Immunization to protect from infection.
In adult birds and many nestlings, APV infection is asymptomatic. Yet these asymptomatically infected birds shed virus and can cause the virus to spread into the aviary and the nursery. Therefore, birds that are taken off your property, exposed to other birds, and then returned to the property, may benefit from the APV vaccine. To properly protect them, they must be vaccinated twice, beginning at least 4 weeks before exposure to other birds. Bird marts, bird shows, and bird club meetings are all potential venues for APV transmission to occur. Remember, a bird shedding polyomavirus may look completely healthy.
Immunization of currently infected or previously infected birds.
Early in our understanding of APV, it was suggested that birds that were shedding virus were incapable of mounting an appropriate immune response.30 It was then suggested that immunization would cause these birds to stop shedding virus. Today we know differently. All the evidence shows that once infected with APV, birds rapidly produce high concentrations of antibody.48 Thus, immunizing a bird already infected with APV will do nothing. Based on everything that we know about virus infections other animals, natural infection with a virus results in permanent immunity. Therefore, it is pointless to vaccinate a previously infected bird as it is already protected. The one exception to this rule is the possibility of vaccinating hens to increase their antibody titers so that their eggs will contain higher antibody concentrations.
Will the APV vaccine protect against infection and disease?
This is an extremely important question that has yet to be answered adequately to my satisfaction. What do we know? We know that several experimental vaccines were successful in inducing a strong antibody response in previously infected birds.51,53,54 However, in birds that did not have evidence of a previous infection, the antibody response to vaccination was minimal.53 In another trial, an experimental vaccine was shown to induce a relatively strong antibody response in antibody negative birds.55 It should also be pointed out, however, that all these birds were in collections were APV had been active previously. As has been discussed, the absence of antibody does not rule out the possibility of previous infection. Thus many of these birds could have been previously infected. The response to the vaccine may have been an anamnestic response and not a primary response. The ability of the current commercial vaccine to induce an antibody response in naive birds has not been made public.
If we grant that the vaccine can induce an adequate antibody response in the naive bird, and again the data is not conclusive that it does, can the vaccine truly protect against infection? In the first study done to evaluate a vaccine, 4 blue and gold macaws were immunized and 2 were used as controls.52 Two vaccinated birds and one of the controls were challenged with live virus orally and intracloacally. The remaining 3 birds were challenged with virus by intramuscular injection. After the initial challenge, none of the birds developed disease. At this point an intravenous injection of the virus was administered and still the birds did not develop disease. The unvaccinated chick challenged by the oral and cloacal route had virus in its cloacal for 2 days. The vaccinated chicks challenged the same way did not. The one unvaccinated chick given virus by an intramuscular injection had viral DNA in its cloaca on day 2 and 3 after infection, the vaccinated chicks were cloaca negative. The vaccinated chicks developed a low antibody titer, the unvaccinated chicks developed moderate antibody titers. Based on this extremely limited trial it was concluded that the vaccine protected against infection. Subsequent vaccination and infection trails have also been reported to have been done, but the data has not been provided for scrutiny by the scientific community.54 In these trails, similar results are said to have been found, but again, the challenged birds did not die.
So where do we stand? The data we have is sketchy at best. An initial trial with too many treatment groups and too few birds, none of which died, has provided questionable results. Other trials have been eluded to, but not made public. Finally, none of the control birds that have been challenged in the APV trails have died. Therefore, we really do not know what this vaccination can do. Until we have better data, I feel that veterinarians need to carefully weigh the cost;benefit ratio with the actual risk of infection and disease on a case by case basis before recommending this vaccine. If the clients birds are at high risk for infection, then use it. In other situations you may choose not to use it at all.
DISEASE PREVENTION
The Nonbudgerigar Aviary
Each aviary will be unique in its composition of birds and management. But disease prevention will always depend on a balance of testing, the use of quarantine, and common sense management techniques.
1. In aviaries where the larger parrot species are being raised. The aviculturalist should be encouraged not to keep and breed budgerigars, lovebirds, and cockatiels. If these species are to be kept, each of these birds should be tested for APV infection. Budgerigars can be tested by serology, lovebirds and cockatiels by blood PCR.
2. Aviculturalists should be strongly encouraged to only raise their own babies and not bring babies from other sources onto their property.
3. Ideally, birds should not be moved off the aviary, exposed to other birds, and returned to the aviary. If this is going to be done, then the returning birds must be quarantined and tested.
4. If birds are going to be moved out of and then back onto the aviary. They must be 9 weeks old or older and vaccinated twice at 4 weeks and 2 weeks before they leave the aviary.
