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Chapter 47 - Salt

Authors: J. Christian Franson and Milton Friend

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Synonyms

Water deprivation, salt encrustation

Cause

Animals become victims of salt poisoning or toxicosis when toxic levels of sodium and chloride accumulate in the blood after they ingest large amounts of salt or, in some species, are deprived of water. For birds, salt sources may include saline water and road salt.

Normally, the salt glands of birds (Fig. 47.1) excrete sodium and chloride to maintain the proper physiologic chemical balance. However, when there has been insufficient time for acclimation of the salt gland to the saline environment, or when salt gland function is compromised by exposure to certain pesticides or oil, the electrolyte balance of the blood may be upset by the excess sodium and chloride, resulting in toxicosis. Salt accumulation on the outside of the body, or salt encrustation, is a greater problem for waterbirds that use very saline waters than is salt toxicosis. Salt encrustation can lead to exertion, acute muscle degeneration, and eventual drowning during the struggle to escape entrapment.

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Species Affected

This infrequently reported toxicosis has affected gallinaceous birds, such as pheasants, and rock doves that consumed road salt and migratory waterbirds forced to use highly saline water. Mortality from salt encrustation most often involves diving ducks.

Distribution

Salt poisoning and salt encrustation can occur anywhere that birds use saline environments. However, salt poisoning may be more likely in northern latitudes where saline lakes remain open while nearby freshwater habitats freeze over and where salt is used for removing ice from roadways.

Seasonality

Salt poisoning and salt encrustation may affect birds at any time of the year. In winter or early spring, terrestrial birds may consume road salt for grit and mineral content. Migratory waterbirds are more likely to be poisoned during late autumn migration after they have spent several months on freshwater nesting grounds. Cold snaps that freeze freshwater areas along the migratory route may force birds to use more saline waters that remain open because of the high salt content. High winds can contribute to salt encrustation by continually covering birds with salt-laden water.

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Field Signs

Clinical signs of salt poisoning may include muscle weakness, partial paralysis, and difficult breathing, all of which can be caused by a variety of other toxicoses. Carcasses may or may not be covered with salt (Fig. 47.2).

Figure 47.1
Click Photo for full-page viewThe salt glands of birds are located just above the eyes (arrows).

 

Figure 47.2 Salt encrustation may completely cover the bird with salt (A), or salt may accumulate on margins of feathers (B).
Click Photo for full-page view(A) Click Photo for full-page view(B)

Gross Lesions

Gross lesions are nonspecific, and they may include reddening of the brain surface (Fig. 47.3), visceral gout (Fig. 47.4), fluid accumulation in the lungs, small hemorrhages on the viscera, and erosions on the surfaces of the eyes.

Diagnosis

Finding a source of salt exposure lends support to a sometimes difficult diagnosis of salt poisoning. Salt on the feathers provides further evidence, but is not in itself diagnostic. Refrigerated blood and frozen as well as formalin-fixed brain are the best tissues to collect for laboratory analysis. Because the body maintains a constant internal environment or homeostasis, sodium concentrations in these tissues normally deviate very little. Therefore, a comparison of sodium concentrations between suspect and reference specimens can be used to support a diagnosis of salt poisoning. Microscopic examination of formalin-fixed brain tissue is also useful when salt poisoning is suspected.

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Control

Birds that are on highly saline lakes can be hazed to freshwater areas, if such areas exist nearby. Road salt should be used sparingly and should be stored out of reach of wildlife. Management practices that may expose birds to compounds that interfere with salt gland function, such as applications of organophosphorus and carbamate pesticides, should be done only when necessary and should be scheduled to allow arriving birds maximum time to adapt to saline environments.

Figure 47.3
Click Photo for full-page viewThe brains of salt-poisoned birds are sometimes very red and congested.

 

Figure 47.4 (A and B) Visceral gout, or accumulation of gritty uric acid deposits on visceral surfaces, is a nonspecific lesion that is sometimes associated with salt poisoning.
Click Photo for full-page view(A) Click Photo for full-page view(B)

Human Health Considerations

None.

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Supplemental Reading

  • Friend, M., and Abel, J.H., Jr., 1976, Inhibition of mallard salt
    gland function by DDE and organophosphates, in Page, L.A.,
    ed., Wildlife Disease: New York, N.Y., Plenum Press, p. 261–269.
  • Trainer, D.O., and Karstad, L., 1960, Salt poisoning in Wisconsin
    wildlife: Journal of the American Veterinary Association
    v. 136, p. 14–17.
  • Windingstad, R.M., Kartch, F.X., Stroud, R.K., and Smith, M.R.,
    1987, Salt toxicosis in waterfowl in North Dakota: Journal of
    Wildlife Diseases, v. 23, p. 443–446.
  • Wobeser, G.A., 1997, Salt and saline water, in Diseases of wild
    waterfowl (2nd ed): New York, N.Y., Plenum Press,
    p. 204–207.

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