Viral papillomas, a veterinary term for warts, are sort of the cooties of the canine world. They spread easily among puppies and young dogs, but don't cause any harm other than unsightliness. Older dogs, with mature immune systems, seldom develop papillomas. Just like kids in the playground spreading cooties, young dogs often spread the virus causing papillomas when roughhousing with each other.
Papillomas might be either round, similar to human warts, or sport a rough surface resembling cauliflower. The Veterinary Partner website describes them as looking like sea anemones. Usually white or grey in color, these warts aren't painful or itchy. You'll have to spot them on your dog, as he won't give you any telltale signs that one has sprouted. The exception could be oral papillomas that interfere with chewing or swallowing.
Viral papillomas usually occur in the mouth or on the lips, but are also found on the eyelids, toes and elsewhere on the body. It's rare for just one wart to sprout -- they usually appear in clusters. Papillomas in the mouth usually disappear faster than those in other areas.
The incubation period for papillomas ranges between one and two months, so you might not be able to pinpoint when your dog was exposed. Transmission of the virus occurs through saliva, so dogs can pick it up not only through play biting, but also from sharing food or water bowls with affected dogs. If your dog comes down with warts, wait at least two months until the last one disappears before allowing him to socialize with other canines. He can't transmit the virus to you or to felines in your household. If he lives with another young dog, odds are that dog will also develop papillomas.
Most warts don't require treatment. They should disappear on their own within a few months, once a dog's immune system matures. However, it's important to have any lumps or bumps on your dog checked out by your veterinarian, since you want to make sure the growth is indeed a papilloma and not a tumor. Don't try to remove canine papillomas with over-the-counter remedies for human warts. If you can't wait for the warts to go away, your vet can surgically remove them or freeze them off. That's also the case in the rare instance when a dog has so many warts in his mouth that he can't eat properly.