What is Proventricular Dilatation Disease?

Proventricular dilatation disease (PDD) was previously known as macaw wasting disease/syndrome, macaw fading syndrome, and neuropathic gastric dilatation. The current name was given because many birds with this disease have a significant increase in the size (dilatation) of a part of their stomach known as the proventriculus. PDD was first reported among macaws in the US in the 1970s, and in the 1980s increasing numbers of cases were reported worldwide. Although initially seen mostly in macaws, the disease was eventually identified in more and more parrot species. To date, the condition has been recognized in over 50 parrot species.

Broadly speaking, PDD causes two types of illness. The first is a gastrointestinal (GI) form that affects the proventriculus and intestines. The second is a neurologic form that affects the brain and central nervous system. These two forms can occur separately or together with one or the other being more apparent. The gastrointestinal form is suspected to be more common than the neurologic form.

What are the Signs of PDD?

Oftentimes birds affected with the GI form of PDD will appear to be sick suddenly. The bird may regurgitate, not eat well, and might pass abnormal stools that contain undigested food, such as seeds. On close examination, the bird will often be thin, indicating that the bird has had the disease for longer than it would appear from the symptoms alone. PDD damages the nerves in the GI tract. As the nerves become more and more damaged, the bowel’s ability to function normally lessens. When the bowel isn’t working correctly, many nutrients pass undigested in the stool.

Some birds may develop broader nervous system signs in addition to the above GI signs, while other birds only develop nervous system signs. PDD may cause a wide range of nervous signs. These clinical signs can appear suddenly or progress slowly. Birds with PDD may be weak and have a reduced ability to perch or walk; one or both legs may be paralyzed; they might have head tremors; and occasionally they have seizures.

What Causes PDD?

Until relatively recently, this was the $64,000 question. For many years, an infectious organism was suspected but PDD did not behave like other infections: it often took months to several years to develop, and some birds in a group didn’t get it. Despite these unusual characteristics, researchers decided that a virus was most likely the cause of this disease, and the hunt for the specific virus began in earnest.

After more than two decades of work, the answer finally came in 2008 when the virus was identified as avian bornavirus. Since that time much has been learned, and researchers around the globe are working towards a better understanding of it.

Which Parrots are Affected by PDD?

The disease has been documented in over 50 parrot species. However, the condition appears most commonly among macaws, Amazon parrots, African grey parrots, cockatoos, Eclectus parrots and conures. Birds of any age can be affected; however, birds are typically 3 or 4 years old when the condition is first noticed. That being said, birds may be as young as 10 weeks and others as old as 20 years when the disease is first diagnosed. As such, owners and veterinarians should consider this disease among any parrot showing clinical signs.

How is PDD Diagnosed?

Neither the GI nor the nervous signs caused by PDD are specific to this disease. In other words, your avian veterinarian will have to consider PDD and many other infectious and non-infectious diseases as the cause of your bird’s illness before a definitive diagnosis can be reached. For example, lead poisoning can cause GI disease and the associated nervous system signs can mimic PDD. The non-specific signs of PDD can make it difficult to definitively diagnose this disease.
For birds affected by either or both forms of PDD, your veterinarian will likely suggest performing routine diagnostic tests such as a complete blood count, plasma biochemistry panel and whole body radiographs (x-rays). These tests will help your veterinarian rule out other infectious and some non-infectious causes of these types of symptoms.

Of the above tests, radiographs (x-rays) are most likely to aid in the diagnosis of PDD, particularly among birds with the GI form. In many of these cases, the radiographs will show that the proventriculus is enlarged. Sometimes a type of radiograph known as a contrast series may be required. To highlight the bowel, your bird swallows a contrast liquid that shows up brightly on the radiograph. A more advanced version of this test uses fluoroscopy; essentially this is a test that generates real-time x-ray images so the contrast liquid passing through the GI tract can be easily monitored.

Even if your avian veterinarian confirms an abnormal structure or bowel function with these tests, a definitive diagnosis of PDD cannot be made. Currently the only way to definitively diagnose this condition is a surgical biopsy of tissue and nerves from the GI tract. Typically the safest and easiest area to biopsy is the crop, which is an outpocketing of the esophagus located in the neck. The tissue is sent to a veterinary pathologist who looks for characteristic signs of nerve damage and inflammation associated with PDD. Unfortunately, even this test is not perfect and a certain proportion of birds with PDD will not be accurately diagnosed using this test. If a surgical biopsy is considered necessary to get a definitive diagnosis of PDD, your avian veterinarian will discuss the risks and benefits and allow you to make an informed decision.

