West Nile Virus
West Nile Virus
Our Challenge for 2004 and Beyond
General Information - West Nile virus (WNV) belongs to a family of
viruses, the flaviviridae, many of which can cause encephalitis, a
potentially fatal inflammation of the brain and spinal cord. WNV is
known to affect birds, horses, and humans - and occasionally other
mammals - and, in an odd case, alligators in Florida that had been fed
WNV-infected horsemeat.
History - The virus was discovered in Africa
in 1937, and was first detected in North America in 1999 during an
outbreak in New York. The "genotype" of the WNV detected in North
America is most similar to the virus found in an outbreak in Israel in
1998, and distinct from other strains of WNV found around the world,
which do not kill birds. Since the summer of 1999, West Nile infections
have been confirmed across all of the United States. The 2002 season
was particularly virulent, racing across many states which had no
previous presence of the virus, and sickening and killing many thousands
of birds, including a much higher proportion of raptors than in earlier
years - as well as many more people and horses than before. It is not
yet known if this huge expansion in impact was caused by a variant
strain, or simply by environmental conditions.
For example, the drought in much of the mid-west
earlier in that year was followed by rain, which may have caused a huge
bloom in mosquitoes with fewer birds (who had, perhaps, gone elsewhere
because of the drought) to keep them in check. The assault on raptors
may have been caused by the lack of other bird hosts or by raptors
eating other infected prey. However, this is all theory. In 2003,
Colorado was particularly hard hit and it struck unusually early in the
season - late spring, rather than the more normal late summer/early fall
- possibly due to heavy spring rains and then very hot weather, perfect
for hatching mosquitoes. So far in 2004, WNV continues its march west,
with Arizona and California sharing the dubious distinction of the
highest number of cases. It has also been identified in horses in Mexico
and some birds in the Caribbean and further south.
Oddly enough, in early October 2002, it was
announced that a raven in northeastern Washington was confirmed to have
WNV; in late October, a case of a crow in Washington was also
confirmed. But Washington then had no cases at all during 2003. Now,
in late summer 2004, the virus has reached Oregon (previously the only
state in the lower 48 with no cases) and is rapidly approaching Lane
County, both from the east and from the south.
Transmission - Birds serve as the host
for the virus, which is spread by mosquitoes to other birds
and animals. So far, it has been shown that only in birds can the virus
reach the level of viremia to reinfect mosquitoes. As of September
2003, West Nile Virus has been identified in 11 species of native owls,
4 species of falcons, and 15 native species of diurnal raptors,
including osprey, vultures, and bald and golden eagles. Altogether,
between captive and wild birds, WNV has been identified in 218 species
of birds in North America, with the crow family (jays, crows, ravens,
and magpies) and, in the last two years, some raptors, seeming to be
especially susceptible. It has also been found in 28 species of
mammals, besides horses and humans, and two species of reptiles. This
is clearly only the tip of the iceberg, however, as in most areas, once
the virus is confirmed present, testing at public expense ceases - and
few rehabilitators or individuals can afford to take on that cost
themselves. Even the list of affected species has not been updated by
the US Geological Service since September of 2003. Though still
considered 'newly emerging,' this disease is clearly now a permanent
fixture in the Western Hemisphere.
There are no verified cases of transmission from
birds or mammals to humans, or person to person (except through blood
transfusions and organ donations, though an infant drinking breast milk
from an infected mother whose milk showed viral particles, so far is
positive for antibodies but has shown no sign of the disease). There
is, however, strong concern and some indication that contact of WNV-contaminated
blood or tissue with an open cut could lead to transmission to people,
and thus extra precautions handling dead or sick birds are warranted.
In a laboratory environment with infected and
uninfected crows housed together, the virus has been shown to travel
bird to bird through, it is believed, direct contact. The virus was
not transmitted from infected birds to uninfected birds when they were
caged separately but in the same room. These results would suggest
that, though the virus is shed in large numbers in both saliva and
feces, aerosol transmission is less likely to occur and has not
apparently been demonstrated to occur between birds so far. However, if
an aerosol were created during washing (spraying with water) of cages
that contained a bird who shed large amounts of the virus in its feces,
then the risk of aerosol transmission to adjacent birds would be
increased.
