Nutritional Myodegeneration in Horses: White Muscle Disease in Foals

Nutritional myodegeneration is a disease of muscle damage caused by selenium deficiency, often with low vitamin E contributing. In horses, it is most often discussed in young, rapidly growing foals, especially those born to mares that were deficient during pregnancy. You may also hear it called white muscle disease because damaged muscle can look pale or streaked at necropsy.

The condition can affect skeletal muscles, which help a foal stand, walk, and nurse, and it can also affect the heart muscle. That is why some foals show stiffness, weakness, trembling, or trouble swallowing, while others develop fast breathing, an irregular heartbeat, collapse, or sudden death. The signs can appear quickly and may worsen over hours.

This is not something a pet parent can confirm at home. A weak or recumbent foal needs prompt veterinary care because several emergencies can look similar, including sepsis, botulism, trauma, and other muscle disorders. Your vet can sort through those possibilities and recommend care that fits the foal's condition, the farm setup, and your goals.

The main cause is selenium deficiency, with vitamin E deficiency sometimes adding to the risk. These nutrients work together as antioxidants that help protect muscle cell membranes from normal oxidative damage. When levels are too low, muscle cells become fragile and break down more easily.

In foals, the problem often starts before birth. A mare eating a selenium-deficient diet during gestation may not provide enough selenium to the developing foal. Risk can also increase when horses are fed low-quality hay, dry winter pasture, or unfortified feeds that do not adequately supply vitamin E and trace minerals.

Geography matters. Some parts of the United States have selenium-deficient soils, so local forage may naturally be low in selenium. At the same time, more supplementation is not always safer. Selenium has a narrow safety margin, and too much can be toxic. That is why prevention should be based on forage, feed, and regional risk, with your vet helping you decide whether testing or targeted supplementation makes sense.

Your vet will start with a physical exam and a careful history, including the foal's age, nursing behavior, the mare's diet during pregnancy, local soil risk, and whether other horses on the property have shown weakness or poor performance. Because white muscle disease can affect both skeletal and heart muscle, your vet may pay close attention to breathing effort, heart rhythm, swallowing, and the foal's ability to stand and nurse.

Bloodwork is often a key next step. Foals with muscle injury may have increased muscle enzymes, especially CK and AST. Your vet may also recommend whole blood or serum selenium testing and, in some cases, vitamin E testing. If heart involvement is suspected, additional monitoring such as an ECG or echocardiogram may be recommended. Ultrasound, urinalysis, or other tests may help rule out look-alike problems and assess how much organ stress is present.

A diagnosis is usually based on the pattern of signs plus test results, rather than one single finding. In severe or fatal cases, necropsy can confirm the diagnosis by showing characteristic muscle changes. Because several neonatal emergencies overlap, fast diagnosis matters. It helps your vet choose supportive care, discuss prognosis honestly, and make a practical treatment plan.

Prevention starts with the pregnant mare's diet. Because affected foals are often born to mares that were selenium-deficient during gestation, it is worth reviewing forage, hay, pasture access, and concentrate choices with your vet before foaling season. In low-selenium regions, a balanced ration may need targeted support, but the exact plan should match your local forage and the mare's full diet.

Vitamin E also matters, especially for horses on dry pasture or low-quality hay with limited access to fresh green forage. Your vet may recommend ration balancing, feed changes, or testing when deficiency is a concern. This is especially helpful on farms with a history of weak foals, poor muscle development, or known regional mineral issues.

Avoid guessing with selenium products. Selenium is essential, but too much can be harmful. The safest approach is to work with your vet or an equine nutrition professional to review labels, total daily intake, and any supplements already being used. If one foal is diagnosed, ask whether the mare, herd mates, or forage should also be evaluated so you can reduce risk for future pregnancies.