By John M. Coluccy, Ph.D., and Nathan Pfost
Wandering through the marsh one morning on the way to our favorite hunting spot on Missouri's Grand Pass Conservation Area, we stumbled upon a large group of roosting snow geese. The birds were so close that we were showered with water droplets as geese flushed in front of us. As daylight broke, we noticed dozens of dead snow geese scattered across the pool where the flock had roosted. We examined several of the dead geese. All appeared to have died recently and were in good condition. We notified the area manager and gave him the birds that we had collected. It was later determined that these birds had succumbed to avian cholera, a highly infectious disease among waterfowl.
Through habitat conservation programs, Ducks Unlimited is working to minimize waterfowl losses to disease and ensure a healthier future for North American ducks and geese.
When thinking of waterfowl diseases, situations similar to the one we encountered often come to mind. However, this isn't always the norm. Waterfowl diseases are extremely diverse and often go undetected because they don't always result in visible effects or death.
A waterfowl disease is generally defined as any condition that affects the birds' normal functions. This broad definition covers virtually every agent that adversely affects waterfowl, including bacteria, viruses, parasites and toxins. Several factors make waterfowl particularly susceptible to disease. First, the gregarious nature of waterfowl during fall and winter places them at greater risk of transmitting infectious pathogens. Secondly, habitat loss in North America has exacerbated this vulnerability by forcing more birds to crowd into fewer suitable areas. Finally, the migratory habits of waterfowl increase the risk of spreading disease great distances from the source of an outbreak. Although diseases affect waterfowl around the world, this article focuses on the major diseases that have caused substantial die-offs of North American waterfowl. (For an update on avian influenza, see page 56).
Caused by the Pasteurella multocida bacterium, avian cholera is a highly infectious disease that can lead to death in as little as six hours, although 24 to 48 hours is typical. Death can be so quick birds may literally fall out of the sky. Under acute conditions, explosive die-offs involving more than 1,000 birds per day have occurred in wild waterfowl. Transmission occurs through bird-to-bird contact, ingestion of contaminated food or water and perhaps through the air. Avian cholera was first documented in North America during the winter of 1943-44, when an epizootic (an epidemic among animals) struck waterfowl wintering at Muleshoe National Wildlife Refuge and other areas of the Texas Panhandle. Since then, avian cholera has caused thousands of waterfowl deaths throughout North America, including recurring outbreaks in the Central Valley and Tulare Basin in California, the Klamath Basin in northern California and southern Oregon, the Texas Panhandle and the Rainwater Basin in Nebraska. Sick birds often appear lethargic and can be approached quite closely before they will attempt to escape. Other field signs include convulsions, swimming in circles and heads thrown back between the wings. Avian cholera is not considered a high-risk disease for humans because of differences in the strains that affect waterfowl and humans.
Duck plague, an acute, contagious disease caused by a herpesvirus, often results in the deaths of infected birds. Duck plague attacks the vascular system, causing hemorrhaging and death within 14 days of exposure. Transmission usually occurs through bird-to-bird contact. Duck plague was first diagnosed in North America in 1967 at a commercial duck farm in New York. Since then, the disease has caused waterfowl deaths across the United States, and reports have been increasing each decade. The first known major outbreak of this disease occurred in January 1973 at Lake Andes National Wildlife Refuge in South Dakota, where more than 40,000 mallards and small Canada geese perished from the disease. Sick birds often seek dense cover because they become sensitive to light. Extreme thirst, droopiness or bloody discharge from the vent or bill may also occur. Other symptoms include loss of wariness, inability to fly and convulsions. Duck plague affects only ducks, geese and swans and therefore presents little risk to humans.
Avian botulism, probably the most well known of waterfowl diseases, results from the ingestion of a toxin produced by the bacterium Clostridium botulinum. This disease attacks the nervous system and can result in death in as little as 24 hours. Major avian botulism epizootics have been reported throughout North America for more than a century. In 1910, avian botulism resulted in the deaths of millions of waterfowl in California and Utah, and an epizootic in 1952 killed 4 million to 5 million waterfowl across the western United States. Avian botulism affects the peripheral nerves that control voluntary muscles, resulting in varying degrees of paralysis, including symptoms such as listlessness, sagging head (sometimes called "limberneck") and drooping wings. Other field signs include paralysis of the inner eyelid, birds propelling themselves across the water using their wings and windrows of carcasses coinciding with receding water levels. While some forms of botulism can affect humans, waterfowl are typically affected by Type C botulism, which has not been found to affect humans. In addition, thorough cooking kills the botulinum toxin in food.
Lead poisoning, which occurs when waterfowl ingest spent lead shot, is a unique disease because it is caused entirely by humans. Ingestion of just a few pellets can cause death, and in some cases, a single pellet may prove lethal. At one time, an estimated 3,000 tons of lead shot were being deposited by hunters in North American wetlands each year, and the number of spent pellets in some wetlands averaged nearly 70,000 per hectare. Within the United States alone, historic annual losses of waterfowl from lead poisoning were estimated at between 1.6 million and 2.4 million birds. Afflicted birds often take several weeks to die and are characterized by an unwillingness to fly, "roof-shaped" wings, severe emaciation, including a condition known as "hatchet breast," and bright green staining around the vent. While some lead hot spots remain and periodic die-offs still occur, the introduction of nontoxic shot has curtailed lead shot deposition in North American wetlands and has become a viable long-term solution to lead poisoning.
While it is unclear how many North American waterfowl actually succumb to disease each year, it is clear that continued wetlands loss will force larger numbers of waterfowl into a smaller number of suitable areas, increasing their risk for exposure to pathogens and major disease outbreaks. Through habitat conservation programs, Ducks Unlimited is working to minimize waterfowl losses to disease and ensure a healthier future for North American ducks and geese.