The "horse" cause of dermatitis is a reaction to an irritant, and is often an allergic reaction. Less common causes are dry skin, celiac disease, and infection. Ironically, soap and detergent are the most common irritants that lead to dermatitis.
In many cases, dermatitis can be treated without having to treat an underlying condition. Anti-inflammatory drugs are often useful and steroid cremes can be useful for some cases, but for some cases can actually exacerbate the condition over the long term.