A. Ringworm infections aren’t actually caused by worms, despite the misleading name, but by various species of fungus. Horses, humans, dogs, and cats can all contract ringworm infections. In humans, specific types of ringworm fungi also cause the conditions commonly known as jock itch and athlete’s foot. Ringworm is zoonotic, meaning humans can contract it from contact with animals and vice versa. They can also contract it from the soil—because infectious spores can exist there, as well—and fomites (objects, such as clothing, that can transmit the infection).
There are several species of ringworm fungi. Dog and cat ringworm infections are usually due to Microsporum gypseum, Microsporum canis, or Trichophyton species. Human ringworm infections are usually due to Trichophyton, Microsporum, or Epidermophyton species. Horse ringworm is typically caused by Trichophyton, Microsporum gypseum, or Microsporum equinum.
As you can see, there is some overlap in ringworm species among animal species, so it’s possible for ringworm infections to be transmitted from cat to human to horse, for example.
Most healthy adult animals and humans have some degree of immunity to ringworm infections, so the lesions will be fairly self-limiting, though they certainly benefit from treatment to increase the speed of healing. Animals or humans that are immune suppressed or otherwise compromised by poor nutrition, etc., are more at risk for generalized infection and may require aggressive treatment with systemic medications rather than just topical treatment.
Therefore, it’s wisest to avoid directly touching your horses or any of their tack or grooming equipment until you have been treating your lesions for three to four days with effective topical antifungal medications.