Equine Dermatology - Useful links below:
https://aaep.org/sites/default/files/issues/proceedings-06proceedings-z9100106000457.PDF
https://thehorse.com/138782/common-equine-skin-conditions-2/
What causes dermatitis in horses?
There are many causes of skin inflammation, including external irritants, burns, allergens, trauma, and infection (bacterial, viral, parasitic, or fungal). Dermatitis may have many signs including any combination of itching, scaling, abnormal redness, thickening of the skin, and hair loss.
https://ihearthorses.com/4-common-equine-skin-conditions-what-to-do-about-them/
