Craniomandibular Osteopathy (CMO, also known as "lion jaw" and "Westie jaw") is a noncancerous, proliferative growth of along both mandibles (lower jawbones), other skull bones, and occasionally forearm bones. It usually affects both sides, but not always. Inflammation is the earliest symptom. The swollen jaw may be very painful. The hinge joints of the jaw are sometimes involved making it extremely difficult for the dog to open his mouth. Fever, drooling and loss of appetite are characteristic. When the mouth is forcefully opened, the dog cries in pain. So-called silent cases (where the puppy appears unfazed) exist. The condition can occur as early as 3 to 4 weeks of age and, rarely, as late as 9 to 10 months. Experienced breeders and veterinarians usually recognize it before 4 months either by clinical signs or by palpation. X-rays of the skull and jaw will confirm CMO.

A simple recessive gene causes CMO. Both parents must carry at least one gene for it. Although studies on Westies, Scotties, and Cairns are in full swing in an attempt to isolate the rogue gene, the birth of an affected puppy provides the only method of confirming both parents as carriers.

Treatment of Craniomandibular Osteopathy
CMO is treatable. Depending on the severity, the amount of medication and length of treatment vary. Four to ten months is the average length of treatment. Many affected puppies will require some dose of cortisone until they are 10 months or older. Most anti-inflammatory drugs work well, but since CMO may require long-term therapy, veterinary advice is essential. Puppies nearly always recover. In severe cases, tube feeding may be required during appetite loss to support nutrition. The disease can be controlled by corticosteroid therapy and buffered aspirin (pain). When treated, the bony swelling often recedes, but some enlargement and impaired chewing may continue for the rest of the dog's life.

Besides Westies, Scotties and Cairns, the condition has been reported in Boston terriers, Boxers, Labrador retrievers, Great Danes and Doberman Pinchers.

The Orthopedic Foundation for Animals and the Institute for Genetic Disease Control maintain CMO registries for terriers in an attempt to diagnose and track the disease.

References:
1. The West Highland White Terrier by Jill Arnel
2. Dog Owner's Home Veterinary Handbook (Howell Reference Books) James M. Giffin, Liisa D. Carlson
3. UC Davis Book of Dogs : The Complete Medical Reference Guide for Dogs and Puppies Mordecai Siegal
4. West Highland White Terriers by Dan Rice

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