Some blood-sucking ectoparasites (specifically hippoboscid flies) taken from infected birds have tested
positive for the virus but it is not yet known if (a) it is because they
were still blood engorged (with virus in the blood meal taken from the
infected bird), (b) if they would test positive after having processed
the blood meal, or (c) if they can, in fact, transmit the disease
through biting. Tests continue on those aspects. However, bird mites
have been documented in experiments to transmit Eastern Equine
Encephalitis virus (another flavivirus) to birds.
More critically for raptors in general, studies by
the CDC during 2002 have shown that (1) WNV can also be transmitted
through eating infected prey; (2) the virus thrives at B70C, so
is not killed by freezing; and (3) there are some birds, e.g., house
sparrows, that can act as asymptomatic carriers of the disease. The disease has been found in several mammal species this past year
(e.g., squirrels in the mid-west), more so than in previous years, and
two species of reptiles.
Human Impact - Regarding human risk,
the mosquito is necessary for transmission of West Nile Virus between
birds and people.
Although thousands of humans have been infected,
the vast majority tested who showed antibodies to the disease never knew
they were infected. For a few, flu-like symptoms arise. However, for
less than 1% of those infected, WNV can cause a dangerous and even fatal
encephalitis - mostly in older (over 50) or immune suppressed people.
Different species of mosquitoes have different predilections to bite
birds or mammals or people. Only female mosquitoes bite, only about 1%
of those carry the disease in any given area, and very few people bitten
will even know they were infected B and mosquito bites can be
prevented. The risk of serious disease or death from WNV is nowhere
near as high as it is from flu.
To avoid mosquito bites, wear long pants, long
sleeves, use a DEET-containing repellant on skin and clothes when
working outside, and avoid being outdoors at high mosquito-activity
times of the day like dawn, dusk and early evening. Remove breeding
opportunities for mosquitoes where water collects. A bigger danger than
the virus itself is over-reaction on the part of the authorities or the
public and the resulting decision to do widespread spraying of
insecticides - as happened in NY in 1999 and Louisiana and Illinois in
2002. This has the potential to cause far more harm than the virus
itself, especially to birds but also to humans. The risk to wetlands
organisms and other insects (and the birds, plants, and economies that
depend upon them) is very high, depending on the type of pesticide used.
Although there are no proven cases where it has
been passed from bird to person via blood-to-blood contact or aerosol
transmissions, the Oregon Health Service and CDC web sites have
suggested hunters wear gloves when handling the meat from dead game.
Furthermore, Dr Ward Stone, NY DEQ veterinary pathologist, whose
laboratory has been handling thousands of suspected cases in the last
four years, requires his staff to wear two layers of latex gloves, a
chain-mail glove (to prevent puncture by bone fragments), and two more
layers of latex gloves over that when dealing with potentially WNV-infected
blood and tissue. He feels that at least one case of infection among
his staff was caused by direct contact.
Although there is evidence to show it can be
transmitted between birds (via direct contact, fecal/oral, possible
aerosol transmission, or ingesting infected prey), the risk of bird to
human transmission is probably minor. There are more and more people
working with sick birds each year, in a variety of circumstances, with
few reported cases - any of which could have easily been from a mosquito
bite. Precautions for hygiene and sanitation are always in order when
handling sick animals and should be satisfactory with WNV.
West Nile Virus in Raptors - Although the
crow family by far remains the most susceptible, West Nile Virus has
caused sickness and death in huge numbers of raptors in many parts of
the United States -- the vast majority, for some as yet unknown reason,
since mid-July 2002. Great horned owls and red-tailed hawks have been
infected by the thousands, especially in the mid-west. There is now
laboratory proof that birds can get the virus by eating infected prey -
and house sparrows are a species of often asymptomatic carriers. Kay
McKeever, who runs the Owl Foundation in Ontario, Canada, has lost over
100 of her breeding owls. Other raptors deemed most susceptible are
goshawks, as falconers have found; and several zoos have lost eagles.
