Selenium is important for a number of equine bodily functions, but too much of this trace mineral can be seriously bad news for horses. How can you tell if your horse is consuming too much selenium? Recent research results indicate that an answer is likely hiding in your horse's mane and tail.
If a horse is suspected of having selenium toxicity or deficiency, there are currently three tests veterinarians can use to assess selenium levels: whole blood, serum, or hair analysis. While most practitioners consider the whole blood test to be more accurate than the serum test, it is only useful if the selenium exposure has occurred within the previous few weeks or months.
Veterinarians can use hair analysis, however, to detect selenium concentrations in exposures that have occurred months or even years before the time of the test, and researchers recently set out to evaluate this analysis more closely.
“Several diagnostic labs use hair analysis to document selenium exposure," the team wrote in the study. "However, there have not been any reported studies in the literature showing segmenting of the hair to determine time of exposure.”
Using an actual field case involving three Quarter Horses suspected of having chronic selenium toxicity, a group of USDA researchers aimed to document selenium concentrations through mane and tail sample analysis.
The horses had been removed from a pasture located near a reclaimed phosphate mine and were suspected of having high selenium levels six months before hair samples were collected. All three horses had grazed in the pasture for six months out of the year in the three preceding years.
All three displayed hoof lesions—a common sign of selenium toxicity. One horse had severe damage to his laminae (the interlocking leaflike structures around the coffin bone that attach it to the hoof wall), resulting in lameness.
Veterinarians and researchers generally recommend owners “be careful with selenium in forage above 5 ppm (parts per million),” relayed Zane Davis, PhD, lead researcher of the study and biochemist at USDA Agricultural Research Service Poisonous Plant Research Laboratory in Logan, Utah. Analyses of several of the plant samples collected from the horses' pasture contained more than 10 times that amount.
Mane samples showed an increase and subsequent decrease in selenium concentrations ranging from 0.7 to 27.6 ppm, while the longer tail samples showed a cyclic pattern that ranged from 0.6 to 47.6 ppm. Essentially, the horses' rotation between the selenium-contaminated pasture and the noncontaminated pasture was reflected in their hair samples. In the shorter manes the researchers saw an obvious increase in selenium concentration followed by a decrease (along the length of the hair). In the longer tail samples they could see a cycle representing the three years that the horses had rotated onto and then off of this contaminated pasture.
The researchers diagnosed the horses with selenium toxicity based on these analyses and the accompanying hoof lesions.
"This was a unique field case because they were exposed over the course of three years, and it showed for the first time that the selenium concentrations are stable in hair," the team concluded. "Sampling and segmenting of equine tail and mane can be a valuable tool to document or determine exposure to excessive selenium in the diet for up to three years post exposure in some cases."
The study, "Analysis in Horse Hair as Means of Evaluating Selenium Toxicoses and Long-Term Exposures," was published in Journal of Agricultural and Food Chemistry. http://pubs.acs.org/doi/abs/10.1021/jf500861p
About the Author - Casie Bazay, NBCAAM
Casie Bazay holds a bachelor of science degree in education from Oklahoma State University. She taught middle school for ten years, but now is a nationally certified equine acupressure practitioner and freelance writer. She has owned Quarter Horses nearly her entire life and has participated in a variety of horse events including Western and English pleasure, trail riding, and speed events. She was a competitive barrel racer for many years and hopes to pursue the sport again soon.