INTRODUCTION
The avian polyomavirus is one of the most significant viral pathogens of
cage birds. It results in substantial economic losses for aviculturalists
and pet store owners each year. The biology of this virus is complex and as
a result veterinarians and aviculturalists alike are often very confused
about how to best prevent this virus infection and once confronted with it,
how to minimize its impact. This confusion is exacerbated by the current
debate that is on going in the research community about the nature of this
virus and its control. The two sides of this debate are represented by Dr.
Branson Ritchie and members of his research team at the University of
Georgia, on the one hand and on the other by Dr. Jack Gaskin at the
University of Florida, Drs. Bob Dahlhausen and Steve Radabaugh at Research
Associates in Ohio, and myself. Dr. Ritchie has used many forums to discuss
his views and feels strongly that vaccination is an important and
economically feasible means of control of this disease. Dr. Gaskin, in a
letter to the editor of the Journal of Avian Medicine and Surgery has
expressed some serious reservations about the usefulness of a polyomavirus
vaccine.15 The issue of testing and its value and which test to use has also
be a source of contention. It is the purpose of this document to address
these issues, both for the aviculturalist and the veterinarian. I feel that
his article is timely, as our knowledge biology and behavior of this virus
has grown significantly in the past few years.
AVIAN POLYOMAVIRUS: A DEFINITION AND
HISTORY
The avian polyomavirus was first recognized in the early 1980's in the
southeastern and southcentral United States 4,9,10 and in Ontario, Canada in
budgerigars.2,3 It was called the Budgerigar Fledgling Disease Virus. It was
found to be a nonenveloped, DNA virus and based on its size, shape, and DNA
content it was classified as a papovavirus.2,3,4,9,10,49 The Papovaviridae
contain two very different virus families, the papillomaviruses and the
polyomaviruses. With further investigation, it was determined that the
Budgerigar Fledgling Disease Virus is a polyomavirus. 26,29,57 Subsequently,
the virus was found to infect many different species of psittacine birds
(parrots) and thus it is generally the convention to call it the avian
polyomavirus (APV).5,17,20,35,45 APV is wide spread and can be found in most
countries of the world where psittacine birds are raised.29,31,32,59,61 As
will become clearly apparent, generalizations about this virus cannot be
made and over simplification about the issues of infection and disease,
while convenient, are often misleading.
AVIAN POLYOMAVIRUS DISEASE
Budgerigars
In the budgerigar, disease and death is confined to nestlings between 10
and 25 days of age.2,3,4,9 Budgerigar breeders first detect this problem in
their flocks when there is a sudden increase in the number of dead nestlings
in the nest boxes. The signs of APV disease in budgerigar nestlings are
somewhat variable. Most often, the young birds experience an abbreviated
course of disease. At death, birds are found to be stunted, to have abnormal
feather development, skin discoloration, abdominal distension, ascites
(fluid in the abdomen), enlargement of the liver with localized areas of
hepatic necrosis (cell death), and scattered areas of hemorrhage. In some
outbreaks, the virus attacks the cerebellum (a portion of the brain) and
these birds will show head tremors. Microscopic examination of the tissues
from these birds reveals virus inclusion bodies in cells of multiple organ
systems, including the liver, spleen, kidney, feather follicles, skin,
esophagus, brain, and heart.2,3,4,9,10,28,29
Not every budgerigar infected with APV will die. Some survivors will
never become outwardly ill and will show no signs of infection. Other
nestlings will fail to develop their primary and secondary wing feathers
and/or their tail feathers.3,18,22 These birds have been referred to as
runners or creepers and this form of the disease has been described as
French molt. It is extremely important to note that another virus, the
Psittacine Beak and Feather Disease Virus, can also cause similar signs. It
is possible that one or more additional diseases, may also cause feather
disease in nestling budgerigars.