So far falcons have seemed to be the least susceptible of the raptors,
but there have been cases in the four main species of North American
falcons. Some species may be as yet unlisted or under-represented, as
there are also some gaps in reporting cases from rehabilitation centers
to the national data banks, especially if the confirmation did not go
through state public health testing labs but was done independently or
were not tested at all. Smaller birds, or birds with the largest or
more remote territories, are also less likely to be found - all of which
means that we do not yet truly know the full
impact this disease in having on raptor or other bird populations.
Clinical Signs of WNV in raptors seem to occur in three phases, specifically -
Phase 1:
Depression, anorexia, weight loss (in proportion to duration of
starvation), sleeping, pinched off blood feathers, elevated white blood
cell count.
Phase 2:
In addition to the above, head tremors, green urates (indicating liver
necrosis), mental dullness/central blindness and general lack of
awareness of surroundings, ataxia (clumsiness or poor equilibrium),
weakness in legs, exaggerated aggression, very high fever, polio-like
flaccid paralysis, excessive sleeping, detached retinas.
Phase 3:
More severe tremors, seizures, and death
Obviously not all birds
show all symptoms - they vary enormously but these are some of the
clinical signs that have been listed by rehabilitators seeing large
numbers of cases the last couple of summers. Equally obviously, there
are other conditions such as head trauma that can produce similar
neurological signs or many diseases that can produce elevated white
counts, fever, or loss of appetite.
As with people, some
birds can get the disease and show no outward signs - blood tests alone
reveal exposure (through the presence of antibodies) or current
infection (levels of the virus in the blood). Birds are likely to be
infective for only a few days and CDC tests have shown it takes 3-4 days
from exposure to develop viremic levels of the virus in the blood even
in asymptomatic cases. Infected birds that do not yet display clinical
signs may be more prone to other accidents, such as flying into windows,
cars or fences - so quarantine protocols of all birds must be enhanced.
Because this is a virus,
there is no one prescribed treatment. But supportive care can be
provided and it is possible for some birds to recover. In general, the
likelihood of recovery depends on what phase the bird is in. Phase 1
birds respond reasonably well to supportive care. Once they reach
Phase 2, some birds respond to supportive care, but others do not
and proceed to phase 3. Complete recovery is uncertain. Birds suffering
from the severe tremors and seizures characteristic of Phase 3
are close to death. Intervention is probably not going to alter the
course. As with other facilities which have been dealing with the
disease, once CRC starts receiving WNV-infected birds coming into the
clinic, room and volunteer time may force triage decisions in
determining the cases with the most potential for survival and the
euthanasia of others.
Recovery -
Recovered birds may have cleared the virus or
they may have become asymptomatic carriers - there is no way of knowing.
We can never say "never," but it is assumed that once a bird recovers
from WNV infection there should be little chance of its being a source
of virus to vectors or directly to other birds. However, we do not know
much at this time about the possibility of persistent WNV infections in
birds: i.e., birds that become acutely infected followed by a chronic
infection until there is a relapse at some time in the future to make
virus available again for mosquito transmission. As a guideline, it is
recommended that "recovered" birds be held inside for two weeks after
they have recovered, not so much to prevent them being a source of virus
to mosquitoes, but mostly to make sure they won't relapse - we don't
know the course of the disease that well yet. In general, however, it is
thought by researchers that recovered birds should have very low virus
counts in their blood - probably not enough to be an immediate infection
source. Oddly, some recovered birds (high antibody titers) have been
found to harbor live virus in their feather follicles - and researcher
do not understand why or what it means. It may be that since WNV is a
summer disease, when birds are molting, and blood circulates into new
feathers (i.e., blood feathers), perhaps as the feathers finish growing
and the blood supply pinches off, there is a detectable residue in the
follicle and surrounding tissue. It is not known if that could become a
source of re-infection, either to the bird itself or to a mosquito.
