Workshop on Avian Vacuolar Myelinopathy
United States Geological
Survey, Biological Resources Discipline
National Wildlife Health Center
October 26-27, 2000
Introduction and Background:
On October 26 and 27, 2000, the USGS/BRD National Wildlife Health Center (NWHC) hosted a
half day technical workshop to discuss potential mechanisms and causes of avian vacuolar
myelinopathy(AVM), an emerging neurologic disease of wild birds in Southeastern
U.S.AVM was first recognized in bald eagles at DeGray Lake, Arkansas in
1994, and 2 years later, the disease was confirmed in a number of coots
on this and another lake in Arkansas (Thomas, et al. 1998. Epizootic vacuolar
myelinopathy of the central nervous system of bald eagles and American
coots. Veterinary Pathology. 35:479-487). Since then, AVM has been confirmed
or suspected in coots on 11 lakes in 5 states (Arkansas, North Carolina,
South Carolina, Georgia, and Texas) and also in several species of waterfowl
(mallards, ring-necked ducks, bufflehead ducks and Canada geese). The
disease is characterized by a specific lesion in the myelin of the brain
and spinal cord of affected birds, but despite extensive diagnostic and
field investigations, the causative agent of AVM is still unknown.
The AVM workshop, sponsored by the USGS Biological Resources Discipline, was convened by
NWHC to enlist the help of scientists with specialized expertise in myelin
biochemistry, neurotoxicology and neuropathology to review the underlying
biochemical mechanisms of myelin function and structure and to further
our efforts to identify the cause and sources of AVM (Attachment 1). Others
in attendance included representatives from the U.S. Fish and Wildlife
Service, Southeastern Cooperative Wildlife Disease Study, the Ross Foundation,
and numerous scientists from NWHC. The workshop began with brief overviews
by participants of previous field investigations of AVM outbreaks, diagnostic
testing, and research studies to date. Several of the pathologists in
attendance also reviewed slides and other case material. The discussion
was then focused on a series of core questions that were posed specifically
to the panel (Attachment 2) and their recommendations for further research
and diagnostic activities.
The invited panel was very encouraging and positive about the work to date on this disease,
and had many good suggestions and recommendations. To briefly summarize
the discussion, the panel concurred that:
- The NWHC pathologists'
morphological characterization of the brain lesion was correctly described
as a vacuolar myelinopathy, compatible with intra myelinic edema.
- The single most
important epizootiologic finding to date was recreation of the disease
in sentinel birds within 5 days of placement in an affected lake.
- Mallards are a
suitable sentinel animal for AVM, as confirmed by sentinel studies.
- The lesion and
epizootiology suggest that AVM is caused by a "chemical substance"
of unknown origin; microbial or other natural toxins are chief suspects.
- Detection of the
lesion in subjects with no apparent clinical signs is not an uncommon
finding in research animals with experimentally-induced myelin disorders
of this nature.
- Myelin structure
and function is similar enough between vertebrates that we might expect
to see the disease in other animals if exposed; a similar lesion has
been observed in animals and humans exposed to certain chemicals (hexachloraphene
and triethyl tin).
- The potential risk
to humans should not be ignored.
- Sentinel studies
are critical at this point and the most logical way to proceed to determine
the source and causative agent of AVM. Woodlake, a small, easily navigated
lake where AVM outbreaks have occurred annually in coots since 1996
is the most appropriate location for study.
- Findings to date
should be communicated as soon as possible in a variety of outlets (professional
meetings, journals etc.).
- Identifying the
causative agent will require multi-year support
Panel Recommendations:
A. Continue and
expand current sentinel studies and bioassays for AVM:
- Determine the
source of exposure by partitioning exposure to food, sediment, water,
and air in sentinel mallards or coots.
- Expand investigations
into the species affected by examining fish and mammals in affected
lakes for evidence of brain lesions.
- Identify the food
habits of AVM-affected birds by collecting and identifying their gut
contents.
- Expand clinical,
clinical pathology and pathology characterization of AVM-affected birds
by collecting additional organs from sentinel birds for pathologic examination;
use intravascular fixative perfusion techniques in a limited number
of sentinels to optimally prepare tissue for pathologic examination;
measure packed cell volumes and body temperature of affected birds;
collect cerebrospinal fluid.
- Document site characteristics
during sentinel studies by collecting samples of surface water for algae
examination and by measuring local water characteristics.
B. Develop a laboratory
animal model for use as a bioassay in epidemiologic investigations.
- Determine whether
mice or ducklings may serve as a laboratory bioassay for AVM. Feed livers
and gut contents from AVM-affected birds to mice and ducklings and also
consider cats and chickens, as neurologic changes may be more easily
detected clinically in these species.
- Use a positive
control in all feeding studies (i.e. hexachloraphene or triethyl tin).
- Note that the tadpole
has been employed previously to assess in vivo the status of central
myelin (optic nerve) and peripheral nerve myelin in normal and myelinotoxic
states. Explore whether the tadpole could serve as a simple biomonitor
of water safety in AVM-affected areas.
C. Conduct site
characterization of affected lakes with paired controls (case control
study)
- Identify characteristics
in common among sites where AVM has occurred by retrospective
analysis of:
water temperature fluctuations
land use, population density, vegetation
limnological parameters
water quality, water softness
physical characteristics
chemical use inventories
meteorologic data
- Test risk hypotheses
developed from retrospective analyses and fill in data gaps by site
assessment and characterization during ongoing AVM outbreaks. Collect
additional data as needed; measure water characteristics during AVM
outbreaks and conduct microbial surveys.
