Susan E. Orosz, PhD, DVM, DABVP (Avian), DECZM (Avian)

There has been discussion on some avian boards about PDD and avian bornavirus and, in particular, there may be some confusion about how contagious this disease is to our beloved birds. We are all learning together as new advances come to light. Almost 30 years ago, the condition of wasting in macaws from dilation of the proventriculus of the stomach was first described. At that time, the disease had a low incidence but if one bird showed symptoms of disease, it then appeared to spread through its flock.

Until recently, clinicians, researchers and pathologists could only make the diagnosis based on dilation of the proventriculus and a biopsy of the crop that showed a lymphocytic, plasmacytic ganglioneuritis. Clinical signs observed are variable and depend upon the competency of the immune system of the avian patient and the severity and distribution of lesions. The most common clinical signs observed are depression, anorexia, loss of body condition, regurgitation, and the passage of undigested food in the feces and these classic symptoms are associated more with New World species of psittacines.

But with the advent of new PCR technologies, our view of this disease and its suspected viral culprit is changing at a fast pace. At this point in time, it appears that avian bornavirus (ABV) is most likely the causative agent of PDD. With the development of PCR primers to test for this agent, we will understand where it locates in the body, how it may be transmitted, and the routes of infection. All this information will help us determine how contagious it is to our birds.

In a survey of 39 healthy parrots, M. Lierz from the Free University of Berlin detected ABV in 27 (45.8%) of the birds based upon blood sample testing. Data from Veterinary Molecular Diagnostic lab suggests that the rate is about 30% of birds (both healthy and ill) that they have tested. This compares favorably with the observed infection rates of Borna Disease Virus (BDV) in mammalian species. This information suggests that we need to form a much different view of PDD—the old view of a disease of low incidence but severe clinical manifestations is outdated. Current research suggests an infection of much greater frequency with a low incidence of clinical disease and a much lower incidence of severe disease.

Such a view of Avian Borna virus (ABV) compares favorably with that of BDV. The infection rate for Borna Disease virus (BDV) in mammals is reported to be >30%. In healthy horses in Germany, a mean infection rate of almost 60% was found compared to the significantly lower clinical disease case rate of approximately 5%. ABV appears to parallel BDV very closely in this regards. The vast majority of ABV-positive birds DO NOT show clinical disease. A much smaller percentage will show clinical disease, but this is quite variable. We must expand our view of clinical ABV disease to that of a continuum of a wide range of clinical symptoms. Many clinically infected birds show only mild clinical signs of disease. Those with the “classic” severe form of PDD represent the extreme of this continuum and are a small percentage of birds that are clinically diseased. But we all know that we want to avoid this in our beloved birds.

We are unsure of how the disease is spread between birds and I have discussed that birds may become infected while still in the egg. We have also had birds that are positive in a household while their friends in adjoining cages have maintained a negative ABV testing status. We must keep in mind that this is an RNA virus. ABV RNA is very unstable in the environment as it is degraded by naturally occurring RNAase enzymes very rapidly. With an agent that has been ever-present in our aviary population, unrestricted for at least 40+ years, one would expect a much higher rate of infection if it was highly contagious or highly pathogenic. Even in a stressed, confined aviary populations experiencing an acute disease outbreak, only about 50% of the birds tested positive in my experience.

Sometimes there is concern that PDD is spreading through a flock and it is unclear what is meant by this. It may be that one bird with clinical signs has been confirmed with PDD and the others have tested ABV positive. These birds would be positive by PCR testing but be normal in outward appearance. The other situation is that one bird has symptoms and then other birds develop the symptoms as well. In those cases, in what I have seen, the number of birds that show outward signs may be as high as 50%.

While this is a contagious disease, it does not appear to run through a flock of birds killing them all quickly like other viruses we have experienced (example-Pacheco’s Disease Virus). On the other hand if 30% of our birds in this country may be affected, what are we going to do with birds that are up for sale or adoption or fostering? At least some will be both avian bornavirus positive and apparently healthy. I would advocate that we do not euthanize them but think through a rational way to tell potential owners, keep the birds as stress-free as possible, and provide quality nutrition to keep them immune-competent. Not every avian bornavirus-positive bird will develop clinical disease or die from PDD, most do not. And by better understanding this disease process, we may be able to eliminate it from our birds.

As we all work together to understand this disease, we will develop better protocols to help our avian friends lead healthful lives. This is what I am seeing now clinically with the birds in our area and I encourage all of us in this dialogue to find a true path of understanding. Our birds deserve this from all of us.