Probably the most common skin disease seen in horses in Texas other than fly allergies is ringworm. Ringworm is not a worm at all but actually a fungus. We see ringworm year round in Texas and the most common symptom is raised patches of hair and then the hair falls out leaving scaly skin. In our practice, ringworm is more common on the horse's face and neck and can spread to other areas without treatment.
The best method to diagnose ringworm is for your vet to culture some of the infected hairs because other conditions such as fly allergies and immune conditions can have the same appearance. It is important to know what you are treating because treatments for allergies and immune conditions will cause ringworm to worsen so an accurate diagnosis is needed. Ringworm is common in young and immunosuppressed horses due to their lack of immunity. Also, ringworm can be a secondary infection that can develop at the site of damaged skin related to minor injuries or fly bites. The disease can be contagious between horses by using the same saddle blankets and halters so these items must be sterilized and not used on any other horses until the infection is cleared. The most effective treatment of ringworm in horses in my experience is bathing followed by a rinse using chlorhexidine. Treatment is usually continued for 30 days or until cultures are negative. It is reported that ringworm is self-limiting in horses and will clear up after several months without treatment. However, a lot of skin damage due to scratching can occur during this period of time so treatment is recommended. Treatment also helps to prevent transmission to other animals.