Research Provides New Insights on Deadly Disease
Michael D. Samuel, USGS-National Wildlife Health Center, Madison, Wisconsin and
Greg Mensik, USFWS, Sacramento National Wildlife Refuge
Avian
cholera is a highly infectious disease caused by the bacterium Pasteurella
multocida. This bacterium has affected more than 100 species of waterbirds
and can kill swiftly, sometimes in as few as 6-12 hours after infection,
although 1-2 days is more common. Bacteria released into the environment
by dead and dying birds can subsequently infect healthy birds. As a
result, avian cholera can spread quickly through a wetland, killing
thousands of birds in a single outbreak. Although the disease may have
occurred in domestic birds in the United States before 1900, it was
not reported in migratory waterfowl until the 1940's when outbreaks
occurred near San Francisco and in the Texas Panhandle. Emergence of
avian cholera as a significant disease for North American waterfowl
began in the early 1970's and since that time the disease has continued
to spread throughout the continent. Disease outbreaks now occur annually
in California's Central Valley, a key wintering area for most waterfowl
in the Pacific Flyway. Management of the disease requires the collection
and disposal of carcasses to reduce transmission to susceptible birds.
During the winter of 1998-99 avian cholera occurred throughout much
of California when nearly 60,000 waterbirds, primarily diving ducks
and coots, were found dead (Mensik and Isola, California Waterfowl,
April/May 1999)
Avian
cholera often affects the same wetlands and bird populations each year.
Outbreaks of the disease also tend to follow the migration routes of
some species, most notably snow geese. These disease patterns have caused
wildlife biologists to hypothesize that bacteria either live year round
in affected wetlands or are transmitted by carrier birds. To investigate
whether wetlands or birds are responsible for disease outbreaks, biologists
at the USGS National Wildlife Health Center (NWHC) in Madison, Wisconsin,
collaborated closely with other biologists from the U.S. Fish and Wildlife
Service, the USGS Western Ecological Research Center (Dixon, California),
Humboldt State University (Arcata, California), Canada, Russia, and
non-governmental organizations including the California Waterfowl Association.
Research
studies focused on wetlands throughout the United States, including
Sacramento National Wildlife Refuge (see sidebar story by Meg Lehr).
Biologists at NWHC tested for living bacteria in water and sediments
of wetlands where disease outbreaks occurred. In many cases, researchers
isolated Pasteurella multocida from samples taken during or shortly
following disease outbreaks. In contrast, samples collected 1-3 months
following outbreaks or in the subsequent fall (6-9 months after outbreaks)
did not contain the deadly P. multocida bacteria. The apparent absence
of P. multocida in these samples indicates the bacteria may not survive
in the environment long enough to cause recurrent annual outbreaks.
Birds
that survive infection and become carriers may be a more likely source
of bacteria. Additional research studies concentrated on lesser snow
goose populations that nest on Wrangel Island, Russia and Banks Island,
Canada. Both populations suffer mortality from avian cholera on their
wintering grounds in California and outbreaks have also occurred at
the Banks Island nesting colony. To investigate the carrier hypothesis,
biologists traveled to the Arctic each summer to sample snow geese on
Wrangel and Banks islands. Blood samples were collected and tested for
the presence of antibodies against Pasteurella multocida. The presence
of antibodies indicated that a bird had been infected with the bacteria,
but survived the infection. Approximately eight percent of the blood
samples contained antibodies following outbreaks that killed thousands
of birds at Banks Island, compared with three percent of those collected
from Wrangel Island, where no outbreaks occurred.
In
addition, researchers used neckbands and radio-telemetry to track snow
goose migration and determine survival rates. About 50% of these marked
birds also were vaccinated against avian cholera. Based on a comparison
between vaccinated and unvaccinated geese, avian cholera mortality reduced
the annual survival during several years of the study. In some years
of the study, avian cholera mortality was equal to hunter harvest. This
finding has important implications for managing Pacific Flyway snow
geese, which have not experienced the same increases as populations
in the mid-continent of North America. In fact, winter surveys of Pacific
Flyway snow goose populations indicate these populations have remained
relatively constant or perhaps declined. Experts once believed almost
all infected birds died due to the swift and lethal nature of avian
cholera. Now they know that avian cholera can occur as a chronic infection
in some bird populations, even in seemingly disease-free areas. The
antibody prevalence following outbreaks indicates that more birds are
infected with avian cholera than previously believed and about half
of the infected birds survive. Birds that survive Pasteurella multocida
infection could be carriers of this lethal disease. If live bacteria
persist in birds for extended periods of time, carrier birds could spread
the disease along their migration routes, explaining the tendency of
avian cholera to track the movement of certain species. The presence
of antibodies does not provide conclusive evidence that snow geese are
carriers. Antibodies only confirm that birds were recently infected
with the bacteria. However, infection may have occurred from a non-lethal
strain of the bacteria, the bird may have eliminated the bacteria following
infection, or antibodies may provide immunity against recurring infection
rather than indicate carrier status.
The
overall results of this collaborative research effort suggest snow geese,
and possibly other species, are carriers of avian cholera, while providing
no evidence to indicate wetlands are the reservoir for bacteria between
outbreaks. Answers are still needed to a number of pressing questions
on avian cholera including when and why low level infection and mortality
escalates into mass die-offs in some wetlands or some years, but not
in others. To confirm that carrier birds occur and are important in
the transmission of avian cholera on wintering areas, additional research
is needed which focuses on isolating live bacteria from the tissues
of birds and determining how and when it is being shed into the environment.
Results of current and future research should bring the community of
waterfowl managers and wildlife disease researchers closer to understanding
the ecology of avian cholera and developing improved strategies to reduce
the risk of outbreaks and level of mortality caused by this disease.
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