Low Selenium Horse Deaths in Central Oregon

Selenium Deficiency linked to the deaths of Horses in Central Oregon

                I wanted to share a story to help educate horse owners on the importance of making sure your horses’ vitamin and mineral needs are being met, especially selenium. When I talk to people in the Northwest, most know that we live in a selenium deficient area. However, when I ask them what they supplement their horse with some people reply that their horse looks fine so they aren’t deficient. But this actually very far from the truth, their horse is not actually “fine”. Vitamin and mineral deficiencies affected many different functions that you can’t see with your bare eyes. When the deficiencies are bad enough is the only time that symptoms will visually show, and when this point is reached your horse is in a ton of trouble. The supportive care to nurse them back to health, if even possible,   will be a substantial cost.


I got a call recently from a local woman, who had had quite the ordeal in the last couple months with her Arabian horses, and was wondering if I might be able to shed some light on the situation. This women breeds, raises, and trains Arabian endurance horses, and also is very active in horse rescue with over 40 horses on her property. So her story goes as follows:

On June 19th, she noticed one of her 4-year-old stallions, Memphis, behaving peculiar. He was standing over the water trough and didn’t move between the time she first noticed him and after finishing her chores. When she went to examine him closer, she realized Memphis was foaming and drooling from the mouth, and had a mouthful of wet hay. She took him to the veterinarian and ran a full blood panel. While at the veterinarian, Memphis had blood in his urine. Sadly, Memphis passed away within 36 hours. When the blood panel was returned his white blood cell count was low, but no glaring problems. Selenium status was not analyzed.

One week after Memphis passed away, the woman had two more stallions become ill. Sawyer (15) and Petey (12) were both found drooling, with a mouthful of hay, over water troughs. Upon discovery, she took them to Bend Equine. When the horses arrived at the vet clinic their swallowing reflex was gone. Complete blood panels and selenium tests revealed that selenium levels were in 3 and 5 ng/mL of whole blood! *** Adequate selenium levels are 160-275 ng/mL of whole blood.*** The stallions were immediately started on IV fluids, received E-Se shots and were started on Horse Guard. Sawyer and Petey received IV fluids for several days to recover, and have both been on Horse Guard since the incident. Both of the stallions fully recovered. Upon re-sampling, Sawyer’s selenium levels 228 ng/mL of whole blood after 3 months of supplementation.

At the beginning of September, a neighbor a mile down the road from the woman, who fed different feed, had a 20-year-old mare become ill. The mare received an E-Se shot, however, she did not receive IV fluids. She passed away within 48 hours. Selenium analysis revealed whole blood selenium concentration of 4 ng/mL of whole blood.

vial of blood

One week later, the same neighbor had another mare become ill. She was an 8 year old daughter of the mare that had passed. The owner suspected that she was extremely selenium deficient like the others and decided to not run any bloodwork on her. She was immediately given a E-Se shot and started on IV fluids. The mare made a full recovery.

After the entire ordeal the owner of the original sick horses decided to test selenium levels on two random mares at her place which received salt blocks but no other form of supplementation. Lilly, a pregnant mare on irrigated pasture and alfalfa hay, was found to be selenium deficient at 98 ng Se/mL whole blood. The deficiency could not only cause problems for the mare, such as a retained placenta, but it also put her foal in grave danger of disease because of a suppressed immune system. If the mare’s selenium status was low enough, the foal could suffer from white muscle disease and ultimately die because muscle damage to the diagram and large skeletal muscles which affect its ability to breath, stand, and suckle.

The second mare tested was Porsche, an older open mare on dry lot being fed Timothy hay and Alfalfa, was found to be selenium deficient at 58 ng Se/mL whole blood. Both of these mares were substantially lower than the threshold of what is considered adequate (160 ng Se/mL whole blood), and although the owner had not seen any signs exhibited these mares had a greater risk of contracting any disease they encounter, and wouldn’t be able perform with optimal muscle function. Without help these mares selenium status would have most likely kept declining and eventually cause potentially fatal selenium deficiency.

