Selenium is an essential nutrient for the health of horses and is necessary for growth, fertility and is important in the proper function of the immune system. However, at high levels selenium can be toxic to animals and humans. Selenium in the soil is absorbed by plants and when animals eat these plants, selenium toxicity can develop.
Three forms of selenium toxicity occur in nature:
- acute toxicity from a very high dose ingested over a short period of time (greater than 500 to 1000 ppm of selenium ingested in a dry ration)
- sub-acute toxicity called blind staggers and caused by the accumulation of 2-5 ppm of selenium in dry matter content
- chronic toxicity called alkali disease or bobtail disease. The minimum lethal dose of selenium for the horse is 3.3 mg/kg of body weight
The acute form is the least common and is characterized by nervousness and fear initially, followed by depression, decreased appetite, diarrhea, fever, generalized muscular weakness and respiratory distress. Death occurs in several hours to several days. The symptoms may resemble those of rabies. There is no treatment for acute selenium poisoning.
Subacute and chronic selenium toxicity occurs in all herbivores. There are limited areas, including the Rocky Mountains and Great Plains, which have selenium levels high enough in the soil to result in toxicity from the ingestion of many plants growing on these soils. Any soil containing more than 0.5 ppm selenium can cause toxicity. The selenium content in some of these areas is as high as 50 ppm. High selenium soil has been associated with shale and on bare outcroppings and can be confined to small distinct areas (only in a few acres in a pasture).
Nearly all plants can accumulate selenium from the soil, however some plants require selenium for growth and are called obligate selenium accumulators or indicator plants, as their presence indicates a high level of selenium in the soil. These plants are unpalatable and are usually only consumed in the spring when lush green plants are rare, or in hay. The common species (see Figures 1-3) of these plants include certain Astragalus spp. (milk vetch), Aster spp., Oonopsis (golden weed), Stanleya (prince’s plume), and Oxytropis lambertii (purple locoweed).
Figure 1: Purple Locoweed (Oxytropis)
Figure 2: Milk Vetch (Astragalus)
Figure 3: Prince's Plume (Stanleya)
The geographic distribution of the most commonly incriminated plants are illustrated in Figures 4-5.
Figure 4: Distribution of Milk Vetch
Figure 5: Distribution of Purple Locoweed
Subacute selenium poisoning is seen predominately in the spring as these plants are among the first to start growing. Animals may be craving green vegetation and consume them even though they are unpalatable. They develop a staggery gait, wander aimlessly, and “head press”, thus the term “blind staggers”.
Chronic selenium poisoning (alkali disease or bobtail disease) is the most common form of the disease in our area and is caused by the ingestion of smaller amounts of the element over a longer period of time. The first recorded case of chronic selenium poisoning recorded in the US occurred South Dakota in the 1850‘s. This condition can occur in the spring and summer on pasture or when horses are on hay containing high selenium plants. Chronic poisoning from selenium salts have also been reported in animals drinking water containing 0.5 to 2.0 ppm of selenium. Horses with chronic selenium toxicosis exhibit weight loss, hair loss (especially obvious in the mane and tail), and lameness in all four limbs. The coronary bands are painful to palpation and the band may separate with excretion of necrotic tissue from the defect. In severe cases the hoof wall may slough off. Provided the hoof wall does not slough the defect will grow down the hoof wall with new growth and eventually leave a normal hoof in 10-12 months (Figure 6). Rotation of the coffin bone (as may occur with founder) is rare and complete recovery is common.
Figure 6: Hoof wall defect in a horse diagnosed with Selenium Toxicosis 5-months previously.
Diagnosis of chronic selenium toxicosis can be made from clinical signs alone, however samples of feed, serum, or tissue can be analyzed to confirm the diagnosis. Serum levels of 1 to 5 ppm are consistent with a diagnosis of chronic selenosis, however horses must be consuming the high selenium diet to be diagnostic. A hoof sample containing 5-20 ppm of selenium is considered diagnostic. Hair analysis is not a reliable test, as it has false negatives. The concentration of selenium varies according to where the sample is taken from the mane of tail, therefore a high selenium analysis could be positive for chronic selenium poisoning whereas a low reading is inconclusive.
Treatment is mainly supportive for subacute and chronic poisoning and there is no treatment for acute selenium poisoning. Low doses of arsenic has been advocated however is usually not practical due to the inherent risk. The treatment of chronic selenium toxicity is aimed at reducing intake of selenium and treating the hoof lesions. Horses should be removed from the high selenium feed and provided a balanced, high protein diet. By the time the chronic form is diagnosed the damage is done and the degree of pathology in the hooves dictates the treatment. Working closely with your veterinarian and farrier is important for an optimum outcome.
Take home message:
- Be aware of the high selenium areas on your pastures. These can be identified by the presence of indicator plants. Your veterinarian, and Extension services are a valuable resource.
- Be very careful of the source of hay, as selenium content can be high in hay. South Dakota is unique in that soils in one pasture can be extremely high in selenium, causing an abundance of selenium in forages, while other areas nearby can be deficient. You can test your forages for selenium to determine if this is a problem in your area.
- If thinning of the mane and tail and unexplained lameness occurs, contact your veterinarian immediately so appropriate measures can be implemented. The longer the horse is exposed to a high selenium diet the more severe the condition becomes.
- Signs of “blind staggers” can be similar to those of rabies so it is important to avoid contact with saliva until a diagnosis has been established, as rabies can be transmitted to humans and is virtually always fatal.