THE APV VACCINE; POSSIBLY A TOOL, NOT A PANACEA

In the past two years a vaccine developed by Dr. Ritchie and co-workers at the University of Georgia and the Biommune company has been on the market. The developers of this vaccine are advocating its use in essentially all parrots, and suggest that if adequate numbers of birds are vaccinated that we can essentially eliminate APV as a problem.1 This is a noble, but flawed, concept and has caused many a bird with no risk of APV disease to be vaccinated and given false hope to aviculturalists that they can protect their nestlings by the use of this vaccine alone.

In somewhat reverse order, consider the following 2 points. First, if the vaccine is effective, which birds can it be expected to protect from infection and disease? Secondly, do we have significant and substantial data to suggest that the vaccine does work?

APV IMMUNIZATION TO PROTECT FROM DISEASE

Adults. If our goal is to prevent APV disease by immunization, then it is essential to understand basic APV biology. As has been discussed, healthy adult parrots rarely if ever develop disease. Thus, vaccinating adult birds to protect them from APV disease is unnecessary.

Nestlings. It is the nestling that when infected with APV will die. Recall, however, that only certain nestling of certain species are susceptible to disease. To protect these nestling, according to the vaccination manufacturer, nestlings should be vaccinated at 5 weeks or older and then again 2 to 3 weeks later.1 They are said to be protected 4 weeks after the first immunization. Thus the vaccine has the potential to protect susceptible chicks from infection and disease in the window of 9 to 14 weeks. A review of Figure 1 demonstrates that we cannot immunize conures at an early enough age to protect them. The same is also true for most macaw and eclectus chicks. Therefore, APV immunization cannot protect most nestlings from infection and death if they are exposed to the virus before the age of 9 weeks. For macaws and eclectus parrots raised in a virus-free environment then moved to a high risk environment at 9 weeks of age, immunization may provide them with protection against infection.

The question then arises, can we immunize adult birds so that they will pass on antibody through the egg yolk and protect their young from infection? This is a valid and important question that does not have a complete answer. In the budgerigar, antibody positive parents still have young that develop disease. It has been shown, in this species of parrot, that antibody is transferred to the egg but does not reach the chicks circulation.40 We do not know if other parrots transfer antibody to their chicks through the egg. If they do, several points need to be considered. First, antibody concentrations in adult blood have to be high enough to result in a significant concentrations of antibody being incorporate into the yolk. Therefore, adult birds would have to be immunized close to the onset of breeding season every year. Any disruption of breeding birds at this time can be expected to have some negative consequences. A second point that needs to be considered is that we do not know if antibody alone will protect from infection. However, if we assume that it does and antibody is transferred to the chick through the egg, then passive transfer may conceivably protect chicks for approximately 5 weeks after hatch.

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.