Description

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 spent lead shot or fishing sinkers and jig heads during normal feeding activities. When the lead reaches the acidic environment of the gizzard (ducks, geese and swans) or the ventriculus (loons), 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.

Distribution

Ducks, geese, swans and loons 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) and raptors, 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 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 lead poisoning include lowered food intake, weakness, weight loss, drooping wings, inability to fly, and green watery diarrhea. In ducks, geese, and swans, necropsy may reveal an enlarged gallbladder, impacted proventriculus, and a cracked, green-stained, peeling gizzard lining, with or without lead shot present. In loons, the ventriculus lining may be stained dark green and will have the piece of lead present. Blood smears may reveal a slight anemia. On microscopic examination, acid-fast inclusion bodies may be seen in kidney tubular epithelial cells.

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, and increased predation due to anemia and weakened muscles.

Diagnosis

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 10 µg/g (dry 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 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., now requires the use of non-toxic steel or bismuth shot for waterfowl hunting.

Significance

The switch from lead to non-toxic shot has significantly reduced the number of birds dying from lead poisoning in Michigan and in the U.S.

Federal (U.S. and Canada) and state governments are addressing the issue of lead usage in fishing tackle and many groups (the Environmental Protection Agency, the National Wildlife Federation and the Environmental Defense Fund) have asked for investigations into the health dangers of lead. Some groups (the Environmental Defense Fund) have requested that lead be prohibited in fishing tackle and some states have considered legislation to do just that. The U.S. and Canadian governments are also considering a ban on lead fishing sinkers. The significance of these actions will be determined at a later date.

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