Selenium is an essential element that is added to many feed supplements. Unfortunately, it has a narrow margin of safety. Too much selenium weakens the hooves, which tend to fracture when subjected to mechanical stress.

All animal species are susceptible to selenium poisoning. However, poisoning is more common in animals such as horses (and livestock) that may graze selenium-containing plants. Plants may accumulate selenium when the soil level is high—generally in alkaline soil with little rainfall. Different levels of selenium are absorbed from the soil depending on the type of plant.

Selenium toxicosis after ingestion of selenium-containing shampoos or selenium tablets is rare in pets. Severity of poisoning depends on how much was ingested and the length of exposure. Poisoning can be chronic or acute (see below). Diagnosis is based on signs, changes in tissues seen after death, and laboratory confirmation of high selenium levels in the diet (feed, forage, grains) or in blood or body tissues.

Chronic Selenium Poisoning (Alkali Disease)

Chronic selenium poisoning, also called alkali disease, usually develops when animals (livestock and horses) consume forages and grains containing selenium for many weeks or months. Affected animals are dull, very thin, and lack energy. The most distinctive signs involve the hair and the hooves. The hair coat becomes rough, and the long hairs of the tail or mane break off at the same level giving a “bob” tail and “roached” mane appearance. Abnormal growth and structure of hooves result in circular ridges and cracking of the hoof wall. Hooves may grow extremely long and turn upwards at the ends. Other effects include anemia, liver disease, fluid build-up within the abdomen, a weakened heart, and reduced reproductive performance. The animal's breath may have a garlicky odor.

Birds can also be affected with longterm selenium toxicosis. Eggs from birds in high selenium areas have a low hatching rate, and the embryos are usually deformed. Effects on development include underdeveloped feet and legs, malformed eyes, crooked beaks, and ropy feathers.

There is no specific treatment for selenium toxicosis. The source and exposure should be eliminated as soon as possible, and the animal given supportive care. Soil and forages should be tested regularly in high-selenium areas.

Acute Selenium Poisoning

Acute selenium poisoning is rare. Animals usually avoid these plants because of their offensive odor. However, when pasture is limited, these plants may be the only food available. Young animals are most susceptible. Signs are different from those of chronic toxicity and are characterized by abnormal behavior, breathing difficulty, gastrointestinal upset, and sudden death. Other signs include abnormal posture and depression, loss of appetite, unsteady gait, diarrhea, colic, increased pulse, rapid breathing, frothy discharge from the nose, and a bluish tinge to skin and mucous membranes. Death usually follows within a few hours of exposure. Multiple organs, including the lungs, liver, and kidneys, are damaged. Treatment consists of supportive care.

Last full review/revision July 2011 by Barry R. Blakley, DVM, PhD; Cheryl L. Waldner, DVM, PhD; Rob Bildfell, DVM, MSc, DACVP; William D. Black, MSc, DVM, PhD; Herman J. Boermans, DVM, MSc, PhD; Cecil F. Brownie, DVM, PhD, DABVT, DABT, DABFE, DABFM, FACFEI; Raymond Cahill-Morasco, MS, DVM; Keith A. Clark, DVM, PhD; Gregory F. Grauer, DVM, MS, DACVIM; Sharon M. Gwaltney-Brant, DVM, PhD, DABVT, DABT; Larry G. Hansen, PhD; Safdar A. Khan, DVM, MS, PhD, DABVT; Garrick C. M. Latch, MASc, PhD; Gavin L. Meerdink, DVM, DABVT; Lisa A. Murphy, VMD; Frederick W. Oehme, DVM, PhD; Gary D. Osweiler, DVM, MS, PhD, DABVT; Mary M. Schell, DVM; David G. Schmitz, DVM, MS, DACVIM; Norman R. Schneider, DVM, MSc, DABVT