These are pictures of ringworm in a 1 month old mixed breed puppy (there were actually 3 puppies from the same litter who all presented for patchy hair loss without itching):
Ringworm is caused by fungal organisms (dermatophytes) that invade and attack the hair follicles, which causes hair loss and variable degrees of itching. The most common form of canine ringworm is caused by an organism called Microsporum canis, which can glow fluorescent green under an ultraviolet Wood’s lamp (pictured above). Crusts of any kind can glow under a Wood’s lamp, so it is important to demonstrate that the hairs are glowing for a diagnosis of ringworm to be confirmed. Microsporum canis can be contagious from dog to dog, among cats and dogs, and even from dogs and cats to humans (a process called zoonosis). Other species of ringworm may pass from the soil or a rodent’s nest to a dog or cat, and in these cases the cat or dog is usually a dead end host (meaning the infection is not contagious).
While ringworm is curable, it often requires months of treatment to resolve, and if your dog or cat has a contagious strain of ringworm, extensive environmental decontamination may be necessary to remove inspected spores from a home environment where the dog lives (ringworm can become a very serious problem in shelter or rescue settings where numerous animals are housed). If your veterinarian diagnoses your dog with ringworm, prepare for a long, drawn out treatment, usually with multiple rechecks to make sure the infection is totally clear before medications and environmental treatments can be discontinued.
Ringworm is also one of the rare skin diseases that we see in dogs and cats where owners can be affected. If your dog is diagnosed with ringworm and you have skin lesions, you may need to see your doctor for treatment, since veterinarians can’t legally treat humans :)