The above tests are particularly helpful for birds who have the GI form. The diagnosis is harder still in birds who only have neurologic signs. Your veterinarian will likely suggest performing most of the above tests; however, if there is no evidence of GI disease, they will likely not perform a surgical biopsy. Generally speaking, neurologic PDD is one of the most difficult diseases to diagnose in avian medicine, and there is no specific test for it. It is essentially a disease of exclusion, meaning that your veterinarian will have to diagnosis it by ruling out, or excluding, other diseases. Your veterinarian will consider many factors such as your bird’s age and species, his risk for other infectious diseases or non-infectious diseases, and the results of screening tests to rule in or out other causes of neurologic disease.

Is the PCR Test Useful for PDD?

Avian veterinarians were initially excited by the discovery of avian bornavirus and the subsequent development of a commercial PCR test to identify it. However, it has become clear when tissue samples are examined by a veterinary pathologist that many birds who test positive for the virus do not have the disease. Equally concerning is the relatively large number of cases where PDD is not identified by the polymerase chain reaction (PCR) test, but the disease is later confirmed by a pathologist. These findings may occur because PDD is caused by a combination of damage inflicted by both the virus itself and the immune system as it tries to rid the body of the virus. Therefore a bird may be infected with avian bornavirus for a period of time and test positive by PCR, but an unknown percentage of these birds will clear the infection and never develop PDD. On the other hand, some birds will be tested for PDD with the PCR at a point when they are no longer shedding the virus, but later go on to develop the disease owing to the inflammatory immune response.

Another possible explanation for the poor correlation between the PCR and the presence or absence of PDD could be the relatively recent discovery of various subtypes (known as genotypes) of avian bornavirus. It is not clear if these genotypes are all detected by the commercial PCR, nor is it clear if some subtypes are more likely to cause disease than other subtypes in certain species of birds. As such, at this point in time, testing birds with the avian bornavirus PCR cannot be recommended since the significance of a positive or negative test result is not known.

Can PDD be Treated?

For many years PDD was considered terminal and the only treatment provided was supportive care. Easily digestible food was provided so as to maximize the amount of nutrients that could be absorbed by the sick bird. Supportive care also involved treating secondary GI infections and maintaining a high level of hygiene to try to prevent secondary infections.

In the past several years, more and more avian veterinarians have reported success in the treatment of PDD by using high doses of powerful anti-inflammatory drugs. These drugs are thought to work by reducing the amount of inflammatory nerve damage that occurs as PDD progresses. Several birds with PDD confirmed by tissue biopsies have survived many months and in some cases several years with this treatment. While this treatment option holds hope for many birds, keep in mind that no scientific studies have confirmed how useful this treatment is, and as such success is considered anecdotal as it is not proven scientifically.

Is there a Vaccine to Protect against PDD? Can PDD be Prevented?

Unfortunately there is no vaccine to prevent PDD. Given the amount and quality of research being performed on this disease at the moment, it is possible that a vaccine may by commercially available at some point in the future.

Other forms of prevention also pose difficulties at the moment. The most important role in preventing infectious disease among pet parrots lies with bird breeders. This is certainly the case for well-understood diseases such as psittacine beak and feather disease (PBFD) and psittacosis, where breeder birds can be tested and removed from the flock and incoming birds can be tested and quarantined. The problem with PDD is our lack of understanding of the organism, and our inability to effectively test for it in a live bird makes it difficult or impossible to recommend much for prevention and control.

Summary

PDD is a serious and often fatal disease that predominantly affects parrots. The disease is caused by a virus known as avian bornavirus. The clinical signs are caused by the virus and the bird’s immune response to the virus. PDD can cause disease of the GI tract and or the central nervous system. Birds affected with the disease may be thin, have abnormal stool, may regurgitate or have anorexia. Bird may also be weak, have trouble walking and potentially develop severe neurological symptoms, such as seizures. Diagnosis is difficult, often requiring a combination of multiple diagnostic tests and the clinical experience of your avian veterinarian. There is no vaccine, and no scientifically proven treatment method has been developed. That being said, a growing body of evidence supports the fact that anti-inflammatory drugs may delay the progress of this disease and potentially greatly prolong affected birds’ lives.