This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
Summarized from a set of articles posted by Jim Jaskie,
Retrievers that lick their paws excessively, horses that "stump suck",
Dobermans that "flank suck" share the same disorder. The disorder is
generally mild and most people never notice it, but sometimes it can
go too far and become a hindrance to normal functioning.
Dr. Judith Rapaport (head of the Child Psychiatry Branch of the
National Institute of Mental Health and author of "The Boy Who
Couldn't Stop Washing") explored this area thoroughly, because of
similarities with a human malady called "Obsessive Compulsive Disorder
(OCD)." This is a disorder that induces unusual behavior such as an
irresistible desire to wash your hands, over and over, even when they
are not dirty.
The experiments at NIMH showed that this behavior is caused by a lack
of Seratonin. This lack can be caused by a genetic predisposition and
also by stress. Proper medication was shown to relieve similar
problems in dogs, horses and people! Some of the reported results were
on Labradors that literally licked the hair off of their paws,
dropping the habit completely after medication.
This research is also a landmark in the understanding of the effect of
some of the neural transmitters and has led to a whole new family of
some wonderful new medicines. This work has already saved dogs, horses
and people from one of nature's less pleasant maladies, and promises
to shed light on other problems such as epilepsy.
The medication that Dr. Judith Rapoport found to work for dogs with
acralick dermatitis as well as Obsessive Compulsive Disorder (OCD) is
Clomipramine (brand name is Anafranil). However, Fluoxetine (brand
name is Prozac) is now being used very successfully for OCD and has
fewer side effects. You should discuss this with your vet, who will be
able to prescribe these medications for your dog if it has OCD.
Some old-ish but very informative articles that describe this problem
are "Chemistry of Compulsion" by Robert Trotter in the June 1990 issue
of Discover magazine and the very thorough but easy to read article,
"The Biology of Obsessions and Compulsions" by Dr. Rapoport in the
March 1989 issue of Scientific American. Only the first article
specifically mentions Rapoport's work with dogs, but if you want to
understand what is really going on, read both articles.