Current Research and Advances in Avian Bornavirus (ABV) and Proventricular Dilatation Disease (PDD) aka Neuropathic Gastric Dilatation or Macaw Wasting Disease (2013)
Jeannine Miesle, MA
Academic researcher in the field of avian medicine. Member of the Association of Avian Veterinarians (AAV).
The author is indebted to Dr. Robert Dahlhausen (Avian & Exotic Medical Center, Milford, OH) for his advice and support on this paper.
Bookmarks / Links:
- Defining PDD
- Borna Disease Virus
- Species Affected by Avian Bornavirus (ABV) Disease
- Periods of Vulnerability
- Signs of ABV Infections and Systems Affected
- Difficulties in Diagnosing PDD
- ABV/PDD Transmission
- Recommendations to Pet Owners, Aviculturists and Clinicians / Screening
For a more advanced article on this topic, please refer to the PDD Resource for Aviculturists and Veterinary Professionals.
Proventricular Dilatation Disease (PDD) is an inflammatory wasting disease found mainly in the parrot family (Psittacines).1 Although in the past it had been considered always fatal, new treatments have transformed this disease from severe to one that is chronic and demonstrates low-to-moderate and occasional symptoms, provided treatment is given early in the disease process. The number of birds (both ill and healthy) testing positive for the Avian Bornavirus (ABV) is staggeringly high—as many as 30%. So if a bird keeper has several birds in his collection, the chances are very good that at least one will be a carrier of the virus. Bird owners can now find peace of mind knowing that PDD can be controlled should their birds test positive.
Independent researchers discovered this virus in 2008, and since then many of them have been studying the source and development of the disease. Their goal is to provide longer, healthier lives for the birds which have been stricken with PDD; hopefully, one day there may even be a cure.
Figure 1: A macaw suffering from PDD (Courtesy R. Dahlhausen)
Known by many names in the past, including Macaw Wasting Disease, it was finally given the name “Proventricular Dilatation Disease” just a few years ago. It was initially reported in the late 1970’s to early 1980’s in the U.S., Germany, and Switzerland.13 Originally thought to affect only macaws, as of 2009, PDD had affected fifty psittacine species, several non-psittacine species, and even wild birds.2,13 Since then, the virus has been detected in samples from all over the world. The expansive bird trade at the time—and continuing even to this day—has been responsible for the rapid spread of this virus, and N. America, Europe, Asia, Africa, and South America have seen the highest number of cases.4 It is now found around the world.
Once regarded as a disease which carries a high rate of fatality but a low infection risk, PDD is now known to be a disease which affects a great number of birds but carries a much lower risk of showing symptoms.2,8 PDD is always fatal if left untreated, and the infected bird will never eliminate the infection.2 However, bird owners need to understand that it is now possible for their birds to become symptom-free and live much longer than had been expected in the past.
During the last 30 to 40 years, researchers and clinicians have been studying PDD in avian species, but the source and development of this disease have proven elusive. Researchers suspected a viral cause, but up until four years ago, there had been no proof. The only diagnostic tools practitioners had were history, symptoms, and tests which did not always provide accurate or dependable results. Crop biopsy and x-rays revealed enlargement of the proventriculus and ventriculus (two-part stomach), and avian veterinarians could also refer to pathology results which usually often showed nerve damage.2,8 Still, clinicians were often unsure of their diagnoses. Only post-mortem examinations gave them absolute proof of the presence of the virus. But now, with more advanced molecular genetic testing methods, practitioners can be certain that their results are accurate.8
Originally discovered in cavalry horses in Borna, Saxony, Germany in 1885, the Borna Disease Virus (BDV) has since been detected in many animals, even in mammals, primates and humans. The Avian Bornavirus, however, is different from the mammalian Borna Disease Virus. It does not grow in mammalian cells; therefore, it is not thought to infect humans or animals.
