Mercury 337

Chapter 45

Mercury

Synonyms

Minamata disease

Mercury has been used by humans for over 2,000 years and

was associated with premature deaths of cinnabar (mercuric

sulfide) miners as early as 700 B.C. More recent human poisonings have been related to agricultural and industrial uses

of mercury. One of the best documented of these cases

occurred in the 1950s in Minamata Bay, Japan, when mercury was discharged into the environment and accumulated

in fish and shellfish used as human food. In addition to

human poisonings, mercury poisoning or toxicosis has been

identified in many other species.

Mercury is sometimes used to recover gold from stream

sediments, and it may pose hazards to wildlife if it is released to the environment during ore recovery. Fungicidal

treatment of seeds with mercury was common in the 1950s

and 1960s, but this agricultural practice has been largely

halted in the Northern Hemisphere.

Cause

Mercury is a heavy metal that is nonessential and toxic to

vertebrates, and it occurs in both organic and inorganic forms.

The organic forms, such as methylmercury, are generally the

most toxic. However, inorganic mercury can be transformed

into organic forms through a variety of biological processes.

Mercury occurs naturally in soils and sediments, but it is

also introduced into the environment by human activities

(Fig. 45.1).

Species Affected

Birds affected by mercury include species that are exposed

to high levels of the metal because of their feeding behavior

(Fig. 45.2). Exposure may occur through accumulation of

mercury in the aquatic food chain, agricultural uses of mercury as a fungicidal seed treatment, and from point-source

industrial and mining discharge to the environment.

Figure 45.1 Sources of mercury contamination.

Mercury

treated seeds

Paper mill

discharge

Fossil fuel

combustion

Incinerator

waste

Sewage

Wetland

Industrial

discharge
338 Field Manual of Wildlife Diseases: Birds

Species group 

and representative species

Loons

Common loon, red-throated loon

Wading birds

Common egret, great blue heron, 

 black-crowned night heron

Pelicans

Brown pelican, white pelican, gannets

Cormorants

Double-crested cormorant

Mergansers

Common merganser, red-breasted merganser

Gulls and terns

Herring gull, common tern

Pelagic seabirds

Fulmars, shearwaters

Raptors

Bald eagle, osprey, golden eagle, owls

Gallinaceous birds

Ring-necked pheasant, chukar partridge, 

grouse, quail

Figure 45.2 Avian species most likely to be exposed to mercury.
Mercury 339

Table 45.1 Reports of mercury exposure associated with mortality and sublethal

effects in free-ranging birds.

Location Species Effect

Sweden Pheasants, partridge, pigeon, Mortality

magpie, passerines

Sweden Goshawk, Eurasian sparrow- Mortality

hawk, white-tailed eagle,

peregrine falcon

The Netherlands Various raptors Morbidity and

mortality

Canada Loons, turkey vulture Mortality

Canada Common tern Poor reproduction

Scotland Golden eagle Poor reproduction

United States Bald eagle Poor reproduction

Canada Loons Poor reproduction

Sources of mercury exposure for birds that live, nest, or

feed in or near aquatic systems include industrial discharge,

acid precipitation, and high mercury levels in fish and sediments. Gallinaceous birds, such as turkey and pheasant, may

be exposed when they consume mercury-treated grain. Raptors, such as golden eagle and owls, may be secondarily exposed when they consume birds or small mammals that died

from eating treated grain.

Major bird die-offs from mercury poisoning are rarely

reported. Mortality from mercury poisoning is more of an

insidious problem involving scattered mortalities. Some instances where mercury has been associated with mortality

or sublethal effects are listed in Table 45.1.

Distribution

Mercury is present in fossil fuels and in some soils and

sediments. The release of mercury into the atmosphere from

burning of fossil fuels, the conversion of inorganic mercury

to organic methylmercury and its cycling in aquatic systems,

and accumulation through the food chain can expose wildlife to mercury and potential toxicity. Problems with mercury

poisoning in birds traditionally have occurred in northern

latitudes in areas affected by acid precipitation, at pointsources of industrial discharge, and in agricultural areas

where mercury-based seed treatments have been used.

Seasonality

Seasonality is dependent only on the movement and foraging of birds that may put them at risk of mercury exposure

while they feed in contaminated habitats.

