Continued from page 1Answers to Commonly Asked Questions Concerning Polyomavirus
Below are some questions submitted by a group of aviculturists concerning the management of polyomavirus.
If a bird has already been infected with polyomavirus, does the vaccine kill the polyomavirus in the bird?
To begin this discussion on polyomavirus, it is important to understand that polyomavirus appears to behave differently in budgerigars than in non-budgerigar psittacine birds. The answers to all the submitted questions will be based on controlling polyomavirus in non-budgerigar psittacine birds. As a general rule, a non-budgerigar psittacine bird that is infected with avian polyomavirus will either die or its immune system will mount an appropriate response and clear the virus from the body. This response is similar to what one would expect with parvovirus in dogs; a dog infected with parvovirus either dies or is able to clear the virus from the body and recovers.
To answer the specific question, experimental data suggest that the vaccine does not hurt nor help birds that are already infected, just as a "flu" vaccine would not help you if you were infected with influenza virus before you were vaccinated. Remember that vaccines are designed to prevent infections not treat them.
If a bird is fed a nutritional diet, housed in an enclosure that is cleaned regularly and is otherwise healthy, can it still be infected with polyomavirus?
Absolutely! Most aviculturists would consider themselves to be relatively healthy and feel that they live in a clean home yet each of us is susceptible to infection by influenza virus (the "flu"), rhinovirus (a common cause of "colds"), rabies virus or herpesvirus (the cold sore virus). Were it not for widespread use of vaccines, these same "healthy" aviculturists would be readily susceptible to infections with poliovirus and human poxvirus (human poxvirus is now considered eradicated, largely due to a worldwide vaccination program). With this said, a good plane of nutrition and a clean living environment should help strengthen any animal's immune system and may reduce the severity of a virus infection, or may help an animal resist secondary invaders (bacteria, fungi, parasites) that can take advantage of a weakened defense system. For example, an aviculturist who consumes a balanced diet, gets plenty of exercise and obtains a normal amount of sleep can still be infected with influenza virus, but they may develop a less severe disease of shorter duration than an aviculturist that eats poorly, smokes and is sleep deprived. As an observation, aviculturists that experience polyomavirus outbreaks frequently mention that their largest, most robust, "healthiest chick" was the first to die.
If birds experimentally infected with polyomavirus do not develop the same signs of disease as those that are naturally infected, then how can a vaccine be developed and tested?
Disease is actually a complex process that involves an interaction between all the microbes found in an animal's body and the animal's response to these potentially infectious agents. There is an in-depth discussion of the factors that control when an infection might cause a disease, and how a bird responds to viruses in Avian Viruses: Function and Control. As a brief answer to the question, many infectious agents (bacteria, parasites and viruses) that cause severe disease in naturally infected animals will cause mild or inapparent disease in experimentally infected animals. For example, parvovirus can cause severe diarrhea and death in naturally infected dogs, yet the same virus recovered from these severely affected dogs may cause mild or inapparent infections in experimentally infected dogs. As another example, panleukopenia virus causes severe disease in naturally infected cats but may cause mild disease in experimentally infected cats. The same situation occurs with polyomavirus in non-budgerigar psittacine birds; virus recovered from birds that have died from polyomavirus causes only lethargy, anorexia and transient diarrhea in experimentally infected birds.
How then was a vaccine developed to control parvovirus in dogs, panleukopenia virus in cats and polyomavirus in birds?
When a virus infects a cell, it begins the process of replication with subsequent production of thousands to millions of new virus particles that are then released from the originally infected cell and move to new cells. When a sufficient number of new cells have been damaged, disease occurs. Vaccines are designed to prevent a virus that does enter the body from starting the process of uncontrolled replication and thereby prevent disease. A virus that does not cause severe disease in experimentally infected animals can still be detected by examining the tissues for the presence of the virus, by detecting a rise in antibody titer indicating that an active infection has occurred or by documenting any mild clinical changes that do occur. The polyomavirus vaccine was evaluated for efficacy by exposing vaccinated and unvaccinated birds to viable (live) virus. Unvaccinated birds exhibited lethargy, anorexia and mild diarrhea, developed a rapid increase in antibody titer and the virus was recovered from their tissues. By comparison, successfully vaccinated birds remained clinically unaffected, there was no significant change in antibody titer and the virus was not recovered from their tissues.
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