Proventricular Dilatation Disease: The Avian Bornavirus Link

Ian Tizzard, Ph.D, Director of the Schubot Exotic Bird Health Center at Texas A&M University, discusses the latest developments regarding Proventricular Dilatation Disease and its theoretical link to Avian Bornavirus.

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Ian Tizard, DVM, Ph.D, Director of the Schubot Exotic Bird Health Center at Texas A&M University has long studied Proventricular Dilatation Disease (PDD) and Avian Bornavirus (ABV). BIRD TALK sat down with Tizzard to learn about the latest developments regarding PDD and its theoretical link to ABV.

What is PDD?
PDD is a virus disease of parrots and some other birds that is characterized by paralysis of the upper intestine and, as a result of that paralysis, food accumulates in the proventriculus. It swells up, and that gives the disease its name. Because the food doesn’t go through the intestine, the bird dies from starvation, so it’s a lethal disease.

What is the history of PDD?
The disease first appeared in the early 1970s and, though it’s not well-documented, it was probably first observed in North American birds in 1973 or 1974. This was a time when a large number of exotic birds were being imported from all over the world. There is a belief that it was a new disease at the time and was imported into North America, but where it might have come from is quite unclear. Imported birds were often mixed together, and so it was impossible to determine the origin of the disease.

What is ABV’s link to PDD?
There is a growing amount of data that supports the idea that Avian Bornavirus (ABV) is the cause of PDD. A number of laboratories both in North America and Europe have been able to reproduce the disease by inoculating this virus. We have found the virus in essentially all cases of PDD, and from what we know of the biology of the virus, it fits the disease. It is my opinion that the data available supports the idea that ABV is the cause of PDD.

How does ABV cause PDD?
ABV does not always cause PDD in all birds. As we’ve learned more about the virus, it’s clear that some infected birds become “healthy carriers.” To all intents and purposes they look healthy, but they shed the virus in their droppings, throats [and] feathers. Some of these [healthy carriers] probably live a normal life span.

Other birds, when they get infected with ABV, die within weeks or months of PDD. So, an obvious question is: what’s the difference? Why do some birds die? Why do some birds survive as healthy carriers? The evidence that we generated recently suggests that this depends on how or when a bird becomes infected. If an adult bird becomes infected through contact with other infected birds, then it will likely develop PDD and die.

We have, however, also found ABV in the eggs of infected birds. This means that this virus may be “vertically transmitted.” This means it can be transferred from the mother to her offspring through the egg. Now one of the consequences of that mode of transmission is that the baby bird does not recognize the virus. The hatchling won’t make an immune response against the virus. For normal viruses, that would be bad news — we want an immune response to protect us from viruses. But, bornavirus is somewhat different than other viruses in that it does not kill cells directly. It infects cells, but doesn’t kill them. (That, by the way, is probably why it took so long to find this virus.)

So, if a bird becomes infected with ABV, and its immune system doesn’t attack those virus-infected cells, then the infection persists. We believe that this is the recipe for chronic long-term healthy shedders. If, however, the bird’s immune system recognizes the virus and the birds’ immune system starts to attack and destroy the virus-infected cells, then this, we believe, will cause the bird to develop PDD.

Is that how a possible vaccination can come about?
If our theory is correct, then vaccines may not be successful, but that doesn’t mean we shouldn’t try. We need to attempt to vaccinate against this disease, as it’s a horrible disease and a bad way for birds to die. So it becomes critical that we try to vaccinate. We also need to look at treatments. That’s what our priority must be. We have to solve this problem, and that will involve either treating or preventing this disease.

What are current treatments for PDD?
The current treatment, a favorite among avian vets, is to try to reduce the inflammation associated with infections with anti-inflammatory drugs. There’s a lot of anecdotal evidence that these [drugs] do help.

There’s also increasing evidence that drugs that suppress the immune disease will also enhance survival and improve the quality of life of the bird. This fits with the idea that it’s not the virus that causes the direct death, it’s the immune response to the virus,and, if you can suppress the immune response, you can cause clinical remission.
We’re also studying an anti-viral drug called Ribavirin, which we know will stop the virus growing in culture, but we have yet to show it’s effective in birds. That may be because the drug doesn’t reach the virus in birds — the problem may not be with the drug, it may be [with] how the birds handle the drug. We have a study underway to see how Ribavirin is handled by a bird.

What’s next?
We have approval to conduct studies looking at vaccines and looking at treatments, specifically anti-inflammatories and immunosuppressants. The problem with anti-inflammatory drugs, while widely employed by avian veterinarians, is that no one has done formal studies to show that they truly work. You can talk to many people whose birds are on anti-inflammatory drugs, and they seem to be greatly improved. While that’s wonderful, I would like to formally prove it.

Maybe anti-inflammatories work quite well, or immunosuppressants work quite well, but what would happen if we combine the two? Would we get even better results? We’re in the early stages of treatment, and I think there is room for experimentation and studies to try to find out how to manage this disease. I suspect we’re going to manage it out of our flocks rather than vaccinate it, but we have to try [to find a vaccine]. 

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