By Andi Cooper
Waterfowl have to contend with illnesses just as people do. While mortality from disease seldom poses a severe threat to waterfowl populations, large-scale outbreaks can claim tens of thousands of birds or more. Ducks and geese are particularly vulnerable to disease in areas where large concentrations of waterfowl gather on degraded or limited habitat. Consequently, conserving healthy wetlands is crucial to minimizing waterfowl losses to disease.
While many waterfowl diseases are similar to those that affect humans, most of these pathogens pose little or no threat to people. Following is a closer look at the most common diseases suffered by waterfowl and where and when they are most likely to occur.
Waterfowl and other aquatic birds are natural reservoirs for avian influenza, or bird flu. These viruses usually settle in the intestinal tract of waterfowl and are shed through the feces of infected birds. Most strains of avian influenza circulate throughout waterfowl populations without making individual birds visibly sick. While bird flu rarely causes widespread mortality in waterfowl, the disease can be catastrophic to domestic poultry operations. In extreme cases of intense exposure, certain strains of avian influenza can be transmitted to people, such as in the case of poultry farm workers in Asia. However, these dangerous conditions do not exist in natural environments, and there have been no documented cases of bird flu being transmitted from wild birds to people.
Avian cholera, a contagious disease caused by the bacterium Pasteurella multocida, is commonly found in both domestic poultry and migratory birds. Cholera infections usually take place within 48 hours of exposure, which typically occurs through bird-to-bird contact or ingestion of contaminated food and water. In wild waterfowl, a predictable seasonal pattern exists in areas where avian cholera has become well established and is closely associated with seasonal migration patterns when birds are densely concentrated. Cholera can produce high annual mortality among waterfowl in certain areas, and some locations suffer waterfowl die-offs each year.
Birds infected with cholera often appear lethargic and drowsy and may suffer convulsions, swim in circles, or be reluctant to fly. Because infected birds die quickly, they are rarely seen by people. Humans can be infected by the same bacteria that causes avian cholera, but only in cases of extreme exposure. For example, poultry farm workers may become infected if there is a severe outbreak at a particular facility, but no cases of avian cholera have been documented from handling wild birds.
Botulism is a common waterfowl disease caused by potent toxins produced by the bacterium Clostridium botulinum. This toxin causes paralysis, which affects the leg and wing muscles of waterfowl and ultimately prevents birds from holding their head and neck erect. When waterfowl reach this stage of the disease, they often drown before succumbing to respiratory failure caused by the toxin.
Botulism is a major mortality factor in waterfowl. It can occur across the United States and Canada, but is most common in the Intermountain West and on the northern plains. In some years, this disease has claimed several million waterfowl across the continent. Typically a fall or summer disease, botulism outbreaks occur when higher temperatures favor the growth of bacteria in soil and decaying organic matter. Although many wetlands support the bacteria that cause botulism, outbreaks typically occur only in areas where the toxin is available to feeding birds.
Waterfowl are initially exposed to botulism when they eat zooplankton or invertebrates that have previously consumed the toxin. Mass outbreaks of this disease can occur when maggots feeding on the carcasses of dead waterfowl concentrate the toxin and are then consumed by other birds, creating a self-perpetuating cycle of death and disease. Once an outbreak has taken place, bacterial spores may remain in the environment for years, thereby increasing the likelihood of future outbreaks in the same area.
The best way to protect waterfowl from catastrophic disease outbreaks is to conserve an abundance of high-quality habitat in areas visited by large numbers of birds. By protecting, restoring, and enhancing wetlands in these areas, we can prevent unhealthy concentrations of waterfowl from gathering on limited habitats. This reduces the birds' susceptibility to infection as well as the potential for disease to spread throughout the larger waterfowl population.
Andi Cooper is the director of foundation relations in DU's Southern Region.