Like humans, horses can be allergic to pollens, spores, and other allergens that appear in the environment seasonally, as well as substances found within the barn such as dust, mites, and animal dander. While people tend to respond to these various allergens by sneezing and developing watery eyes, while horses react by getting very itchy skin or developing hives. This leads to constant scratching and chewing which can cause trauma to the skin and extreme discomfort. Generally, symptoms worsen with age and can be controlled, but not eliminated.
The fungal skin disease dermatophytosis has come to be called ringworm because of the appearance of the skin lesion that characteristically occurs with this disorder: a circular area of hair loss with a red, raised outer rim. These lesions result from an inflammatory reaction to the fungus. Most often, dogs and cats are infected by the Microsporum canis fungus, but other types of fungi cause ringworm infections as well.