds by using a technique called a Western Blot assay. Indeed we found that POD-affected birds all made antibodies against one or more proteins in infected bird brain. These proteins were absent in normal birds brains and appeared to belong to a virus. Initially, we had no idea what virus we were detecting but we found it in 22 of 25 PDD cases and two of 25 apparently healthy birds. We seemed to have a diagnostic test!