Contrary to what the name suggests this is not a parasite but a Fungal infection of the skin and hair shafts caused by dermatophyte fungus (skin loving fungus). It loves hot, humid conditions though it can be seen throughout the year and is only sometimes itchy.
Ringworm is a zoonosis, this means it can be transferred between humans and animals. It is a highly contagious infection which can be transmitted from horse to horse, human to horse or contact such as stables, trailer and grooming equipment. The fungus itself is quite resistant to environmental factors and can be on the skin for up to 3 weeks before any signs actually develop and remain in environmental areas such as stables for years.
Usually symptoms start as small raised spots from which the hair is lost. The fungus then spreads from these spots usually becoming scurfy with a thick dry scab. The lesions themselves typically affects areas such as the face, saddle area and girth.