D. Communicate
findings to date as soon as possible.
- Increase communication
about ongoing AVM investigations with other federal agencies, including
CDC, USDA, EPA.
- Publish preliminary
sentinel findings immediately as a short note to alert biologists, managers,
and others to the site specificity of AVM and the short time of onset.
- Consider writing
a short communication in an international outlet such as Lancet or Science,
describing the disease, calling for similar reports, and addressing
the potential human health risk; panel may consider submitting this
as a group.
- Consider describing
AVM in a special symposium at the Society of Toxicology annual meeting.
E. Formally invite
the workshop participants to serve as a technical advisory panel on AVM;
consider including expertise in epidemiology, limnology, algal toxins,
statistics and public health (risk assessment).
Actions to be taken as a result of the workshop:
NWHC will take immediate
action on several of the suggestions and recommendations that resulted
from the workshop.
A. A sentinel study
has just begun on Woodlake, NC and will be expanded. Fish will be sampled
biweekly and examined for brain lesions, and attempts will also be made
to collect mammals opportunistically. Core body temperature and PCV's
will be measured in affected birds, in addition to other blood parameters
currently being measured, and a sample of all tissues will be collected
and fixed upon necropsy, including GI contents for later identification
of food items. A sample of affected coots will be anesthetized and then
perfused with fixative for light and electron microscopy; cerebrospinal
fluid will be collected from these birds prior to perfusion. Water samples
will be collected periodically to survey algae populations and various
water characteristics will be measured weekly.
B. Feeding studies
in mallards removed from the site will also begin soon as previously planned
to partition possible sources of exposure. Food items and water will be
collected from Woodlake during AVM outbreaks, transported to North Carolina
State University, and administered to mallards
held in pens. The mallards will be observed for signs of disease; sick
birds will be euthanized and brains will be collected for histologic examination.
After 7-10 days of exposure, all birds, including apparently healthy birds,
will be euthanized and their brains collected for histologic examination.
C. Experiments are
being planned to feed livers and GI contents from affected birds to mice
and ducklings (in collaboration with FWS and SCWDS) to identify an appropriate
laboratory animal model, and these studies will begin as soon as possible.
Triethyl tin will be used as a positive control as it has been show
to cause as similar lesion in mallards previously. Tadpoles may also be
considered as a model.
D. Previous proposals
have been submitted for site characterization studies (case control epidemiology
studies) and will be submitted to various outlets again, but to date no
funding has been obtained.
E. A short communication
on the preliminary sentinel findings will be written and submitted for
publication immediately. Other outlets will be considered, including letters
to Lancet or Science and special symposia. Efforts will be made to alert
other federal agencies about AVM and our investigations, particularly
when outbreaks occur.
Acknowledgment:
We are grateful to the U.S. Geological Survey, Biological Resources Discipline,
for funding this workshop on AVM.
Attachments:
AVM Workshop -
Invited Panel
Dr.
Pierre Morell
Neuroscience Center, CB#7250
University of North Carolina
Chapel Hill, NC 27599
|
Dr. Gabriele
ZuRhein
Dept. Pathology and Lab Medicine
University of Wisconsin
505 Service Memorial Institutes
1300 University Ave.
Madison, WI 53706
|
Dr.
Kinuko Suzuki
Pathology and Lab Medicine
401 Brinkhous-Bullitt Bldg
University of North Carolina
Chapel Hill, NC 27599
|
Dr.
Karl Jensen
Environmental Protection Agency
Natl. Health and Environmental Effects
Research Triangle Park, NC 27709
|
Dr.
Raymond Baggs
Dept. Of Laboratory Animal Medicine
University of Rochester Medical Center
Rochester, NY 14642
|
Dr.
Peter Spencer
Center for Research on Occupation and Environmental Toxicology
Oregon Health Science University
3181 SW Sam Jackson Park Rd. L606
Portland OR 92701 |
Dr.
Brian Popko
Neuroscience Center, CB #7250
University of North Carolina
Chapel Hill, NC 27599
|
Dr.
Ian D. Duncan
Medical Sciences
School of Veterinary Medicine
2015 Linden Dr. W
Madison, WI 53706 |
Dr.
Diane Henshel
School of Public and Environmental Affairs
Environmental Neurotoxicology
Spea Building 1315 E. 10th St.
Bloomington, IN 47405
|
|
Other Workshop Participants:
Tom
Augspurger
US Fish and Wildlife Service
Ecological Services
Raleigh, North Carolina
|
Dr.
Joe Nix
Ross Foundation
P.O. Box 335
Arkadelphia, AR 71923 |
Dr.
John Fischer
SE Cooperative Wildlife Disease Study
College of Veterinary Medicine
University of Georgia
Athens, Georgia |
|
National Wildlife
Health Center Participants:
Tonie
E. Rocke, Ph.D., Epizootiologist
Kimberli Miller, DVM, Wildlife
Disease Specialist
Nancy Thomas, DVM, MS, Endangered
Species Pathologist
Carol Meteyer, DVM, Wildlife
Pathologist
Louis Sileo, Ph.D., Pathologist
Christopher Brand Ph.D.
Field Investigations Branch Chief
Evelyn Galban, graduate student
Susan Smith, microbiologist
Kathryn Converse, Ph.D.
Wildlife Disease Specialist
Robert McLean, Ph.D. Director
NWHC
Facilitator:
Tom
Corcoran
12132 SE Foster Place
Portland, OR 97266
For more information contact: Dr. Tonie E. Rocke, National Wildlife Health Center at 608-270-2451. |