Avian Borna Virus Really is the Cause of PDD


In the summer of 2008, two research groups one based at the University of California, San Francisco, and one at Columbia University in New York, and working independently, used highly sophisticated molecular techniques to show that parrots suffering from proventricular dilatation disease (PDD) were infected by a previously unknown virus they called Avian Barna Virus or ABV. Other groups such as those atTexas A&M's Schubot Center confirmed and extended these findings by isolating and growing this virus from the brains of birds that died from PDD. Scientifically however, none of these researchers had "proved" that ABV is the cause of PDD. All we had done is shown a close association between infection with ABV and the development of PDD. But some birds that are infected with ABV remain in apparent good health, while this virus cannot be detected in some birds with PDD. It was theoretically possible that another, unknown agent caused PDD. For this reason, it was necessary to formally prove that ABV causes PDD by inoculation of pure ABV into normal birds.


The first steps were taken by Gancz and his colleagues who showed that typical PDD developed in cockatiels that had been inoculated with brain tissue containing ABV. This was a great first step but not the final proof since other agents within the brain tissue might possibly have affected the results. The group at Texas A&M were however able to isolate the virus from eight different cases of PDD, and culture the virus for several generations in tissue culture. The pure virus culture was then administered to two Patagonian conures by mouth and injection. 66 days later one of these birds died showing the classical signs of POD.The other conure had lost so much weight by this time that it had to be euthanized for humane reasons and it too was found to be suffering from typical POD. Necropsy, histopathology and molecular biology all confirmed that these birds had clinical POD and ABV was present in large amounts in their brains. ABV is proven to be the cause of POD.


All these new discoveries have been very exciting but this virus continues to kill precious rare birds. What can we do about it? There are four areas of research that we are investigating. First, we need a good diagnostic test so we can detect infected birds. Second we need to know how the virus spreads between birds so we can stop it. Third we need to be able to treat and hopefully cure clinical POD. Finally we need to develop a vaccine to completely stop this virus in its tracks.


Two diagnostic tests are now available.

They measure two different things. Thus the PCR (Polymerase Chain Reaction) test can detect the virus (strictly speaking, its genetic material) in bird tissues, secretions and droppings. This is a very sensitive and specific test. We believe that it is best used to detect the virus in a bird's droppings. Unfortunately, our experience also confirms that an infected bird does not shed the virus in every dropping, every day. Thus if this test is positive, it confirms that the bird is infected with ABV. If the test is negative, it only means that there was no virus in that specific dropping. It requires multiple fecal PCR tests to "prove" that a bird does not have ABV. The number of tests required might depend on the species involved. For example, we have observed that as few as half of a group of infected cockatiels are shedding the virus at any one time. Three tests will thus identify only about 88% of infected birds. This problem notwithstanding, we suggest that the fecal PCR test is best used to identify and separate infected birds from non-infected birds. Repeated testing does not disturb the birds and it is possible to perform a single test on pooled fecal samples from multiple birds in an aviary.

The second test we have developed is a blood test known as an immunoblot. This test detects the response of a bird to ABV infection. In our experience this test becomes positive long after the bird begins to shed ABV but around the time the bird develops clinical illness. We think that this test may best be used as a diagnostic test to confirm PDD in sick birds. It appears to be positive in at least 90% of PDD cases. Unfortunately, immunoblots are slow and expensive so we are working on developing a simpler, cheaper and faster test.