Selenium deficiency is called white muscle disease, and it can afflict all large animals, though the young and fast growing are more prone to it than adults. In all animals, symptoms will include weakness or stiffness in the legs in minor cases, up to pain or complete inability to walk. Eventually, the animal will die if left untreated.
Treatment for this type of white muscle disease is simply supplementing with selenium through an injection or oral gel, and noticeable improvement is usually seen within 24 hours of treatment.
Another type of white muscle disease affects the heart, and those symptoms are difficult to see, but will include symptoms similar to pneumonia such as difficulty breathing, frothy nasal discharge and fever. The prognosis for this form of the disease is usually not good.
Providing a combination of a mineral block with selenium and also loose minerals with selenium will help your animals, but in severely selenium deficient areas, additional supplementation may be needed. Check with your vet to find out how your area is for selenium and whether or not you need to supplement.