Not all budgerigars appear to be equally susceptible to infection and
disease. In one study in the United States, English budgerigars were rarely
found to be infected with APV although they were housed with other birds
shedding the virus.44
Nonbudgerigar parrots
Nonbudgerigar parrots are also susceptible to avian polyomavirus
infection.5,17,20,32,33,58 Some are highly susceptible to disease, while
others rarely if ever develop disease (Table 1).37 APV-disease in these
birds occurs at different ages in different birds (Fig. 1). In conures,
deaths typically occur in birds less than 6 weeks of age. Deaths in macaws
and eclectus parrots occur in birds 14 weeks and younger. Most, possibly
all, of the nestlings lost are being hand fed.5,17,20,39 Infected nestlings
appear healthy, show very few premonitory signs, and then die suddenly. When
signs do occur, they proceed death by only a few hours. Observant owners may
notice delayed crop emptying, weakness, a generalized pallor, or bruising
under the skin in the preceding hours before death. Yellow discoloration of
the urates is another rare observation.5,17,20 Dr. Susan Clubb was able to
predict which birds would die, up to 24 hrs before their death, by pulling
out growing feathers. Birds developing disease would bleed extensively from
the feather follicle.5
Necropsy findings commonly include generalized pallor with subcutaneous
and subsersoal hemorrhage and enlargement of the spleen and liver. Less
commonly, acites and pericardial effusion may be present. Microscopic
examination of the tissues reveals extensive areas of necrosis (cell death)
in the liver. Virus inclusion bodies are found in the spleen, mesangial
cells of the kidney, and Kupffer cells of the liver. Necrosis of splenic
cells is often massive. Less commonly, virus inclusions are found in other
organ systems including the feather follicles.5,17,20 An immune complex
glomerulopathy occurs in a significant percentage of the birds with this
disease. These complexes contain antibody and viral proteins.36,43
Lovebirds
APV disease in lovebirds is distinct enough to merit special attention.
Like the budgerigar, this disease occurs in nestling birds and inclusion
bodies can be found in multiple organs. Unlike the budgerigar, birds up to 1
year of age can also be affected.32 This unusual age susceptibility has not
been fully explained. However, in at least some of these older birds,
concurrent infection with PBFDV is also occurring and may permit APV disease
in a bird that would otherwise be resistant to it.34,41
INFECTION VERSES DISEASE
It has become evident that infection and disease are not synonymous. Many
birds are infected by the virus but only a certain, and sometimes small,
percentage of these birds will develop disease.6,7,30,44,48 Whether disease
will develop is dependent on the species of bird infected, the age of the
bird infected, whether that bird is also infected with the PBFDV, and other
factors that remain unclear. Birds that are infected and do not develop
disease still have virus replication within their bodies and shed virus in
their droppings for a period of time.6,7,30,44,48 The length of time that
virus shedding occurs, again, depends on the age the bird at the time of
infection and the species of the bird.6,7,48
Infection in Budgerigars
As previously mentioned, in the United States, the English variety of
budgerigar appears to have some resistence to APV infection.44 The most
devastating outbreaks of disease occur in large commercial aviaries of the
American variety of budgerigar where birds are bred in rooms containing tens
or hundreds of free-flighted birds. Both nestling and adult budgerigars are
susceptible to infection. Death, however, is confined to young birds between
the ages of 10 and 25 days. The nestling mortality (death) rate is often
high and may approach 100% when the virus is first introduced to an aviary.
If there is no intervention, in subsequent breeding seasons mortality rates
will decline but production will always remain depressed.34,37,38
Birds that survive infection may have abnormal feathering or appear
completely healthy.18,32,38 Survivors shed virus in their droppings and
probably their skin and feather dander for up to 6 months after infection.38
Virus shedding stops with the onset of sexual maturity or during the first
breeding cycle. The infection cycle is then maintained through the shedding
of virus by nestlings and young adult birds. Thus, birds are exposed to the
virus immediately after hatch and have virus circulating in their blood by
the time they are 7 to 10 days old.37 Fledglings and young adult birds are
also important sources of virus exposure for other birds when they are taken
to bird shows, bird marts, and sold to pet stores.
