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Selenium, a mineral that is naturally found in the soil in many regions of the world, plays an important role in supporting the horse’s immune system. Horses that graze in areas with an adequate amount of selenium will usually ingest the mineral as they eat pasture vegetation, but those living in selenium-poor areas may become deficient unless they are given supplemental selenium. However, too much dietary selenium is just as dangerous as not enough, so it’s important to guarantee the right level in a horse’s diet.

A study conducted at the University of Kentucky used 28 mature horses in an experiment designed to see whether horses grazing pasture that was marginal in selenium would react differently to vaccines, compared with horses that were supplemented with additional dietary selenium. The horses were divided by age and sex and randomly assigned to a dietary selenium treatment. Then they spent 35 weeks on a dietary program that depleted selenium. For the next 29 weeks, some of the horses were given additional dietary selenium supplements of either organic or inorganic types.  All horses were then given vaccinations, and the researchers collected blood samples to evaluate immune response.

In previous research, scientists have found evidence of a suppressed immune response in horses with low levels of selenium. In this experiment, evaluation of the response of the horses to vaccination was not effective, although antibody production was unaffected. Other tests in this study suggested that cell-mediated immune function, which is responsible for the activation of immune cells, was suppressed in the horses with low selenium consumption.

Laurie Lawrence, Ph.D., who led the study, suggested that it is important for horse owners to read feed labels so they can avoid both oversupplementation and undersupplementation of selenium for all classes of horses. Having hay and pasture soils tested is a good idea also, as many horses will not need extra selenium. Lawrence also pointed out that results could have been different if the horses had been undergoing more intense exercise and were under performance stress.

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