5. Traffic control in the aviary should be such that APV has a limited chance of movement from adult birds to nestlings.
6. All new birds entering the aviary must be quarantined and tested for APV by PCR before they are put in with the breeding birds. Appropriate species should also be tested for PBFDV.
Should all adult birds on the aviary be immunized? This is an important and difficult question. In the author's experience, if APV has previously been present in the aviary, most adult birds (60-90%) will have been previously infected and are naturally immune.38,44,48 Immunization of these birds would be of little benefit. If APV has not been present in the aviary, then an immunization program might be of benefit if the aviary is at high risk for exposure.
Should all adult birds in the aviary be tested for APV infection? In an ideal situation where money was not a factor, the answer would be yes. In addition, appropriate species should also be tested for PBFDV. PBFDV infected birds will shed both PBFDV and APV continually.41 Thus, testing for PBFDV in the appropriate species will eliminate both the threat of PBFDV and reduce the threat of APV. In general, virus shedding in birds other than budgerigars and cockatiels lasts less than 12 weeks. Unfortunately, some very rare individuals may shed virus for longer periods of time.6,44 This author has identified a pair of nanday conures and 2 Bourke parakeets that were found shedding virus on three cloacal samples 6 months apart.44 If long-term virus shedding is an actual phenomenon, even in an extremely small percentage of infected birds, testing of all birds or careful nursery management will be essential in preventing nestling exposure.
Another management tool that may prevent APV disease in the nursery would be to pull all eggs from the adults and incubator hatch them. As has been discussed before, this author feels that egg transmission is either rare or nonexistent.
Preventing APV Infection and Disease in Budgerigar Aviaries
1. Make sure that APV is not already present. Select a representative number of birds in the collection and have them tested by serology for evidence of infection.
2. All budgerigars entering the aviary should be seronegative.
3. Carefully restrict all movement of birds on and off the property.
a. If the aviary is a commercial aviary, dealers, feed sales persons, delivery trucks, and other bird breeders should be banned from the aviary entirely. Young birds taken to the bird dealer and rejected should not be returned to the aviary.
b. If the aviary is primarily breeding show budgerigars, then all birds going to the show should be quarantined until the end of the show season and tested by serology before they are returned to the breeding colony.
c. A modified-live vaccine may be available sometime in the future for budgerigars. This vaccine may prove useful for show budgerigars. Show birds would need to be immunized at least a month before the show season was to begin. Until the value of this vaccine is proved, these birds should be tested by cloacal swab or blood PCR before being returned to the collection.
d. The potential use of a modified live vaccine in a commercial flock has been suggested, but its actual value will need to be proved. Immunizing thousands of birds will be labor intensive and potentially very expensive. Again, it will only benefit aviaries that are initially free of the disease and not infected birds.
Preventing APV Disease in the Pet Store
The pet store is one of the most common places where APV outbreaks occur. Most pet stores get their birds from multiple sources, they sell budgerigars, lovebirds, and cockatiels, the 3 species that are most likely to be shedding virus, and many stores will acquire susceptible species when they are still nestlings. To avoid disease, pet stores can use several strategies.
1. The easiest and best method for preventing APV disease in the pet store is to buy only weaned nestlings. These birds will be old enough that if infected with APV they will not develop disease.
2. If unweaned nestlings are to be purchased, they should be raised outside of the store until weaned.
3. If nestlings must be in the store, they should be separated from all other birds, and a person designated to take care of them and no other birds. The public should not be allowed to handle these birds.
4. Vaccination may be helpful in macaws and eclectus parrots immunized at 5 and 7 weeks old, if they are not brought into the store before they are 9 weeks old.
CONTROL OF APV OUTBREAKS
Control in the Nonbudgerigar Aviary.
In most cases, once APV is introduced to a nursery it spreads rapidly, so that by the time the first case is recognized most of the nestlings are already infected. This concept is important for 2 reasons. First, vaccination at this point will do no good. Second, testing during the outbreak will only prove that the virus is widely disseminated. To save money, in most cases, the aviculturalist should be encouraged to reserve testing to determine when shedding is stopped and the chicks can be sold.
Little can be done to keep exposed chicks from disease. However, efforts should be made to improve hygiene, spread out birds, use individual syringes for hand-feeding individual chicks. The most important element to control of APV outbreaks is to stop bringing babies into the nursery. Chicks can be left in the nest to be raised by the parents or pulled and sent to another facility to be raised. It remains unclear why, but parent-raised chicks (excepting lovebirds and budgerigars) are not reported to develop APV disease. Surviving chicks will shed virus for 8 to 14 weeks, rarely as long as 16 weeks. All chicks should be found negative by blood PCR and then held for an additional 2 weeks before being sold.
