If you have a working terrier, you may find that your dog will, at some time, develop a small round sore with hair loss on its face or foot, or perhaps somewhere else.
The area will probably be not much larger than a quarter. There will be no cut, but the area will be red and sore-looking, perhaps with a slightly raised edge and slighly darker-colored patch in the middle.
This is classic ring worm.
Despite its name, ring worm (or ringworm) is a FUNGUS, not a worm, and it is easy and cheap to treat, though it takes a bit of time for the hair to regrow. It is very common, especially on the foot pads of digging dogs where small abrasions and lots of dirt create opportunities for the fungus to catch hold and grow.
A vet will put a dog under a "wood's lamp" (a UV lamp) to see the fungus glow (even though more than half of all ring worm infections do not glow and this is an imperfect diagnosis). Then they will "culture" the infection and charge you for that. Then they will sell you an expensive ointment.
If the symptoms are as described, you can skip all this cost and go straight to a cheap Tenactin foot ointment or spray (the same thing you use for toe fungus and jock rot) that you can get at any drug store or pharmacy. This is the exact same stuff the vet was trying to sell you -- without the markup.
Ringworm has several forms, but Tenactin works for all of them. If you cannot find Tenactin, look for another anti-fungal topical ointment or spray with miconazole and/or clotrimazole.
A quick note on ring worm fungus types. There are about 30 types of ring worm, but the three most common types are: Microsporum canis, which is generally gotten from a cat or another dog; Microsporum gypseum which is often gotten from contaminated soil; and Trichophyton mentagrophytes which is usually caught from rodents and their burrows.
As you can see, while all dogs can get ring worm, working terriers are likely to be particularly susceptable because of what they do and who they do it too.
The good news is that ringworm does not spread all that easily, so it will probably stay localized and it will generally not jump to other animals or to you. It can, it should be said, but it probably won't. Risk is very low.
There is no instant "cure" for ring worm, as the hair on your dog will take time to grow back, but if you put fungal medicine on two or three times a day for a week or so, the fungus itself will die off pretty quickly. The hard part is going to be keeping the dog from licking the ointment off, which may require a cone-like "Elizabeth" collar for a few days or perhaps a basket muzzle. You may have to crate the dog for a few hours every day if you have other dogs that will lick the sore foot of the afflicted dog.
A final note: While I prefer "Fast Actin' Tenactin", "Tea Tree Oil" is sold in health food stores and also works, though it is perhaps a bit slower and a bit more expensive.