Avian polyomaviruses have a worldwide distribution and are known to infect Psittaciformes (parrots), Passeriformes (weaver finches, canaries), Galliformes (chickens and turkeys) and Falconiformes (falcons and hawks).
An avian polyomavirus has been reported as the most common infectious agent causing death in unvaccinated young psittacine birds in the United States. Some affected birds die without developing any clinical signs of disease, while others die 12-to 48-hours after developing clinical signs that may include depression, loss of appetite, weight loss, delayed crop emptying, vomiting, diarrhea and bleeding under the skin. These changes can also be associated with other infectious and noninfectious disease processes.
In addition to chicks, adult psittacine birds are readily susceptible to infection, can become ill, and some may die. Many affected young birds die, while most infections in adult birds are unrecognized or infected birds manifest subtle clinical changes such as transient lethargy, a poor appetite and diarrhea with the surviving birds developing antibodies to the virus.
Infections classified as "subclinical" (not obvious upon typical examination) are common in adult and young birds. Experimental and field data suggests that the incubation period can vary from several days to several weeks depending on the route of virus inoculation and the age and species of the bird.
Inhalation or ingestion of contaminated excretions and secretions (fluids from the respiratory tract or crop) from infected birds is thought to be the primary method of virus dissemination through, and among, flocks.
Most affected birds shed viruses for several days to several weeks, and in rare cases birds may shed for longer periods. Feather debris (dust) should be considered an important method of virus dissemination from birds with polyomavirus-induced feather abnormalities. Transmission of polyomavirus through the egg has been confirmed in budgerigars but not in other psittacine birds.
Indirect contact with virus contaminated environments or goods is considered important in the transmission of polyomavirus. This virus is thought to be durable in the environment and should be considered viable when outsider the host for at least six months if not years.
Specifically, polyomavirus outbreaks have been linked to:
1) inadequate quarantine procedures
2) virus contaminated nest boxes
3) virus contaminated incubators
4) shipment of unvaccinated or incompletely vaccinated birds to brokers or pet retailers
5) mixing unvaccinated birds from numerous locations
6) exposing unvaccinated flock residents or neonates to infected birds or a contaminated environment and returning them to the aviary without quarantine.
In Germany, free-ranging Falconiformes have been shown to be susceptible to avian polyomavirus, suggesting that infected free-ranging birds could disseminate this virus.
The best way to control or prevent polyomavirus associated problems is through vaccination. To find an avian veterinarian in your area that can vaccinate for polyomavirus, contact Biomune (913-894-0230).
The strategies for using the avian polyomavirus vaccine are similar to those used to control other common viral diseases, like parvovirus in dogs, panleukopenia virus in cats or poliovirus in humans. If you are purchasing a companion bird, seek a breeder that only sells vaccinated birds.
To reduce polyomavirus infections within the aviary, it is essential to vaccinate the adult birds. Once a breeding flock is vaccinated, the population of birds at-risk is substantially decreased, and the likelihood of a progressive cycle of transmission among these adult birds is reduced. This in turn lessens the chances that the adult population will serve as a source of virus for neonates in the nursery.
If virus activity in the breeding aviary is reduced through vaccination, then careless avicultural practices (i.e., no quarantine procedures, bringing birds from other aviaries into the nursery, allowing visitors with direct or indirect contact with birds access to the nursery) become the most likely route by which the virus enters the aviary. If the flock is vaccinated and the virus is not inadvertently introduced to the nursery by people or exposure to a contaminated fomite, it is unlikely that any neonates will be exposed to the virus. It is crucial for young birds that will be leaving the aviary to be protected before they are exposed in the pet trade to unvaccinated birds which may be shedding the virus.
Vaccinating Companion Birds
If a companion bird is maintained in complete isolation, which is not a very realistic scenario, it has minimal risk of being exposed to polyomavirus. However, isolation means that the bird, and its keeper, never leaves the home to go to the veterinarian, groomer, club meetings or any location where direct or indirect exposure to other birds might occur. Complete isolation also means that all of the food, toys, perches and enclosures for the bird are purchased from mail-order catalogs that ship from warehouses that do not house birds. These items may be contaminated with polyomavirus if they are kept in the same airspace with birds.
It seems more logical to vaccinate companion birds rather than attempt to maintain such rigorous isolation, although isolation is the only method to reduce a bird's exposure to the infectious agents for which vaccines are not yet available. It is interesting to note that companion dogs and cats that are maintained in relative isolation (those that are confined to indoor living) are routinely vaccinated to protect them from common infectious diseases.
