Proventricular Dilatation Disease (PDD)

Proventricular Dilatation Disease, or PDD for short, is a disease of the nervous system that has been reported in more than 50 species of psittacine birds. Originally described in macaws in the late 1970′s, the list of psittacine species being affected has grown to include some of our most popular pet birds. African Grey Parrots, Amazon parrots, Cockatiels, Budgerigars, Cockatoos, Conures, Eclectus parrots, Lovebirds, Meyer’s parrots, Pionus Parrots, Quaker parrots and Senegal Parrots have all been diagnosed with PDD.

What causes PDD and what signs will an infected bird show?

Despite excellent hygiene, valid quarantine procedures, and in the absence of new additions to the flock, PDD can still occur in an aviary. In 2009 researchers analyzed brain, adrenal gland and gastrointestinal tract tissue from birds diagnosed with Proventricular Dilatation Disease, and using high throughput pyrosequencing and real-time PCR (two sophisticated laboratory tests that help identify species DNA), were able to identify a virus related to the Borna virus. This avian bornavirus was found to affect the bird’s nervous system– commonly the nerves that supply the bird’s proventriculus (the bird’s true stomach) and intestinal tract. As a result of this viral infection the proventriculus does not have the ability to contract normally and food stops moving through the intestinal tract in its usual rhythmic pattern. Over time this malfunctioning proventriculus dilates up to ten times its normal size as it fills with seeds or food. At the same time the bird does not get adequate nutrition from its food and loses weight in spite of eating. These birds will eventually show a “razor thin” keel bone, indicating atrophy of the breast muscle secondary to weight loss. Eventually affected birds become sick with some showing signs of lethargy and depression. In some infected birds, owners may notice regurgitation or the passage of undigested seeds (for those who feed seed) in the droppings. The virus may also affect other areas of the nervous system such as the brain and spinal cord. When this occurs birds become weak and uncoordinated with an inability to walk or fly normally. They will be unable to grab with their feet and fall over easily. Once birds start showing the described clinical signs the prognosis is guarded. It should be noted that one unusual feature of this avian bornavirus is that some infected birds may become ‘healthy’ carriers after exposure and carry this disease for years before they ever show clinical signs and become ill.

What is the incubation period and how is the disease transmitted?

The incubation period for PDD varies greatly. In some birds it can be as short as one week to three months from time of exposure to time when clinical signs start. In other birds it may be up to four years or longer. It is believed the PDD bornavirus is transmitted from the feces of infected birds and picked up orally by susceptible birds. This can include fecal dust (dried fecal material) carrying the virus from one cage to another within an aviary.

My bird has been exposed to PDD—will he/she get sick?

After exposure to the PDD avian bornavirus a bird can follow several scenarios:

(1) The exposed bird will become ill with the disease in one to sixteen weeks.

(2) It will become a carrier bird- a healthy bird not showing clinical signs, but one who carries the virus. These birds may start showing clinical signs up to five years after initial infection.

(3) The bird’s immune system will effectively rid the body of the virus and the bird will remain healthy.

How does one definitively diagnose PDD?

Currently the only way that practicing veterinarians can definitively diagnose PDD in a live suspect bird is with biopsies, usually of the intestinal tract. These biopsies show diagnostic changes at the cell level that an avian pathologist can associate with PDD. Case history, clinical signs, blood profiles and X-ray studies with Barium contrast may also be helpful. Researchers are presently working on a specific blood test to detect PDD in infected birds without the need for surgical biopsies. The fact that six different genotypes (the genetic constitution of an organism) of the avian bornavirus have been identified complicates this search.

How can PDD be treated?

To slow progression of the disease and ensure adequate nutritional support, owners can feed high quality, easily digested foods such as pellets or slurries made from moistened pellets. As well, Non-Steroidal Anti-Inflammatory Drugs (NSAIDS) are used to diminish the tissue inflammation associated with the virus and can help restore intestinal function so that infected birds can digest foods properly again. Some birds have shown apparent cures after nine months of NSAID treatment, while others will succumb to the disease no matter what is therapeutically tried. Good hygiene and a stress free environment are recommended in order to help prevent secondary bacterial infections.

PDD has been diagnosed in my aviary-now what do I do?

The first thing is-don’t panic and don’t throw in the towel. You are not alone! PDD is a major cause of disease in aviaries across the United States and research is ongoing to develop diagnostic tests and possible vaccines to help diagnose and prevent this disease respectively. The goal is to reduce the incidence of PDD much as we have with Polyoma virus and Psittacine Beak and Feather Disease. These advances are forthcoming. In the meantime research has shown that improving aviary ventilation and increasing space between cages will help decrease incidence in an aviary. If one bird from a breeding pair dies of PDD it is recommended that the cage mate be isolated and monitored for signs of disease. As new diagnostic blood tests become available this bird can be tested to determine carrier status. Most veterinarians do not suggest euthanizing this exposed bird and some advocate making this bird a pet bird as it may never become ill.

Peter G. Fisher, DVM