Scrapie is the disease most like Mad Cow (BSE,MCD) and is a prion based infection. Johnes, is a bacterial infection of the gut by the same bacteria that causes TB in humans. In cattle the Johnes will cause obvious distress, watery foul diarrhoea, etc. in sheep it's not always obvious. This may be where you're getting confused as it can be a chronic wasting type disease. If your sheep are eating well and are not infected with worms, you could test for Johnes. Scrapie wasting is due to not eating. Scrapie infected sheep don't always show many symptoms, odd behavior or trouble controling how they walk (a peculiar hopping gate not related to feet problems) are common symptoms. If your sheep are healthy good producing animals I'd be surprised to hear they had either disease.