No one knows if there are
likely to be birds that will have permanent neurological damage,
even if they have survived the virus infection. Each case is, of
course, different - but if, during recovery, a bird plateaus for an
extended time, showing no sign of improvement, it's likely that's as
much as they are going to recover. When to euthanize must be a case by
case decision. In a small CDC study during 2002 that experimentally
infected some non-releasable birds with the 1999 virus, none of the
birds showed any clinical signs of illness, though their blood showed
viremic levels within 3-4 days; on necropsy, the only internal sign was
an enlarged spleen. We do not know if that might have a long-term
effect or if it returns to normal.
During 2003, Dr Patrick
Redig from the Raptor Center at the University of Minnesota conducted a
study with non-releasable red-tailed hawks using a new DNA vaccine made
from the 2002 virus isolated from a raptor. The birds were then sent to
the University of Louisiana, where they were experimentally infected
with the virus. This research is only beginning, as no one knows how
much vaccine is needed, or how much virus is necessary to reach an
LD50. Unfortunately, it has become impossible to find enough
non-releasable wild raptors that have not been exposed to WNV, so the
study is now continuing using domestic quail. Their susceptibility may
be completely different. Even if the vaccine proves effective, getting
it manufactured for the small captive bird market - or even
rehabilitated birds - will be difficult. Obviously, the horse market is
much larger and the equine vaccine is the one that has also been used in
birds - though no one has done a challenge test in any species except
crows (where it was not effective).
Treatment Protocols -
Because WNV is a virus, the primary treatment is supportive care (fluids, nutrition, warmth). In addition, treatment of the associated
spinal/brain inflammation with non-steroidal anti-inflammatories
such as Meloxicam, Banamine or Celebrex can be indicated. In
histological examination of brain tissue, what
is seen most often is necrosis and vacuolization, with varying degrees
of inflammation. Steroids such as Dexamethasone are not recommended
because of immuno-suppressive concerns. Vitamin B1 (thiamine) may
be helpful and is often routine with neurological conditions. Vitamin E
is possibly helpful, but care should be taken with dosing, as the
fat-soluble vitamins can be overdosed. During 2004, an experimental
treatment using a human blood pressure medicine has been tried with
varying results.
Prevention of the Disease in Captive Birds - The dose
of the equine vaccine (made by Fort Dodge) that has been used in
birds has induced no apparent negative side effects. Many zoos and
education programs have vaccinated their birds and, at least in some
birds tested so far (some raptors and cockatiels), antibodies have been
produced. We have no idea if this confers protection on vaccinated birds
- e.g., although there is some indication that it has in sandhill
cranes, it definitely does not in crows. However, it does not appear to
do any harm and it is thought that it may prepare the immune system to
react more vigorously if the bird becomes infected. During 2003,
vaccinated goshawks at a breeding facility did get sick but few died,
whereas earlier goshawks had been shown to be quite susceptible.
CRC has vaccinated all of
its resident raptors with the equine vaccine, starting in early 2003,
with boosters in 2004. We have not been inoculating birds we release,
due to the cost of the vaccine (between $15 and $30 per bird, depending
on size and whether two or three shots are given), the extra time in
captivity that would be necessitated by the vaccine protocol (2 or 3
shots given 3-4 weeks apart) after a bird has been determined to be
releasable, and the unknown efficacy of the vaccine in birds.
Protective Measures -For collections of birds,
the first defense is to protect them from mosquitoes by moving them
indoors, covering enclosures with mosquito netting, and/or using a
USDA-approved carbon dioxide mosquito trap. We have screened all the
enclosures housing our permanent resident birds and now that WNV is in
Oregon, we have also moved our crow and raven inside, into screened
cages in our clinic building. It is also critical to isolate infected
birds in mosquito-proof areas away from other birds that may be at risk,
as well as removing any ectoparasites that might possibly transmit the
disease, and to incinerate carcasses of dead birds. Long-term, we must
be prepared to deal with an ongoing threat of West Nile virus for the
foreseeable future.
