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