Dogs have no way of communicating their maladies verbally, so the sight of a dog chewing its own tail is an alarming one, and it means one of a number of problems, including an allergic reaction to parasitic infestation or simply boredom. Pet owners who are unable to determine which of those is the case should take their dogs in for a checkup at the veterinarian because once a dog develops a "hot spot," or a wet, red irritated spot that happens with persistent biting, the cycle becomes even more difficult to stop.
Allergies that cause a dog to bite its tail include skin irritations such as contact dermatitis that occur when dogs get into substances such as soap or pesticides. This is why it is crucial to keep a dog out of the yard for 24 to 48 hours after fertilizing.
Dogs also get bored. If a dog does not get enough activity in the form of outdoor exercise or stimulation, it often develops repetitive behaviors that mimic obsessive-compulsive disorder, and chewing on its tail is one sign.
Parasites like fleas often set up shop on the tail and start chewing, so the dog's chewing is just returning the favor and trying to dislodge the parasite involved. It's not easy to see ticks, and mites are microscopic, so just because the pests are not visible does not mean they are not there. Talking to the veterinarian is a must when the situation continues without remedy.