It has been suggested that egg transmission of APV occurs in the
budgerigar.10,56 This conclusion is based on 2 observations. First, intra
nuclear inclusion bodies where reported in day-old nestlings suggesting that
these birds had virus growing in them before they hatched.2 Secondly, in a
clinical trial, eggs were removed from a flock of budgerigars experiencing
an outbreak of disease and placed under the hens of a clean flock. The young
from these eggs subsequently developed disease. This author's experience,
however, does not support this conclusion. I have not seen inclusion bodies
in birds less than a week old. Also, there is another interpretation for the
results of the clinical trial. If the transferred eggs were contaminated
with virus, then the chicks could have been exposed at hatch. Additionally,
budgerigar hens eat the egg shells. Thus they could have become infected and
then passed the infection onto their young. In a paper I presented in
Utrect, The Netherlands, I found very low concentrations of APV DNA in some
embryos and very young nestling budgerigars.37 This data has also been used
to suggest that egg transmission occurs.56 These birds never developed
disease and subsequently, I found that one of the reagents used in this work
was contaminated with viral DNA. Therefore, at this point in my
understanding of APV disease in the budgerigar and other species, I feel
that there is only very limited and circumstantial evidence that egg
transmission occurs.
Dr. Branson Ritchie, citing my data, has stated that budgerigars are the
only bird that is continuously infected with APV and remain sources of virus
for life.56 In justifying this conclusion Dr. Ritchie cites one of my
publications,38 but ignores another.42 In the first publication,38 I found
that virus DNA could be detected in tissues of budgerigars at least to the
age of 4 years. Virus concentrations were highest in 6 month old birds, but
diminished in birds breeding for 4 months and were even lower in birds
continuously breeding for 17 months. Although virus DNA was found in birds
of all ages, it was not clear that the older experienced breeding birds were
actually shedding virus. In my second study,42 I took older breeding birds
that we new had been infected with virus and rested them from breeding for 7
months. These birds were then allowed to breed and their young were
monitored for signs of infection and the development of antibody to the
virus (an indication of infection). None of the 107 young birds produced by
these previously infected budgerigars developed disease. Therefore, we must
conclude that older experienced budgerigar breeders are not sources of virus
infection and even if small concentrations of virus DNA can be found in
their bodies, they do not actively shed the virus.
Infection in nonbudgerigar parrots
Susceptible birds infected with APV infection will die. Rarely, a
susceptible bird will have transient signs and survive.5 In birds resistant
to disease, infection is unapparent. In these birds, viral DNA can be first
detected in blood after which it is detected in the cloaca.7,8,48 Cloacal
samples may intermittently be negative, but generally the blood will remain
positive.7,8,48 When the bird is about to stop shedding, the blood will
become negative and within a week or two, cloacal swabs will also become
negative.48 The length of time that birds are blood and cloacal positive is
dependent on the species of bird and the age that it was infected. It
appears, for the most part, that the older the bird is at the time of
infection, the shorter the duration of shedding.7,8,48
Conures: Many, possibly most, conure
nestlings exposed to APV at six weeks of age or younger will develop disease
and die. In birds older than six weeks, APV causes an unapparent infection
(Fig. 1). In conures, unapparent infections are best detected by examining
the blood for virus DNA. Virus shedding can be expected for 4 to 8 weeks in
most birds, but up to 16 weeks in the rare individual.7,8
Macaws: Macaws are susceptible to APV
infection and disease up to approximately 14 weeks of age, after which
infection is unapparent. Peak mortality in macaws occurs from 4 to 8 weeks
of age (Fig. 1). Unapparently infected birds will become blood positive and
cloaca positive. In a recently completed study, 2 blue and gold nestlings
that survived infection shed virus 14 weeks. Two fledgling red-fronted
macaws shed virus for 10 weeks. Adult blue and gold macaws and hyacinth
macaws shed for 6 weeks or less. The nestling birds became blood negative
first, then negative on the cloacal swab.48
Eclectus parrots: Infection of
otherwise healthy nestling eclectus parrots will cause their death if they
are less than 14 weeks old (Table 1). Specific studies on the length of
virus shedding in these birds have not been done.39
Cockatoos: As a general statement,
cockatoos of any age are highly susceptible to infection with APV, but are
extremely resistant to disease. Healthy adult cockatoos are not expected to
ever develop APV disease and the same is true for nestling cockatoos under