This story helps to drive home the point of how crucial it is to supplement with selenium in Northwest! I have heard people from the Willamette Valley say that they don’t need to supplement their horse because they buy their hay from Central Oregon. I am here to tell you that they are dreadfully wrong. If you have a selenium deficient horse you may or not be able to see subtle signs. Does your horse have tight muscles; is he prone to tying up; does he have a dull hair coat or act lethargic? All are signs that your horse is selenium deficient. However, I use the analogy of an iceberg many times. Like an iceberg, in your horse’s body there are many things that you can’t see effected by selenium deficiency that a negatively impacting your horse’s health. If you have want to assure yourself that your horse is selenium deficient without supplementation give your veterinarian to test your horse’s whole blood selenium status. Supplementing with Horse Guard, at around $0.30 per day, is pretty cheap insurance for the optimal health of your horse. It could save you costly vet bills and possibly the death of a beloved companion.

Horse Guard


About Dr. Kelsey J. Nonella Ph.D., P.A.S.

Kelsey J. Nonella, Ph.D. is an equine nutritionist who was riding horses before she could walk. Her love for horses drives her to help educate people on what their horses’ needs in order to have happy, healthy horses. Kelsey went to Cal Poly receiving a Bachelor’s of Science in Animal Science and then onto West Texas A&M, where she got her Masters and eventually her Doctorates in Equine Science. At A&M, Dr. Nonella did extensive research on Selenium within horses. Click here to view her research. Kelsey’s colleagues have mentioned her as one of the United States equine Selenium experts.
View all posts by Dr. Kelsey J. Nonella Ph.D., P.A.S.

6 Responses to “Low Selenium Horse Deaths in Central Oregon”

  1. Lisa says:

    I live in the Willamette Valley and feed central oregon hay. I had a gelding tie up this past summer. They said put him on horse guard 2x daily. I have all 4 horses on horse guard 1x daily unless the are training then do 2x daily. I had always been told central oregon had high levels of selenium.

    • mm Dr. Kelsey J. Nonella Ph.D., P.A.S. says:

      Thank you for the question. It is a common misconception that I hear from people in the Willamette Valley that central Oregon hay is adequate. When, actually hay from Central Oregon typically as low in selenium as hay in the valley. The only way to be absolutely certain of the selenium content of your hay is to get your hay analyzed for selenium. The reason central Oregon hay is deficient is because of the volcanic soils which contains little to no selenium, whereas the valley is selenium deficient because the selenium is leached out of the soil from the rain. In my Ph.D. research on selenium, we fed a great timothy orchard blend from Madras, OR. It was an extremely high quality hay, but only contain 3% of your horse’s selenium requirements.
      However, your horses are receiving correct amounts of selenium from their daily dose of Horse Guard, so you have nothing to worry about. Hope this helps.

  2. Judy Neill says:

    We’re any of the horses that were test on any kind of mineral supplement prior to the tests?

    • mm Dr. Kelsey J. Nonella Ph.D., P.A.S. says:


      Prior the incidences, the horses were only receiving grass hay from Culver, Oregon, and had access to trace mineral salt block. Thank you for your question.

  3. Beth Matanane says:

    What would have caused the downward spiral of deficiency to the levels of the dead horses? And why only these individuals considering this breeder had 40 plus horses in her care? And then to include the neighbors horses? Seems there must be more to the mystery. Can you elaborate please?

    • mm Dr. Kelsey J. Nonella Ph.D., P.A.S. says:


      Thank you for the questions. I didn’t talk about the diet that the horses were on before becoming clinical. All 40 horses were receiving grass hay from Culver, OR and had access to a trace mineral salt block. Even though these horses were receiving a mineral block, the average horse would only receive 3% of their selenium requirement from the block. When she tested her hay she found that there weren’t even detectable levels of selenium in her hay.

      As I noted in the article, after the ordeal with her poor horses and her neighbor’s horses, she randomly tested two horses in her herd and found that they were both selenium deficient. I suspect the horses that got fatally deficient must have been exposed to some stressor, such as a pathogen, that caused their body to use the little selenium that was present. Selenium is very important for immunity, and affects the production of immunoglobulins M and G, and assists in oxidative bursts of phagocytes. It is also key in the production of glutathione peroxidase, which is an enzyme important in removing the free radical, hydrogen peroxide from the body.

      Thank you for your questions and interest.

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