I did a bit of research on this in sheep when starting my Animal Science degree, but have never encountered it pigs. In sheep, the deficiency of selenium causes white muscle disease - also known as nutritional muscular dystrophy.
As vitamin E and selenium work in conjunction with another, supplementing with additional vitamin E is the first step - though this will take a couple of weeks at least to take effect. Good pasture and fodder which includes peas and beans etc provide plenty of vitamin E, whereas other food sources, like silage and root crops aren't so good. Also, if you store feed for too long, or in poor conditions, the vitamin level can drop.