Lead has no known beneficial biological function and is toxic to animals and humans when high enough concentrations are absorbed by the body. Waterfowl and other birds are particularly susceptible to lead poisoning because they often consume shotgun pellets, bullet fragments, and fishing sinkers made of lead. The lead is broken down in the gizzard or stomach (depending on the species) and absorbed into other tissues. Lead toxicity has been recognized in humans and domestic animals for at least 2,500 years, and it was first reported in waterfowl in the United States in 1874 in Texas.
Lead poisoning has resulted in major die-offs in waterfowl and many other species of birds. The impact on upland birds and their predators is not clear, but new studies are being initiated to determine their susceptibility. Bald eagles and California condors are species of particular concern when it comes to lead poisoning. Lead toxicity that does not result in death can suppress the immune system and lead to death by infection with other disease agents like bacteria and fungi. Humans rarely experience lead poisoning as a result of eating lead-poisoned birds.
Lead poisoning has been reported in every major species of waterfowl in the United States, but it is most common in ducks, geese, swans, and loons. It also occurs with less frequency in upland game birds such as mourning doves, wild turkeys, pheasants, and quail. Eagles are the most commonly affected of the land birds; other raptors can also experience lead poisoning. Any animal, humans included, can suffer from lead poisoning following ingestion of hazardous amounts of lead, but this toxicity is much more common in birds than in other wildlife.
Lead poisoning occurs throughout North America. It has also been reported in migratory birds throughout Europe, Japan, Russia, and Australia. Birds can experience lead toxicity at any time of year, but losses are more common after a period of accumulating lead from the environment. For example, swans, geese, and puddle ducks experience their highest mortality in January and February while deaths due to lead poisoning in diving ducks are more commonly reported in the spring.
Ingestion of lead shotgun pellets is the most common source of lead poisoning for wild birds. Less commonly, birds can experience toxicity following the consumption of other lead objects such as fishing sinkers, lead bullets or their fragments, mine waste, and paint chips. Raptors suffer from lead poisoning as a result of feeding on prey containing lead. The acidic environment of the gizzard or stomach causes the objects to erode, and the lead is absorbed into the blood stream and transported to tissues throughout the body. Lead is not absorbed from pellets or bullet fragments lodged in the body.
Clinical signs vary with the species and the amount of lead absorbed. Waterfowl suffering from lead poisoning usually exhibit loss of appetite, lethargy, and greenish diarrhea that stains the feathers around the vent. They will also experience progressive muscle weakness, which first causes weak flight, then an inability to fly, followed by inability to walk, coma, and death. Affected birds often hold their wings in a "roof shaped" position, followed by wing droop. These chronically affected birds will also exhibit progressive weight loss and will become emaciated. In affected Canada geese, the neck may appear bent during flight, and the head is often swollen. Raptors experiencing lead poisoning usually show nonspecific signs including generalized weakness, depression, and weight loss. They may also exhibit neurological signs and labored breathing. Sudden death without other clinical signs can occur in any bird that ingests large quantities of lead.
At necropsy, waterfowl will often have food or sand impacted within the esophagus and stomach, while raptors usually have an empty stomach. The gall bladder may be engorged with bile, and the normally yellow gizzard lining may be green from bile staining. Lead pellets may or may not be found in the stomach.
Demonstration of lead in the stomach at necropsy or using X-rays can be helpful, but is not diagnostic for lead poisoning. Post mortem diagnosis is reached by measuring lead levels within the liver or kidney. Blood lead levels can also be used for ante-mortem diagnosis of lead poisoning.
Treatment by removing the lead from the stomach and using lead neutralizing agents can be attempted in captive birds and individuals of rare species, but success rates are low unless the toxicity is addressed early. Treatment is usually not attempted in wild free-ranging birds.
The use of lead shot for hunting waterfowl has been banned in the United States since 1991, and this has resulted in a decrease in lead poisoning in waterfowl. Government agencies are considering banning the use of lead fishing sinkers and lead projectiles of all sorts in upland and big game hunting because these practices continue to contaminate the environment. When outbreaks of lead poisoning occur, sick and dead birds must be removed promptly to prevent scavengers from becoming poisoned as well. Wildlife managers must also attempt to prevent birds from using the contaminated area.
Friend, M. Lead. Pages 317-334 in M. Friend, and J. C. Franson, technical editors. Field Manual of Wildlife Diseases: Birds. United States Geological Survey.
Michigan Department of Natural Resources. Wildlife Disease. Lead Poisoning. http://www.michigan.gov/dnr/1,1607,7-153-10370_12150_12220-26676--,00.html
National Wildlife Health Center. 2011. Lead Poisoning. United States Geological Survery. http://www.nwhc.usgs.gov/disease_information/lead_poisoning/.
Locke, L. N., and N. J. Thomas. 1996. Lead Poisoning of Waterfowl and Raptors. Pages 108-117 in A. Fairbrother, L. N. Locke, and G. L. Hoff, editors. Noninfectious Diseases of Wildlife, Second Edition. Iowa State University Press, Ames, Iowa, USA.