most circumstances. In a recent study, it was found that citron-crested and
umbrella cockatoo nestlings exposed to the virus at less than 3 weeks of age
developed abnormal feathers. These birds showed transient signs of a
systemic illness, then recovered with supportive care. Older birds and other
cockatoo species remained healthy, although nearly all of them became
infected.48 Virus shedding, as determined by cloacal swab, lasted 8 to 10
weeks. Virus could be detected in the blood consistently until just before
shedding stopped. In this group of birds, cloacal swabs were not
consistently positive and several birds that were originally cloacal
positive became negative and then positive again.48
I have documented 2 cases of APV disease in nestling cockatoos that
resulted in their deaths. Both birds were also infected with the PBFDV.47
APV INFECTION AND DISEASE IN ADULT
PARROTS; THE PBFDV CONNECTION
APV readily infects adult parrots.30,36,44,48 Most infections, probably
greater than 99.9% of then, are completely asymptomatic. These birds become
infected, shed virus for a period of time, and never look or act ill. APV
disease, however, has been documented in adult birds.24,25,46,50,58 So why
do these few adult birds develop APV disease? The answer in most cases is
that they are immunosuppressed with a concurrent infection of PBFDV.24,25,46
The author has documented an outbreak of APV disease in adult eclectus
parrots.46 All birds had PBFDV. Disease has also been identified in adult
cockatoos,24,25,34,58 again when these birds were tested for PBFDV, they
have been found to be positive. I have previously mentioned that young adult
lovebirds can die with APV disease. Again, concurrent infections with PBFDV
may be the explanation for why. In the authors experience, on every occasion
that APV outbreaks have occurred in lovebirds, PBFDV could also be found in
the aviary.
PBFDV-infected birds are a common source for APV in an aviary.44
PBFDV-infected birds, like AIDS patients have a poorly functioning immune
system. Therefore, if they become infected with APV they cannot clear the
virus. Some of these birds will develop full blown APV disease and die. Most
will become persistently infected. These persistently infected birds will
then shed virus continuously from their skin and in their feather dust. This
constant virus shedding contaminates the environment and makes it likely
that it will be tracked into the nursery.
ARE CAIQUES MORE SUSCEPTIBLE TO APV
INFECTION EVEN AS ADULTS, THAN OTHER PARROTS?
One of the first reports of APV disease in adult birds documented an
outbreak where an eclectus, a painted conure, and 3 white-bellied caiques
died.50 These birds clearly had APV-disease. They were not, however, tested
for the PBFDV. So we do not know if this means that they were normal birds
that have a predilection for APV disease, or were birds infected with PBFDV
and were immunosuppressed. Since that time, the author has heard of a number
of deaths in adult caiques. However, none of these birds were tested for
PBFDV. Thus, the answer to this question remains elusive and requires
further investigation. Because PBFDV may not cause histologic evidence of
disease, the author feels that it is essential that when APV disease occurs
in adult birds or in species where it is not normally a problem, that they
be tested by DNA probes for the PBFDV.
TESTING
Currently there are 3 types of tests available for detecting APV
infection in birds; serology, examination of blood for virus DNA, and
examination of cloacal swabs for virus DNA.
Serology
Serology is the examination of the liquid portion of blood (plasma or
serum) for antibodies that are made specifically against a virus, bacteria,
or fungus. If a bird is infected with APV and survives, it will develop
antibody to the virus.38,44,62 Antibody can be detected in the budgerigar by
9 days after infection, in most other birds antibody is not present in the
blood until 2 to 3 weeks after infection.37 Antibody concentrations rise
very quickly and by 4 to 6 weeks after infection reach maximal
concentrations.48 Antibody to APV can be detected in the blood for months to
many years after infection depending on the species.38,44 Budgerigars
maintain an antibody titer for life.38 Cockatiels probably only maintain
antibody titers for about 6 months.44 However, for most parrot species,
antibody can be detected for at least 2 to 3 years following infection.44
So what does APV serology tell us? In the budgerigar, it tells us that
the bird was infected with APV. If the bird is a young adult it is probably
still shedding virus. If the bird is an older experienced breeder it is not
shedding virus and most likely will not. A positive antibody titer in a
cockatiel means that the cockatiel has been infected within the last 6
months and this bird may be shedding virus. In other parrots, it tells us
very little. If the bird has antibody, then we know that it has been
infected with virus, but we do not know whether the bird is shedding virus.
