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.
Assess the area and share your findings and concerns with your vet. Notice whether the areas are painful or itchy. Treat undiagnosed hair loss as contagious until proven otherwise. Be careful of spread through tack and equipment. You can try an antiseptic shampoo on the area. Treat for several days, remove all scabs and scale, let stand 10 minutes, rinse well and dry. If there is not improvement in a week.
Do not treat with harsh antiseptics without veterinary guidance.
Do not inadvertently spread bacterial or fungal agents to your other horses through sharing tack or equipment.
If your vet suspects that hair loss is a result of bacteria or fungus, they may suggest additional diagnostics and treatment and will discuss prevention in your other horses.
What is the appearance of the patch or patches of hair loss?
Does the horse live with or near other horses?
Can you expand the affected area by picking hairs or crusts on the edges?
Is the skin crusty or does it have any lesions or scales?
Are there multiple affected areas?
Hair Loss, Traumatic
Hair Loss, Generally
Chemical Burn, Caustic Substances
Effluvia of Haircoat
Shampoos, Antiseptic