Within the clinic,
housing rehabilitation birds in individual cages, without direct
contact, sanitizing (careful cleaning with diluted bleach or commercial
antiviral solution) of cages between use by different birds, and careful
handling and sanitary disposal of fecal material should greatly reduce
the risk of direct bird-to-bird transmission. CRC will use its screened
outside ward as a quarantine area, and will help regulate temperature in
there by putting plastic around it, if needed, though the mosquito
season should coincide with warm weather. Quarantine protocols will be
very important.
Control of ectoparasites
on intake is part of quarantine procedures with any new patients, and
particularly important with suspected WNV cases. Starting in 2002,
Cascades Raptor Center has stopped accepting killed or live wild prey
for food, and will not accept free range chickens/ roosters. Even
donations of mice from the Forest Service or other sources must be from
completely enclosed or screened buildings. We have checked with all our
current chick, quail, rat and mice breeders to ensure their facilities
are completely screened, and we've screened our breeding facilities as
well.
Although no transmission
to people directly from birds has been documented, a bird with the
disease can shed the virus in both saliva and feces. So once we have
cases in the CRC clinic, staff should wash hands even more
frequently, wear latex gloves when cleaning hospital cages and wash
their gloved hands between cages, dispose of cage newspapers to an
outside receptacle immediately, and wash laundry frequently, so feces do
not dry and aerosolize.
For the public - In finding or handling dead birds, minimize any direct contact.
Wear gloves if possible, place the bird in a sealable plastic bag such
as a Ziplock, and freeze. Wash hands thoroughly after handling.
Testing of dead birds in Oregon is funded for corvids
only (crows, jays, and magpies - probably ravens) that have been dead
less than 24 hours and that are part of a general die-out in a
particular area - i.e., that has been going on for 2-3 days. Collection
information is available at www.oregon.gov/DHS/ph/acd/diseases/wnile/birdform.pdf.
Pre-approval must be
given before the birds can be shipped for testing. When finding dead
birds that meet the above criteria, contact your county health
department or vector control agency. Locally, call Lane County Public
Health (682-4041) or call Environmental Health (682-4480). They will take your information and
contact Dr Emilio DeBess, State Public Health Veterinarian, at the
Oregon Department of Health Services: 503/731-4024. If the specimen is
accepted for testing, you will be given instructions on how to send it
to OSU Veterinary Diagnostic Laboratory, 30th and Washington
Way, Magruder Hall - Rm 134, PO Box 429 - Corvallis OR 97339-0429. They
only accept birds Monday through Thursday. For more information, see
www.oregon.gov/DHS/ph/acd/diseases/wnile/wnile.shtml.
No one in Oregon seems
able or willing to do testing on live birds. IDEXX offers a WNV
antibody blood test for $50, but actually sends it to Cornell for
running and their backlog is undoubtedly large. Such diagnostics,
though expensive, would help us in differential diagnoses where typical
treatment for CNS problems from, say, head trauma, would be
contra-indicated if the signs are from WNV. For the most part,
rehabilitators have simply had to get familiar with the disease and how
it manifests, and treat cases primarily with supportive care.
For more information
about West Nile virus, visit the following web sites:
www.cdc.gov/ncidod/dvbid/westnile/index.htm.
www.cindi.usgs.gov/hazard/event/west_nile/west_nile.html.
and
www.westnilemaps.usgs.gov/ for WNV maps.
(Much of the foregoing has
been taken from the FAQ sheets of The Raptor Center at the University of
Minnesota, the CDC, Wildlife Health Center, and Oregon Health Department
web sites cited above, as well as personal communications with people
working at the CDC, Wildlife Health Center, and vets and rehabilitators
who have been working with raptor cases the last three summers, or who
have been in contact with experts working in the field.)
Prepared by Louise Shimmel
Cascades Raptor Center, Eugene OR
(541) 485-1320
info@eRaptors.org
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