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Proventricular Dilation Disease: What's New?
Michael Taylor DVM and Bruce Hunter DVM MSc
Proventricular dilation disease (PDD) is an infectious disease of several orders of birds. It has been known to occur in imported parrots since the late 1970's. Over the past five to six years PDD has been more frequently diagnosed at laboratories throughout North America and Europe. This rise in occurrence comes at a time when importation of wild caught birds has all but ceased. PDD outbreaks can be devastating, especially in indoor aviaries.
A virus has been found in the feces of affected birds. The virus has been concentrated By Dr Branson Ritchie and his colleagues at the University of Georgia and used to infect birds to produce clinical PDD and thus demonstrate transmissability. This encapsulated virus has been difficult to work with and awaits further identification.
The virus causes damage to the autonomic nervous system of the host. Of specific interest is the virus's tropism for the myenteric portion of the autonomic system. Damage associated with viral replication in the ganglia of the myenteric plexus leads to abnormalities in the control of gastrointestinal motility. Digestion is impaired with the consequence that classically infected birds slowly lose body condition before finally starving to death. Other portions of the autonomic nervous system in the brain, adrenal gland and the heart may also be affected.
The current "gold standard" for the positive diagnosis of PDD is the finding of lymphoplasmocytic infiltrates in ganglia and associated nerve fibers of the myenteric plexus of the gastrointestinal tract.
Early detection of PDD has, in the past, been difficult to impossible. One of the objectives of our research has been to examine techniques that might allow early detection of this disease so that infected individuals could be removed from the flock preventing further new cases of the disease. We have shown that chronically infected birds are present in an exposed population acting as reservoirs of the virus. Could these birds be detected? For the past three years we have endeavoured to answer these and other questions about PDD.
An Update on the Findings So Far
We have demonstrated the utility of barium contrast fluoroscopy as a technique to examine the abnormal motility of the psittacine gastrointestinal system. Dzuik and Duke first described the normal motility of the turkey ventriculus. We described, for the first time, the use of modern video fluoroscopy in the macaw and found very similar motility patterns to those described for the turkey ventriculus. Avian pathologists have suggested from routine histopathogic studies that PDD appears to affect the gastrointestinal tract in a random, segmental fashion. This was confirmed in our study for the first time, both fluoroscopically and histologically. Involvement of the crop, esophagus, proventriculus, ventriculus and duodenum in all possible combinations was recorded. The most commonly observed lesions were dilation of the ventriculus, duodenum and proventriculus. Crop dilation was especially common in African Grey Parrots. Distal esophageal (post ingluvial) dilation was a rarer finding in all species but did occasionally occur. Ventricular motility was also assayed and found to be a good early indicator of PDD. The psittacine ventriculus has a vigorous and complex pattern of motility that is grossly similar to the turkey. Birds suffering from PDD evidence changes in the coordination and effectiveness of these contractions at an early stage of the infection. Weight loss and death were most frequently associated with advanced ventricular and/or duodenal motility problems. Fluoroscopic lesions were compared to crop biopsy and/or necropsy findings. A important finding from the fluoroscopic study was the detection, for the first time, of naturally occurring, chronically infected parrots. We currently have several chronically infected birds in isolation who have been living with PDD for over four years. We continue to believe that it is the chronically infected parrot who poses the greatest risk to aviculture.
Frequently Asked Questions About PDD
What do you recommend for diagnostic confirmation of PDD?
Useful for gross examination of the proventriculus, isthmus region and ventriculus from the left caudal thoracic and left abdominal air sacs. Especially useful for confirming inflammation present on the serosal surface of the proventriculus and ventriculus in early cases and the dilational changes of more advanced disease.
The crop remains the only really accessible portion of the gastrointestinal tract from which to harvest samples of the myenteric plexus. It is easily exposed via routine surgical technique and a large sample is possible. The only disadvantage is that the segmental nature of this disease means that false negatives will occur.
Proventricular biopsies are contraindicated due to the very thin wall and accessibility, morbidity/mortality concerns. The ventriculus is somewhat more approachable and can safely be biopsied due to its thick muscularis, but problems of sample size (did I get any nerve tissue?), the segmental nature of PDD, as well as the relatively more invasive nature of the procedure compared to crop biopsy remain.
Hypoalbunemia is a consistent finding in advanced cases of PDD. Increased Lipase levels have been a frequent but inconsistent finding in PDD cases. This may reflect the segmental involvement of the duodenum in this disease.
Excellent for demonstrating motility abnormalities far earlier than still radiographic contrast techniques; however enough nerve pathology must already be present to cause recognizable motility abnormality. Superb for demonstrating crop, esophageal and duodenal effects of PDD. The examinations can be recorded for analysis and review. The disadvantage is that fluroscopic equipment is expensive and only available at teaching hospitals or specialized clinics.
Also note that anesthesia is absolutely contraindicated in motility studies as it will impair or completely stop ventricular contractility. Even benzodiazapine tranquilizers such as midazolam were found to completely eliminate ventricular contractions in our study.
Are there other conditions that might mimic PDD?
Yes. Certain diseases may appear similar to PDD.
· chronic lead or zinc toxicity
· megabacterial ventriculitis
· fungal ventriculitis
· papillomatous disease involving the pro- ventriculus / ventriculus (partially obstructive)
· ventricular foreign body leading to ventriculo- duodenal outflow problem
· other bacterial ventriculopathy
By using a panel of diagnostic tests it is possible to rule out the presence of these other conditions. In our screening study we have found that the incidence of these other conditions is far less common than PDD.
What is your recommendation for dealing with PDD in an indoor aviary?
It is important to stress that not all exposed birds will become infected. Our current understanding of the virus strongly suggests that it is not hardy outside of the host and rapidly succumbs to dessication and disinfection. It is also clear that this disease is more devastating in indoor environments than in outdoor housing.
Many birds who lose a mate are themselves chronically infected birds. The finding of the first dead bird with confirmed PDD in the aviary should immediately raise the question: Who is the bird in the nearby environment who is shedding the virus while appearing clinically normal?
We screen suspicious birds after reviewing the epidemiologic history of the aviary. Crop biopsies and fluoroscopy, analyzed together, have been the highest yield tools at this time. We believe that chronically infected birds must be identified and removed from the aviary to stop the spread of the virus. Hygiene is critical to the prevention of PDD transmission in the indoor aviary. Avoid crowding of cages, use a two bowl system for food and water bowls and avoid spraying or aerosolizing feces in the aviary. Maintain a secure caging system to prevent birds from escaping and defecating on other cages.
Have you documented a bird recovering from PDD?
I have yet to see a clinical, confirmed case of PDD recover. Some birds may clinically improve but turn out to be chronically infected on close inspection.
Is there any treatment for PDD?
There is no known treatment for PDD at this time (November 1998). Prevention of new cases remains the best strategy. Chronically infected birds must be identified and removed and effective hygiene protocols implemented.