Testing in Live Birds
Until the avian polyomavirus vaccine was registered by the USDA, control of polyomavirus outbreaks was problematic because of the commonality of virus activity in psittacine birds and the inherent difficulties in reducing potential exposure to this environmentally stable virus by maintaining closed aviaries, practicing extraordinary hygiene and attempting to detect and isolate transiently-infected birds.
Techniques originally developed at the University of Georgia College of Veterinary Medicine to facilitate this latter task include assays to detect anti-polyomavirus antibodies and a DNA probe test to detect polyomavirus nucleic acid. An antibody assay can be used to determine if a bird has been previously infected with the virus (the speed of decay of detectable levels of antibodies varies with the individual bird).
A lack of an antibody titer may indicate either that the bird has not been infected, that it was infected in the distant past and the antibody titer has decreased below a detectable level, or that it has been infected but has not developed an antibody response (considered rare with avian polyomavirus). Polyomavirus nucleic acid can be detected in cloacal swabs (fecal samples) taken from psittacine birds during an outbreak. This allows birds that are excreting nucleic acid to be isolated from the remainder of the flock until the infection has resolved in the positive birds. Polyomavirus-specific DNA probes also can be used to detect viral nucleic acid in environmental samples collected from a bird's living area (home, nursery, incubators, etc.) that may have been contaminated with the virus.
If one chooses to use any diagnostic test for polyomavirus, it is important to ask what question is to be answered. If the question is whether a bird has been previously infected with polyomavirus, then the test that is most likely to provide this information is an antibody assay. Antibody titers will persist long past the time that a bird has cleared an infection.
If the question is whether a bird represents an immediate threat to other birds in a group, then the best test to run is a DNA probe assay on excrement. If testing is chosen for attempted control of polyomavirus in lieu of vaccination, then using both a test to demonstrate if polyomaviral nucleic acid can be detected in excrement, or blood, and a test to demonstrate if virus-neutralizing antibodies are present is recommended.
Confirming an Infection
Neonates that die from avian polyomavirus are usually in excellent overall condition and may have full crops and alimentary tracts, indicating a rapid illness and death. Typical abnormalities noted at necropsy include enlargement of the liver and spleen and hemorrhage under the skin and on the surface of many internal organs.
The most accurate way to confirm the presence of avian polyomavirus infection is the recovery of the virus in cell culture; however, this procedure is time-consuming and expensive. Other techniques that can be used to document an active polyomavirus infection include: the demonstration by electron microscopy of virus particles in affected tissues; demonstration of a four-fold increase in antibody titer in paired samples run at the same time in the same laboratory; specialized staining of suspect lesions using viral-specific antibodies; or the detection of viral nucleic acid in affected cells using polyomavirus-specific DNA probes.
To find an avian veterinarian in your area that can test for polyomavirus contact the Infectious Disease Laboratory at the University of Georgia College of Veterinary Medicine (706-542-8092).
Controlling an Outbreak
Steps to controlling a polyomavirus outbreak should include isolating clinically affected birds and any birds to which they have been exposed, vaccinating the adults and neonates to stimulate flock immunity and cleaning and disinfecting the contaminated facility.
While vaccinating during a polyomavirus outbreak has been shown to be advantageous, it should be stressed that deaths may continue in neonates until flock immunity has been increased, generally 2 to 3 weeks after the last booster vaccination.
Great care should be exercised when handling birds during an outbreak to make certain that aviary personel do not disseminate the virus.
Several therapies, including various immunostimulants and antiviral drugs designed for other viruses, have been suggested for the treatment of birds with avian polyomavirus. Included in the group of frequently mentioned therapies are interferon (a nonspecific immunostimulant), acyclovir (an antiviral drug with specific activity against some herpesviruses) and AZT (an antiviral drug with activity against some retroviruses).
Anecdotal evidence suggests that some of these therapies may be effective in the treatment of birds with clinical signs suggestive of avian polyomavirus. However, none of these treatments has been demonstrated to be effective in birds with documented (demonstration of virus in affected tissues) avian polyomavirus infection. Of these speculative therapies, interferon may be the most promising.
As is the case with many viral-induced diseases in companion animals, vaccination plays a pivotal role in reducing the incidence of avian polyomavirus infections. However, because no vaccine is 100% effective, vaccination should not be expected to completely combat the deleterious effects of poor management or hygiene. Other procedures that will reduce a bird's exposure to this virus include: