Contact dermatitis is an inflammatory skin reaction resulting from direct-contact with an irritating agent and horse's skin. Moisture (humid weather, sweating, bathing, etc.) enhances the reaction. Pruritus might be mild to severe, sometimes painful. Primary lesions include the development of papules and vesicles, followed by a wide range of types of secondary lesions.
There are a wide range of causes of contact dermatitis---as it can be caused by anything from a particular clothing-like material (cotton, jute, chrome, hemp, etc.) used in blankets, fly sheets, leg wraps, towel, saddle pad, girth, or even something worn by the handler), grooming tool (particular brush), topical insecticides and shampoos, stall bedding, medication, and certain types of plants found in the pasture or within the stable yard or within reach of the horse (contact dermatitis-causing plants).
The location of the reaction can usually aid in the determination of its cause. A reaction in the girth region is likely related to the material the girth is made from. A reaction on the lower limbs of the horse would most likely be the cause of contact-dermatitis causing plants or a reaction to leg wrap material.