Proventricular dilatation disease (PDD) is a disease that can affect the nervous system of birds, particularly parrot species. It was originally described in macaws in the late 1970s and was dubbed “macaw wasting syndrome,” due to the rapid weight loss these birds displayed. Since then, PDD has been documented in more than 50 species of psittacine birds encompassing all major groups of parrots.
The disease affects the nerves of birds, especially those of the gastrointestinal tract. Typical signs include regurgitation, weight loss, and passing undigested food in the droppings. Seeds and nuts are particularly difficult to digest and may be passed whole. As the nerves that supply the proventriculus (a bird’s true stomach) become more severely affected, the proventriculus dilates and becomes flaccid. It may reach 10 times its normal size. As the proventriculus distends, normal digestion and motility are lost and the bird begins to waste away as they have difficulty absorbing the nutrients in their diet. Death eventually comes from severe malnutrition. Other birds may have nerves affected elsewhere in their body. Depending on which nerves are affected, these birds may have trouble with balance, walking, or flying. Some birds may even have seizures.
While PDD appeared to be contagious, the infectious organism responsible for the disease remained a mystery for decades. Diagnosis of the disease relied on seeing typical clinical signs and finding the telltale inflammation around nerves from the gastrointestinal tract on a surgically collected biopsy (usually from the crop). These crop biopsies are still considered the gold standard for diagnosing PDD; however, in 2008 researchers isolated a virus from birds affected with PDD. The virus was similar to borna disease virus, which causes neurologic signs mostly in horses and rarely in humans and other mammals. This new virus was named avian borna virus (ABV).
Controlled studies have proved that avian borna virus causes PDD, but the situation is a little complicated. Since the initial discovery, at least six different genotypes, or strains, of ABV have been discovered. Currently, at least two of those genotypes have been shown to cause disease in parrots and related species. The significance of the other genotypes is unknown, as is how many others are yet to be discovered.
Blood tests are now available that can screen for ABV. As more birds and even whole aviaries have been tested, it has become clear that many clinically healthy birds harbor the virus. While some of these birds may go on to develop typical signs of PDD, many will remain healthy for years. Why some birds develop signs while others do not is unknown. It may be related to the particular strain of ABV that infects them or perhaps other factors that affect their overall health status. There have been reports of parrots in single-bird households that have become sick after more than 15 years with no exposure to other birds. These birds were likely exposed to ABV prior to entering the household and remained as asymptomatic carriers until something else either triggered the virus or weakened the bird’s immune system, allowing the virus to activate.
Treatment of PDD is largely supportive. Feeding highly nutritious, easily digestible pellet-based diets can help the birds maintain weight and reduce the amount of regurgitation. For more severely affected birds, liquid hand-feeding diets can be offered or the pellets can be made into a slurry to make them even easier to digest. Though several anti-viral medications have been tried, there are currently no known drugs that effectively kill ABV. Non-steroidal anti-inflammatory drugs (NSAIDs) can control the inflammation the virus causes around the nerves and slow down the progression of the disease. Though many birds do continue to decline, others have been apparently cured after nine months of therapy with NSAIDs. Recent research has revealed an association between ABV and chronic feather plucking, with the incidence of ABV significantly higher in birds displaying feather destructive behaviors. Whether NSAID treatment would be able to help these birds still remains to be seen.
Breeders, aviaries, and small flock owners can help to minimize the spread of ABV by separating carriers and affected birds from those that are free of the virus. The virus can be shed in the droppings and potentially even feather dander, so maintaining excellent hygiene can also minimize cross-contamination. Carrier birds that are clinically healthy can still be used as breeders, since they can produce unaffected chicks. Hand rearing of these chicks can further improve the chances of them growing up virus free. As we learn more about avian borna virus and proventricular dilatation disease, our ability to diagnose, treat, and even prevent the disease will continue to improve.