Continued from page 3If you are not recommending that budgerigars be vaccinated for polyomavirus, then why is it discussed that budgerigars spread polyomavirus? Is the polyomavirus that infects budgerigars the same as the virus that infects non-budgerigars?
The polyomavirus that infects budgerigars is the same virus that infects non-budgerigars. However, budgerigars respond differently to polyomavirus infection than do non-budgerigars. Non-budgerigars infected with polyomavirus will either die or recover. During the infection and recovery period, non-budgerigars may shed the virus for days to weeks (in rare cases several months) exposing other birds in the aviary to the virus. By vaccinating, you create a barrier that prevents new birds from being infected which stops the spread of the virus within a particular population of birds.
Controlling polyomavirus infections in budgerigars presents a different set of problems. A budgerigar infected with polyomavirus has been considered (correctly or incorrectly) to be infected for "life" and can shed the virus for long periods of time. In some flocks, it has been shown that 100% of the budgerigars are infected with polyomavirus.18
This high prevalence of polyomavirus in some budgerigar flocks, and the fact that an infected budgerigar continues to shed virus for extended periods, renders vaccination alone of questionable value in controlling the virus in a budgerigar flock. Additionally, the cost of the vaccine makes it difficult to economically justify in commercial budgerigar flocks. None-the-less, it is important for the avicultural industry to control polyomavirus in budgerigars because this group of birds is probably serving as a reservoir for the virus. For the past year, we have been evaluating an economically feasible testing and vaccination program to control polyomavirus in budgerigars. We are currently testing this program in flocks of budgerigars and our data is encouraging. If our data continues to be positive, we could have a testing and vaccination program that would allow budgerigar producers to establish and maintain polyomavirus-"free" vaccinated budgerigars within several years.19
Will the vaccine ever be less expensive?
According to the vaccine's manufacturer, the wholesale cost of a vaccine is completely dependent on the volume of vaccine used. The cost of producing, testing and distributing a vaccine is relatively fixed. Basically, each dose of the vaccine that will be used is divided into the fixed cost of production to determine the price per dose of vaccine. If we as a community were using as many doses of the polyomavirus vaccine as the chicken industry (tens of millions of doses per week) the vaccine would be pennies. If we were using as many doses of the vaccine as dog and cat owners (tens of millions of doses per year) the vaccine would cost dollars. As it is, we are only using tens of thousands of doses of vaccine per year. While I cannot speak for the manufacturer of the vaccine, I feel certain that the cost per dose would be substantially less if a group of veterinarians or aviculturists contracted with the manufacturer for the production of 500,000 doses of the vaccine per year.
If I vaccinate all of my birds, is there a 100% chance that I will never have polyomavirus in my closed aviary?
We do not have a 100% chance that a mechanic will properly repair a dysfunctional car, and repairing a car is a "cake walk" compared to predicting the interaction of a vaccine, an animal and a respective virus. My 1984 Blazer has been to the mechanic (an excellent mechanic I might add) 3 times over the past year to repair a clutch problem. What were the chances that the clutch would be repaired with the first attempt? Certainly, there is no such thing as 100% when dealing with biologic systems (birds, viruses, vaccines, etc). However, we have not seen a polyomavirus outbreak in a properly vaccinated aviary (adults and neonates vaccinated) and that includes aviaries in which we vaccinated to stop an ongoing polyomavirus problem.16
We used three aviaries in the field evaluation of the polyomavirus vaccine that were experiencing severe polyomavirus outbreaks at the time of vaccination. One aviary lost approximately 30% of their neonates in two consecutive breeding seasons. The affected aviculturist was preparing to leave aviculture and I convinced them to try one more breeding season using the vaccine. Following vaccination, they have had two very successful breeding seasons without a single loss to polyomavirus. In another aviary, polyomavirus killed 90% of the neonates in a 4 month period. In the two subsequent breeding seasons after vaccination, the flock has produced more than 500 chicks with no deaths from polyomavirus.16
Can a vaccinated bird "test positive" for polyomavirus?
Yes or no depending on the type of test that is used. To understand the answer to this question, it is critical that the reader comprehend that the polyomavirus vaccine contains whole virus (the viral proteins and its nucleic acid) which has been chemically inactivated (similarly, most rabies vaccines contain whole rabies virus which has been chemically inactivated). Because the virus in the polyomavirus and rabies vaccines have been chemically inactivated, they will not cause disease in a vaccinated bird. However, vaccination can cause any test which is designed to demonstrate virus exposure to be positive (remember a vaccinated bird is exposed to inactivated polyomavirus and a vaccinated dog or cat is exposed to inactivated rabies virus, thus a test designed to detect exposure can be positive). Vaccination will not cause a positive test in any assay (culture, histopathology) designed to detect an active virus infection.
Any bacteria, virus, antibiotic, toxin, chemical, etc. which enters the body (entering the body would be defined as passing through a mucosal barrier) is taken by various components of the blood and filtered out of the body through the liver, spleen or kidneys. While an agent is present in the blood and is being filtered out of the body, it can be detected by various types of tests. DNA probe-based tests are particularly sensitive at detecting small quantities of target nucleic acid in a sample. For example, the DNA probe test for polyomavirus can detect as few as 10 copies of the small segment of viral DNA it is designed to detect. When correctly used, a test which can detect such a small quantity of target is extremely useful. When improperly used, a DNA probe-based test is subject to severe misinterpretation. Such misinterpretation is rampant in the veterinary, diagnostic laboratory and avicultural communities. DNA probe tests indicate only that the small segment of nucleic acid they are designed to detect is present in a sample, they do not indicate whether or not viable virus is present in the sample.20,21
When a bird is vaccinated, it is injected with more than 1,000,000 virus particles, each of which can contain inactivated nucleic acid. A DNA probe test will detect this inactivated nucleic acid. The immune response that the vaccine is designed to elicit occurs when defensive cells in the blood "engulf" the inactivated virus and are then filtered out of the body primarily through the spleen. While these cells are being cleared out of the body, a DNA probe test performed on blood or the cut surface of the liver, spleen or kidney can be positive because it has properly detected the presence of target nucleic acid, even though the nucleic acid is inactivated.
Vaccination should not cause a DNA probe test performed on excrement to be positive. However, if a vaccinated bird swallows polyomavirus and then a swab of the excrement is tested for viral nucleic acid, then the swab could be positive. Consider that the gastrointestinal tract is a long hose which starts at the mouth and ends at the cloaca. If you were to swallow polyomavirus (for analogy purposes only, do not try this at home) and test your feces for the presence of polyomavirus nucleic acid using a DNA probe test, you might be positive. Vaccines are designed to stimulate the immune response and any assay designed to detect antibodies to the virus should be positive for a defined period after vaccination.
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