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The cause of proventricular dilatation disease (PDD), a fatal
neurological disorder that affects mainly captive parrots, is avian
bornavirus (ABV). A group led by researchers at the Schubot Exotic
Bird Health Center of the Texas A&M University College of
Veterinary Medicine & Biomedical Sciences confirmed this
revelation in a recent study.
The study, published in the March 2010 issue of the journal
Emerging Infectious Diseases, is based on the fulfillment of Koch's
postulates for ABV. The postulates are a set of criteria that must
be met to prove that a particular pathogenic agent causes a
First identified in 2008 in birds affected with PDD, ABV has
been suggested as the possible cause of this disease. However, thus
far, conclusive evidence demonstrating the virus actually causes
PDD has not been presented.
Establishing such a causal relationship between ABV and PDD, the
researchers explain in the study, would require satisfying Koch's
postulates, that is, "isolation of the agent [in this case, ABV]
from infected birds; its propagation in culture; and, after
reintroduction of the isolate into a susceptible host,
manifestation of the disease."
The researchers have demonstrated precisely these steps.
The group isolated ABV from the brain tissues of eight parrots
with PDD. The virus was then propagated under laboratory
conditions; specifically, the virus was grown in a culture of duck
embryonic fibroblasts. Fibroblasts infected with the virus were
then injected into two PDD-free Patagonian conures. One Patagonian
conure was injected with fibroblasts that did not contain the
Patagonian conures infected with ABV developed clinical signs of
PDD. Further, brain tissues from these birds tested positive for
ABV. The conure that was not infected with the virus did not
"It's the final act in proving that the virus actually causes
PDD," said Dr. Ian Tizard, director of the Schubot Center and head
of the research group, commenting on the successful experimental
reproduction of the disease in healthy birds.
The study, funded by Texas A&M University's Richard M.
Schubot Endowment, represents a major step forward in understanding
PDD, a disease that has befuddled scientists for more than 30
First reported in the late 1970s, PDD affects more than 50
species of parrots as well as other birds. Many of these species
are endangered and are raised in captivity, making PDD a serious
threat to their conservation.
The disease is characterized by damage to the nerve supply in
the organs in the gastrointestinal tract. This affects birds'
ability to digest food, resulting in the accumulation of undigested
food in the proventriculus, the first part of the stomach, which
consequently dilates (hence the name of the disease).
Common clinical signs include weight loss, regurgitation of
undigested seeds and loss of appetite. PDD can also damage nerves
in the brain and spinal cord, resulting in neurological symptoms
such as imbalance and lack of coordination. The disease eventually
results in death.
Further studies at the Schubot Center will focus on the origin,
prevention and treatment of the disease.
These future projects include examining raccoon fecal droppings
as a possible source of viral origin, testing the ability of
antiviral drugs to inhibit the growth of the virus grown in duck
embryo fibroblast culture and a multicenter trial to test the
effectiveness s of anti-inflammatory drugs (for example,
cyclosporine) to prolong the survival of PDD-affected birds.
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