If the bird was infected recently, then it probably is shedding virus. If
the bird was infected over 16 weeks ago, then it is probably not shedding
virus, unless it is also infected with PBFDV. Therefore, with the exception
of the budgerigar, serology is generally not very helpful in detecting virus
shedding birds. Unfortunately, this test has been inappropriately used in
the past. The author is aware of people who have killed or given away their
seropositive birds without understanding that they were not necessarily
shedding virus.
The author is also concerned that not all serologic assays are the same.
The test used by most investigators is a virus neutralization assay. This
test measures both IgG and IgM and appears to be very accurate.14,30,38,48 A
complement fixation assay has also been made available for testing parrot
serum (Texas Veterinary Medical Diagnositic Laboratory, College Station,
TX). In a comparison between the virus neutralization assay and the
complement fixation assay, the complement fixation assay was only in
agreement with the virus neutralization assay 60% of the time.41 At this
writing the author strongly discourages veterinarians from using this
complement fixation assay for APV serology.
PCR assay of cloacal swabs and blood.
The polymerase chain reaction, or PCR, is an assay that has become an
incredibly important tool for the diagnosis and control of infectious
diseases. This assay takes a low concentration of the APV DNA and amplifies
it to a concentration that can be detected. Therefore, as few a 10 copies of
the virus can be detected if the test is properly preformed.35 The
sensitivity of this test is one of its greatest strengths as well as one of
its greatest weaknesses. The potential problem with this assay is that even
the smallest contamination of the sample, either at the collection site or
in the laboratory will result in a negative sample becoming positive.
Therefore, if one is testing multiple birds, it becomes very easy to get the
sample from a negative bird contaminated with the feather dust or dried
feces from a positive bird.48
Which PCR assay is better?
The original discovery that APV could be detected in the live bird was
made by Dr. Frank Niagro at the University of Georgia.30 He and his
collaborators found that APV could be detected in cloacal swabs of
unapparently infected birds. This technology was licensed to the Research
Associate Laboratories (Drs. Dahlhausen and Radabaugh) and has been offered
by them for 5 years. During this time, these scientists have modified and
improved this assay and have discovered that APV DNA can also be detected in
the blood of birds recently infected with APV.6,7 The blood-based PCR assay
has been heavily criticized by Dr. Ritchie and he has also questioned its
scientific validity. His criticism is unfounded.
Both the blood and cloaca PCR -assays will pick up most birds shedding
virus. So which one will you choose to screen birds? In a recently completed
study, Drs. Dahlhausen and Radabaugh and myself compared cloacal virus PCR,
blood PCR and serology.48 Of all the birds (>50) that were examined with
multiple tests, both tests picked up all but 1 of the birds that
seroconverted. Not all birds were positive on both tests each time. In
cocaktoos and conures, it was found that birds stayed consistently positive
with the blood PCR, while a several were intermittently positive on the
cloacal swab. As virus was cleared from the bird, the blood test became
negative first and the cloacal swab became negative 2 to 4 weeks later.
Therefore, for these species, I recommend that the blood PCR be used as a
screening tool. If it is positive, the bird should be retested in 2 to 3
months, if negative, the bird should be quarantined for 4 additional weeks
and then will be considered free of virus shedding. In macaws, we found that
in most situations both tests were positive. Virus shedding and viremia
stopped almost simultaneously. In the future, Research Associates may offer
a PCR assay that screens both blood and cloacal samples from the same bird
in the same reaction. This should be the most sensitive assay of all.8
It has been said that blood PCR testing of live birds following
vaccination or swabs of tissue in recently vaccinated birds that die, will
detect fragments of DNA from the vaccine. This assertion is totally invalid.
Recent work by Drs. Dahlhausen and Radabaugh has shown that viral DNA is
never present in the blood of nestlings vaccinated for APV. The veterinarian
must therefore conclude that if a bird is blood PCR positive, vaccinated or
not, that it is infected with APV and is most likely shedding virus.
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
immunized 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.
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.
There is a $5.00 per sample for this assay.
