We hope that does not happen to you, but if you have observed that your pet drags its tail across the ground rubbing the anal area, you are probably in need of an urgent doctor visit!
This behavior in the dog is popularly attributed to the presence of stomach parasites (worms), but actually this is a error. This attitude is due to an alteration that the dog is undergoing in its perianal glands, which, as its name indicates, are located on the sides of the anus.
These glands have a viscous substance that would identity properties about the dog recognizable among its kind, hence the common behavior of dogs sniffing the anal area mutually, so you could compare this as a talk between people, only in this case the dogs meet and exchange information about their sex and sexual or health status, always through smell.
Although no agreement has been reached among professionals, it is estimated that the alterations that could suffer perianal glands, and that would lead to dog to drag its tail across the ground, would happen when the substance they contain solidifies a little or becomes infested, thus hindering its segregation through the glands, causing in the dog aches and pains that he would try to correct by rubbing his tail on the ground to try to empty them.
If not treated in time, this problem could worsen until it appears ulcerations in the perianal area called fistulas, so if you observe this behavior in your pet, do not attribute it to worms, consult immediately to a veterinarian who will be able to detect the real problem and act in the best way.