Answer

By Your Horse

Vet Malene Jørgensen replies:

It sounds as if he has atopic dermatitis, also known as atopy.
This means he has a hypersensitivity disorder towards inhaled or absorbed allergens such as pollen, moulds, or dust. The horse may or may not be itchy, but some horses can scratch until they bleed.
The lumps you’ve seen in his skin are probably nettle rash, which is a symptom of an allergic reaction. Your horse can also develop respiratory disease, which is an asthma-type condition, as well as headshaking, coughing or snorting. The dark areas on his skin are a complication to itching and scratching, which is called hyperpigmentation. The skin can show other symptoms, such as thickening and hair loss. There’s a risk of infection if the skin is broken.
The diagnosis can be confirmed with a ‘prick test’, where small amounts of allergens are injected into the skin. Later, the skin will temporarily swell up in a small lump over the site where the horse reacted to an allergen. You can also ask the vet to take a blood sample from your horse to test for the allergy. The steroid treatment he is on at the moment will only keep the symptoms away while he is on the drugs and long-term steroid treatment can be ultimately ineffective even at doing this. Other options you can try include supplements with fatty acids in, and oils such as flaxseed or evening primrose may also be effective. You may want to ask your vet to refer you to a qualified veterinary homeopath or herbalist.