Researchers in Utah have found that selenium concentrations can be detected in equine tail hair samples for up to three years after exposure, which means that episodes of toxicosis can be identified long after the fact.

Selenium, a naturally occurring trace mineral, is an essential nutrient, but it is poisonous at high levels. Prolonged grazing in areas where soil contains high concentrations of the mineral can lead to selenium toxicity, characterized by a sparse mane and tail, weak and brittle hooves, a dull coat and other problems. Hair samples are commonly used to test a horse’s selenium concentrations, but only recently did researchers learn how far back these measures were reliable.

The discovery came when scientists at the United States Department of Agriculture’s Poisonous Plant Research Laboratory in Logan, Utah, collected mane and tail samples from three horses showing signs of selenium toxicosis and one seemingly healthy pasturemate. 

Analysis of long tail hairs taken from the horses revealed a distinct pattern: In the affected animals, selenium concentrations spiked, dropped and spiked again in repeated cycles. Investigating possible sources of intermittent selenium exposure, the researchers discovered that for six months each year the horses grazed in a pasture where a stream contained high concentrations of the mineral, but for the rest of the year they were kept in other pastures where selenium concentrations were not high. In the longest tail hairs of the affected horses, a corresponding pattern could be detected as far back as six such cycles—or three years. Hair from the healthy horse, who had only recently been moved to the pasture, did not show a similar pattern.

“The tail hair from one of the mares was 36 inches long,” says Zane Davis, PhD. “The tail hair on the other horses was about 24 inches long. Tail hair on horses grows at an average rate of 0.9 inches per month while mane hair grows at about 0.7 to 0.8 inches per month. Consequently, you can determine the timing of a toxic exposure at some previous time dependent upon the length of the hair samples.”

The situation that led to toxicosis in these horses is uncommon, says Davis. “In the area where these horses were poisoned, land managers typically know which areas are a risk and use them cautiously by grazing less susceptible species for shorter periods of time. However, in states such as Wyoming and the Dakotas, forages from areas with high selenium are sometimes harvested and sold. Horses then ingest high levels of selenium without the owners knowing.”

Nonetheless, Davis says, this case has significant implications for other horses. “We were able to document exposure three years [later]. In addition, the study showed that the selenium concentrations are stable in the hair.” This may help diagnose cases of selenium toxicosis even years after the exposure occurs. 

Reference: “Analysis in horse hair as a means of evaluating selenium toxicoses and long-term exposures,” Journal of Agricultural and Food Chemistry, July 2014.

This article first appeared in EQUUS issue #447.

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