Left: Lead sinker, fishing tackle and gizzard stones removed from loon gizzard. Center: Gizzard stones and lead sinker in loon gizzard. Right: Radiograph showing ingested lead fragments in the ventriculus of a bald eagle.
Lead poisoning has been recognized as a mortality factor in waterfowl since the late 1800's. Lead poisoning cases today are either the result of ingestion of bullet fragments, spent lead shot or fishing sinkers and jig heads during normal feeding activities. When the lead reaches the acidic environment of the gizzard (loons, ducks, geese and swans) or the ventriculus (eagles), it is worn down, dissolved, and absorbed into body tissues. Once the lead reaches toxic levels in the tissues, muscle paralysis and associated complications result in death.
Eagles, loons, ducks, geese, and swans are the animals most commonly affected by lead ingestion, however, upland game birds including mourning doves, wild turkeys, pheasants, and quail are occasionally affected. Lead poisoning has also been noted in small mammals (raccoon) presumably from the ingestion of lead contaminated prey. In ducks, geese and swans, lead poisoning is most commonly seen during migration in the late fall and early spring. In heavily contaminated areas, cases may be seen at any time of year.
Transmission and Development
When lead is ingested, the fragments, pellets, sinkers, or jig heads lie in the gizzard or ventriculus and begin to erode. The lead enters the circulatory system and mimics the movement of calcium. It becomes stored within the bones and is excreted via the bile into the feces.
Clinical Signs and Pathology
Clinical signs of chronic lead poisoning include lowered food intake, weakness, weight loss, drooping wings, inability to fly, and green watery diarrhea. Clinical signs of acute lead poisoning are similar to those seen with chronic lead poisoning but the bird does not experience weight loss because the lead levels become elevated quickly due to the quantity of lead consumed. In ducks, geese, and swans, a necropsy may reveal an enlarged gallbladder, impacted proventriculus, and a cracked, green-stained, peeling gizzard lining, with or without lead shot present. In loons, a necropsy examination shows a bird in poor physical condition, an enlarged gall bladder and a green-stained gizzard lining with a piece of lead present. In eagles a necropsy will reveal an enlarged gall bladder. Small lead fragments may be present in the ventriculus, but in many cases the lead has passed through the digestive system by the time of the bird's death. The bird may be in very good to poor physical condition depending on whether the bird has been acutely or chronically poisoned. Blood smears may reveal a slight anemia. On microscopic examination, acid-fast inclusion bodies may be seen in kidney tubular epithelial cells.
In loons usually a single jig head or sinker is found and in eagles a small number of small lead fragments may be present. In waterfowl a dose as small as 1 pellet can result in anemia, while a lethal dose of 5 or more pellets can result in death due to heart attack or muscle paralysis. Occasionally cephalic edema (swollen head) may be seen in Canada geese.
It has been proposed that the mortality directly due to lead poisoning may be secondary to the losses due to "non-lethal" effects of lead such as reproductive problems, increased susceptibility to disease and infection, behavioral changes and increased predation due to anemia and weakened muscles.
Antemortem diagnosis can be made on suspect lead poisoning cases using blood lead levels. Microscopic examination of red blood cells for red fluorescence has also been used. The most accurate postmortem diagnosis is lead analysis of liver and kidney tissue. Liver lead levels in excess of 6 µg/g (wet weight) are considered diagnostic for lead poisoning.
Treatment and Control
In most instances it is not possible to treat the affected birds for lead poisoning. However, if a bird is found in the early stages of lead poisoning, there is a treatment which appears effective. This consists of removal of the lead from the gastrointestinal tract with enemas, laxatives, emetics, or surgery to prevent further absorption. Chelating agents (CaEDTA) can then be used to remove the lead from the body by the formation of non-toxic complexes excreted by the kidney.
Control of lead poisoning problem areas on land consists of plowing the areas to lessen the availability of spent shot to birds. Michigan, as well as all of the states in the U.S., requires the use of non-toxic shot for waterfowl hunting. Several states have restrictions on the use of lead sinkers and jigs and a few have restrictions that prohibit the use of lead bullets. In Michigan, there are no restrictions on the use of lead fishing tackle or lead ammunition used for upland game bird, small game or big game hunting.
The switch from lead to non-toxic shot has significantly reduced the number of waterfowl dying from lead poisoning in Michigan and in the U.S. Mortality still continues to be seen however in loons and eagles, as the sources of lead consumed by the two species are still legal to use (lead sinkers and jigs and lead bullets, respectively).
Federal (European, Canadian and U.S.) and state governments are addressing the issue of lead usage in fishing tackle and ammunition. These foreign governments (Canada, Great Britain and Denmark) and the U.S. government (3 National Wildlife Refuges and 6 states which include New Hampshire, Massachusetts, Maine, New York, Vermont and Washington) have restrictions in place on lead fishing tackle that can be sold and/or used. Many groups (the National Wildlife Federation, the Environmental Defense Fund and the American Bird Conservancy) have asked for investigations into the health dangers of lead and its use by anglers and hunters. The Environmental Protection Agency has been petitioned and sued by various organizations to ban the sale of lead shot, bullets and sinkers. Legislation has been introduced in Washington, D.C. to exclude ammunition and fishing tackle from the Toxic Substances Control Act.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.