by Bruce Batt, Ph.D.
Waterfowl can contract diseases that may weaken them temporarily or lead to a quick death. We don't often see chronically sick waterfowl in the wild simply because weakened birds often remain in cover as they try to recover from whatever ails them. Or they may die quickly or be caught by predators that naturally target individuals lagging behind flocks of healthy birds.
Migration itself is a severe biological stressor that sorts the weak from the strong. If a bird is injured, or sick, on northern staging areas in the fall, it will likely be left behind to become a meal for a hawk, an eagle, an owl, or a mink as it struggles to sustain itself in an increasingly unfriendly environment. During spring migration the same outcome occurs less conspicuously because stragglers aren't concentrated in patches of open water in frozen rivers or ponds where we can easily see sick birds. Also, the weakest birds have already been claimed during the winter by other causes, including hunters.
But some acute diseases kill waterfowl much more rapidly and in large numbers. The greatest scourge is botulism, a disease that has been a major concern for waterfowl managers since Ducks Unlimited's founding in 1937. In some years hundreds of thousands of waterfowl are killed by botulism. The bacteria that cause botulism occur naturally in soil and are normally benign except under certain environmental circumstances that usually occur in the heat of the summer. Exposed mud flats dry out in the heat, depleting oxygen in the soil, which allows the bacteria to become infected by a virus. The bacteria may then produce one of the most toxic poisons in all of nature.
This toxin is released when a dry mud flat becomes flooded by a wind tide or rain shower. To waterfowl, especially dabbling ducks, this looks like an ideal place to feed. When ingested incidentally by waterfowl feeding in these areas, botulism toxin attacks the birds' nervous systems, causing them to lose control of their muscles, which often results in drowning. Decaying carcasses soon become infested with fly maggots that accumulate, but are not affected by, the toxin. The deadly maggots look like tasty food items to other ducks that ingest them and suffer the same grim fate. It is not uncommon for the explosive spread of botulism to quickly claim tens of thousands of birds before wildlife agencies can respond.
Exotic Snails and Worms Threaten Waterfowl
A relatively new but growing disease in water birds, especially coots and scaup, is caused by a snail infected with tiny trematode worms. Both the snails and the trematodes were introduced to North American waters from Eurasia in the 1870s. The snails and worms have persistently expanded their range since their arrival. Outbreaks have occurred in the Great Lakes states and Montana. In the navigation pools on the upper Mississippi River, some 70,000 waterfowl have been killed by this disease since 2002. And these infected snails are becoming more numerous in central Minnesota, where waterfowl mortality is now being recorded every year. While still relatively small numbers of birds are currently being affected, waterfowl deaths caused by this disease are unwelcome in populations of scaup and other species. Just how far these deadly organisms will spread and what their ultimate impact on waterfowl will be remains to be seen.
The usual response by waterfowl managers is to pick up and bury or burn the carcasses of dead birds as quickly as possible to break the "maggot cycle." But this is quite ineffective in many situations, especially when sick birds die in wetlands with large stands of emergent vegetation like bulrush or cattail. Whenever possible, waterfowl managers also keep water on botulism-prone areas to prevent mud flats from drying out. Unfortunately, managing water levels in this manner is difficult if not impossible on many large wetlands prone to botulism outbreaks, so this disease will likely remain a serious threat to waterfowl in some years, just as it apparently has for centuries.
The other disease of great concern to waterfowl managers is avian cholera, which is among the primary causes of death from disease in all wild birds. This bacterium was transmitted by domestic fowl to wild bird populations during the late 1940s, and sporadic die-offs have occurred in waterfowl since the 1970s in Canada and the United States. Avian cholera bacteria are now endemic in waterfowl populations and appear to be carried by birds that have natural resistance to the disease.
Waterfowl are at greatest risk of contracting avian cholera while concentrated in large numbers on key staging habitats, especially in cold weather. The Rainwater Basin of Nebraska is a place where waterfowl are at particular risk of this disease. Less than 10 percent of this region's original wetlands remain intact, and spring staging populations of geese, sandhill cranes, and ducks have soared on these habitats in recent years. Major die-offs of thousands of waterfowl occurred here during the 1980s and 1990s, and managers fear another severe outbreak could occur again in this area. In an effort to keep that from happening, Ducks Unlimited and its partners are working hard to restore as many wetland basins in this region as possible to spread out staging waterfowl and hopefully lessen the potential impact of future outbreaks.
Waterfowl suffer a variety of other afflictions, which are not a threat to populations but can be deadly to individual birds. Among these diseases is duck plague. This often-fatal virus is carried by naturally resistant birds or transmitted from domestic to wild flocks. Only two major die-offs caused by this disease have been documented in recent decades, one in South Dakota, where 40,000 mallards died, and another in New York, where a few thousand black ducks and mallards died.
Other localized waterfowl deaths occur when ducks and geese eat post-harvest cereals or other grains contaminated by a fungus that produces a poisonous "aflatoxin." In addition, lead poisoning still occurs in certain wetland areas where lead shot persists and can be ingested by feeding waterfowl. Fortunately, incidence of lead poisoning is now diminishing as nontoxic shot regulations have been in place for many years. And lastly, waterfowl are regularly infected with many types of intestinal, blood, and skin parasites. While most of these parasites occur at nonlethal levels in ducks and geese, particularly severe infestations likely kill some birds, especially ducklings and goslings.
Disease is a natural source of mortality in waterfowl, but the birds are most susceptible to illness when they are under stress. And in many cases, the most severe waterfowl die-offs caused by disease can be directly linked to habitat loss. Thus the best way to ensure waterfowl—and their populations—remain healthy is to provide the birds with an abundance of high-quality wetland habitat, where they can feed, rest, and seek shelter from the elements.
What about Avian Influenza?
Just six years ago, the threat of Highly Pathogenic Avian Influenza (HPAI), dubbed H5N1, was a serious concern to public health officials worldwide. This deadly illness spread rapidly in domestic fowl throughout Asia, Africa, and Europe, and there was serious concern that wild migratory birds might carry the virus to North America.
In response, federal agencies in Canada and the United States established extensive monitoring programs in every province and state to detect any occurrence of H5N1 in wild birds. But by the end of 2009, more than 400,000 wild birds had been sampled in the United States without a single case being found. Nevertheless, human avian influenza deaths continue to occur in Asia, and the possibility remains that this virus could arrive in North America. For more information, visit our page on bird flu facts.
Dr. Bruce Batt served as DU's chief biologist for more than a decade before retiring in 2007. He continues to write extensively about waterfowl and conservation subjects for a variety of publications.