After the outbreak has stopped, a close inspection of the aviary must be done. Possible sources of the virus need to be identified and tested or eliminated from the aviary. Extensive cleaning and disinfection of the nursery will also have to be done. In aviaries where the underlying source of disease has been eliminated, subsequent breeding seasons can be free of the disease.
Control of APV in Budgerigar Aviaries.
The cycle of infection and disease in the budgerigar aviary is maintained by virus shedding of young adult birds and nestlings.35 The shed virus contaminates the environment and young birds are probably infected as soon as they hatch. To break the cycle, breeding should be stopped, the young birds removed from the aviary, and the experienced adult birds moved to a clean environment. After several months, if the facility is adequately disinfected, the established breeders can be put to work again.42
It is important to note that disinfecting a small barn, shed, or other wooden structure and wooden nest boxes is difficult at best. The use of formaldehyde gas may be necessary. This type of disinfection must only be done by someone with extensive experience with this highly toxic agent.
APV INFECTION AND DISEASE IN NONPSITTACINE BIRDS
There is no doubt that one or more avian polyomaviruses can infect nonpsittacine birds. Several species of passerines have been documented to have classical APV disease.11,12,13,19,21,27,63 In the authors experience, flocks of Gouldian finches are perhaps at greatest risk. Again in the author's experience, mortality is limited to nestling and young adult finches during one breeding season, but is not seen again in the following year. Surviving birds have moderate levels of antibody that will neutralize a lovebird derived APV. APV DNA was detected in the tissues of one finch with PCR primers derived from the psittacine APV sequence, suggesting that this bird was infected with a psittacine variant. However, other studies suggest that another significantly different virus may also infect passerines.
Recently, a green aracaris has been documented with APV disease. Sequence analysis of this virus suggests that it was a psittacine APV that for some unknown reason crossed over into an aberrant host. As the bird's mate never developed evidence of infection, it was postulated that the infected bird may have been immunosuppressed.23
It is extremely disturbing, that APV has recently been documented in chicken in Europe60 and the United States.16 How this virus has reached these populations is not known. This author, however, was provided with sections of a house sparrow (Passer domesticus) from Maryland.41 This bird had characteristic lesions of APV disease, raising the possibility of APV infection in wild birds.
CONCLUSIONS
The avian polyomavirus is a single virus with a broad host range. Its ability to infect and cause disease in birds is dependent on the age of the bird, the species of the bird, the immune status of the bird, and other poorly understood factors. It is first necessary to understand the complex biology of this virus before the practitioner or the aviculturalist can begin to choose the appropriate strategies to control it. Sadly, many unsubstantiated claims have been made about this virus, APV testing, and the value of the APV vaccine. These claims have cost time and money to disprove and worst of all have created confusion in the aviculture and veterinary communities. It is hoped that this article will result in an open and frank discourse about what we know and do not know about the control of APV. None of us know all there is to know about APV and new findings will undoubtably modify our understanding of it. It is therefore essential that all views in the discussion of this virus and disease be heard and that all possibilities be considered.
APV serology (virus neutralization assay)
c/o Dr. David Phalen
Department of Large Animal Medicine and Surgery
Texas A&M University
College Station, TX 77843-4475
This assay is run once a week and takes 4 days till completion.
Serum or plasma separated from the blood is necessary for this assay.
Blood and cloacal PCR for APV and Blood for PBFDV
Research Assocaites
100 Techne Center Drive
Suite 101
Milford, Ohio 45150
513-248-4700
Table 1. Relative Species Susceptibility to APV Disease: Psittacine Birds
Highly Susceptible.... Macaws Conures Eclectus Parrots Budgerigars Ring-necked parakeets Caiques Lovebirds
Infrequently Reported with Disease.... Cockatiels Lories Amazon parrots Hawk-headed parrot
Disease is Rarely or Never Seen.... Cockatoos Quaker parrots African Grey Parrots
Table 2. Risk Factors Associated With APV Outbreaks
1. Exposure at bird shows, sales, and fairs.
2. Movement of birds in and out of the aviary.
3. Mixed collections of birds. Especially those containing lovebirds, budgerigars, and cockatiels.
4. Psittacine Beak and Feather Virus infected birds on the premises.
5. Chicks from various sources being raised in the same nursery.
6. Birds of susceptible ages in pet stores.
7. Failure to quarantine new birds or inappropriate quarantine procedures.
8. Failure to test new birds brought into the aviary.
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December 12, 1997