Both Borna Disease Virus (BDV) and Avian Bornavirus (ABV) are responsible for infections of the Central Nervous System.4,2 However, ABV affects multiple organs: the brain, gastrointestinal (GI) tract, liver, kidneys, heart, peripheral blood vessels, and lungs.3 Some birds show only slight changes in behavior, but others suffer severe symptoms, and some of those eventually die.4, 2 Some display mild, occasional symptoms and never receive treatment, while others may be infected over a long period of time and never exhibit any signs.2
Figures 2 and 3: Two radiographs showing distended crop with poor motility and enlarged proventriculus and ventriculus. (Courtesy R. Dahlhausen)
Another way in which Avian Bornavirus (ABV) differs from other viruses is its method of attack. Most viruses invade a host cell, destroy it, then move on to infect more cells. The Bornavirus, however, does not destroy the cell, so the infected cells suffer very little damage.2,3 Because the cell is not destroyed, the virus is able to avoid being recognized by the host's immune system.3 As a result, the virus can remain in the body's cells for an indeterminate amount of time, possibly even as long as the animal lives." 2 This leads to the continual, persistent infections for the duration of the animal's life. 3
There are periods in the life cycle of the birds during which they may be more apt to contract the virus than at other times. Unweaned nestlings are more susceptible to the virus than older birds; the incubation period for them is two to four weeks.5
Another vulnerable time is the breeding season, and symptoms tend to recur during that time. The stresses of reproductive activity depress the immune system, allowing symptoms to flare up. Clinicians frequently see an increase in new cases and a recurrence of signs in existing cases during these periods. One clinician has treated a patient who had experienced three severe relapses of gastrointestinal symptoms during consecutive breeding seasons.2
PDD is described as a sporadic disease which tends to cycle from active to dormant states throughout the bird's life. The bird may experience bouts with the symptoms for months, even years, after diagnosis. 2 Even single pet birds, housed in the same environment for many years, have been diagnosed with the disease. This might be due to the state of the bird's immune system, stress due to malnutrition, the presence of another disease the bird is fighting at the same time, its reproductive activity, or improper care.2 Stress is generally accepted as the primary trigger in the activation or recurrence of Avian Bornavirus (ABV) disease, and it will accelerate the spread of the virus.2
Figure 6: Signs of self-mutilation in an African Grey (Courtesy R. Dahlhausen)
Because so many of the birds with Avian Bornavirus (ABV) do not actually exhibit symptoms, many go undiagnosed for years. Only if they begin to show signs might they be seen by a practitioner. There is a wide range of signs, from infrequent episodes of mild disorders to sudden and severe illness.11 A bird may experience just some of the symptoms, not necessarily all. And different birds may show different symptoms. Some birds become symptomatic years or decades after becoming ABV-infected, and some never show signs at all, but may continue to shed the virus and thus infect other birds.4
Clinicians often wish to rule out other diseases before they screen for ABV infection. The bird's own system can make diagnosis difficult. If the bird isn't PCR-tested for the presence of ABV, or for the specific genotype (subgroup) that is causing symptoms, the virus will not show up on the test results. Also, the bird might not be shedding the virus at the time of testing, so results may be negative.1
Researchers have concluded that the virus is transmitted by the fecal, oral, and nasal routes. 5 The bird must “come in contact with saliva and bodily excretions of infected animals” in order to contract the virus.2
However, birds do not need to ingest fecal matter or vomitus to become infected. The virus can be transmitted by becoming airborne when the infected individual regurgitates and defecates and the material is allowed to stand and dry. Even if this material is cleaned up immediately, infection can result if another individual eats food or breathes air near the site of the vomitus or droppings. The virus continues to be shed even after symptoms cease and can be detected many weeks after infection. 8
Although some clinicians consider the virus to be highly contagious at all times, some believe it is more likely to spread in homes and aviaries in which good hygiene is not practiced. Overcrowding and insufficient ventilation lead to the spread of the virus. 2 This is a chronic disease; symptom-free, long-term survival does not mean that the bird has been cured. It still carries the virus and can transmit it to other birds. Isolation from other birds is essential.2
Vertical transmission, from an infected breeding hen to the embryo in the egg, has been demonstrated by at least two different researchers. They are attempting to find out how and when this happens in the course of egg production.4 Breeders, in particular, who have ABV-positive birds in their aviaries are concerned about vertical transmission.
Figures 7: A necropsy photo showing the wasting of the breast muscle and body. (Courtesy R. Dahlhausen)
Many people have very little knowledge about the deadly consequences of untreated Avian Bornavirus (ABV) infection. We know now that this disease cannot be prevented or cured, but it does respond favorably to treatment and medications. For bird owners, this means their birds can live a normal, healthy, symptom-free life, even with PDD. Informed breeders and bird-owners have the power to curb the spread of this disease by closely monitoring their flocks and preventing or discontinuing the reproduction of PDD-positive birds. Hopefully, bird-keepers have gained sufficient awareness about the dangers of ABV and PDD to motivate them to be vigilant regarding the signs of PDD.
For a more in-depth article, please refer to the PDD Resource for Aviculturists, or contact the author through Avian Web.
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