The most common reason for a defined area of hair loss on the body is trauma, which is often caused by other horses (a bite or kick). These patches usually have smooth edges of healthy hair that cannot be easily plucked out. Kick injuries may be shaped like the soles of hooves. Bites can look like, well, bite marks. Often the skin is damaged badly enough to peel. These lesions are usually in locations you would expect: the topline, neck, etc.

However, hair loss caused by bacteria or fungal infection can be hard to differentiate from trauma. For this reason, treat any hair loss situation as contagious until you have discussed it with your vet. Multiple horses can be affected by infectious agents and disease can be transmitted through tack and contact.

Fungal and bacterial hair loss tends to have a less defined scaling around the edge of the patch, and is most commonly found on the head, back, girth area and top-line. Hair loss can also occur with chemical or thermal burns and a variety of other conditions.