Field Signs

Clinical signs of mercury poisoning in birds have been

documented primarily from experimental feeding studies, and

they include incoordination, tremors, weakness, ruffled feathers, and drooping eyelids. Experimental exposure of birds to

high levels of mercury has caused acute death in less than 1

hour with few signs of intoxication. In free-ranging birds,

most cases of mercury poisoning are probably more insidious, resulting in an emaciation syndrome and a variety of

sublethal effects that may act together to cause eventual death

(Table 45.2).

Gross Lesions

Birds suspected of having died of mercury poisoning often are emaciated, but no other gross lesions are noted.

Diagnosis

A diagnosis of mercury poisoning as cause of death can

seldom be made on the basis of mercury concentrations in

tissues alone. A complete necropsy examination with appropriate laboratory evaluations should be done by a qualified

diagnostic laboratory. A diagnosis is generally based on total

mercury concentrations of 20 parts per million wet weight

or more in the liver or the kidneys and by the presence of

microscopic lesions in tissues consistent with mercury poisoning. A definitive diagnosis is difficult, however, because

the amounts of residues that would indicate mercury poisoning have not been determined for most bird species. Also,

seabirds may naturally accumulate and tolerate higher levels

of mercury than nonmarine birds. Another confounding

factor is that selenium, which is an element that is essential

to health, has been found to reduce the toxicity of mercury,

and residues of both of these elements are often found in

birds. A thorough history of field observations and background information about potential agricultural and industrial uses of mercury is an invaluable supplement to the specimens submitted.
340 Field Manual of Wildlife Diseases: Birds

Control

Prevention of exposure is required to control the lethal

and sublethal effects of mercury poisoning in avian populations. Elimination of mercury discharge in industrial, mining, and sewage wastes, reduction of fossil fuel (especially

coal) combustion, reduced inputs to (and thus releases from)

municipal incinerators, and elimination of agricultural uses

will reduce the amount of mercury entering the environment

as a result of human activities. One factor to consider in the

development of new wetlands is that the accumulation of

mercury in aquatic biota is enhanced when terrestrial habitats are flooded. Little control is possible over low-level exposure to naturally occurring sources of mercury from soils

and sediment.

Human Health Considerations

Mercury is a well-documented human health hazard.

Avoid exposure to elemental mercury, which is volatile and

can be inhaled in significant amounts in enclosed areas, mercury-based seed treatments, and mercury-contaminated food.

One should wear gloves when handling carcasses, but birds

thought to have died of mercury poisoning present no special hazard because the mercury is biologically bound to tissues within the carcass.

J. Christian Franson

Table 45.2 Sublethal effects of mercury exposure from experimental studies.

Species Effect(s)

Pheasants Decreased egg weight, fertility, and hatchability

Starling Microscopic kidney lesions

Mallard duck Microscopic brain lesions, skeletal deformities;

reduced clutch size, hatchability, embryonic

growth; behavioral changes

Black duck Reduced clutch size and hatchability

Red-tailed hawk Neurologic signs of weakness and incoordination

Supplementary Reading

Eisler, R., 1987, Mercury hazards to fish, wildlife, and invertebrates: a synoptic review: U.S. Fish and Wildlife Service

Biological Report 85(1.10), 90 p.

Hecky, R.E., Ramsey, D.J., Bodaly, R.A., and Strange, N.E.,

1991, Increased methylmercury contamination in fish in newly

formed freshwater reservoirs, in Suzuki, T., and others,

Advances in mercury toxicology: New York, N.Y., Plenum

Press, p. 33–52.

Heinz, G.H., 1996, Mercury poisoning in wildlife, in Fairbrother,

A., and others, eds., Noninfectious diseases of wildlife (2nd

ed.): Ames, Iowa, Iowa State University Press, p. 118–127.

Thompson, D.R., 1996, Mercury in birds and terrestrial mammals,

in Beyer, W.N., and others, eds., Environmental contaminants

in wildlife: interpreting tissue concentrations: Boca Raton,

Fla., Lewis Publishers, p. 341–356.

Wren, C.D., Harris, S., and Harttrup, N., 1995, Ecotoxicology of

mercury and cadmium, in Hoffman, D.J., and others, eds.,

Handbook of ecotoxicology: Boca Raton, Fla., Lewis

Publishers, p. 392–423.