Research Provides New Insights on Deadly Disease

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.