Derfen is to be applied as soon as there are signs of SSRD (or any other skin problems) or even if your horse is already affected. Derfen can be applied at any state of the infection.

No matter how the conditions of your horse may be, Derfen has an instant effect on the skin by hydrating and soothing.

However, in case of severe mud scale, it is recommended to use a veterinary anti-parasite lotion prior to applying Derfen in order to get optimal results.(se "directions")