CMO in West Highland White Terriers

Craniomandibular Osteopathy (CMO), often called “Westie Jaw” or “Lion Jaw,” is a painful developmental bone disease that primarily affects young West Highland White Terriers. Although the condition can occur in several breeds, Westies are among the dogs most strongly associated with the disorder.

What Is CMO?

CMO is a disease in which abnormal bone growth develops around the skull and jaw, especially the lower jaw (mandible), the bones surrounding the ear, and sometimes the jaw joint itself. Instead of normal bone development, affected puppies produce excessive immature bone tissue, causing swelling, thickening, and severe pain.

The condition usually appears in puppies between three and eight months of age, during periods of rapid growth. In many cases, the abnormal bone growth slows or stops once the dog reaches maturity at around one year old.

Why Westies Are Prone to CMO

Researchers believe CMO has a strong genetic basis in West Highland White Terriers. The disorder has long been recognized as an inherited disease within the breed, and related terrier breeds such as Scottish Terriers and Cairn Terriers may also be affected.

The disease was first documented in Westies in the late 1950s, and veterinarians quickly noticed the unusually high number of cases within the breed. Because of this strong association, CMO became widely known as “Westie Jaw.”

Symptoms of CMO

The most common signs include:

• Swelling of the lower jaw

• Pain when opening the mouth

• Difficulty eating or chewing

• Excessive drooling

• Fever

• Lethargy

• Weight loss

• Crying or sensitivity when the jaw is touched

In severe cases, the jaw joint can become partially fused, making it difficult or impossible for the dog to open its mouth normally.

Owners often first notice that their Westie puppy suddenly refuses food or cries while chewing toys or kibble.

Diagnosis

Veterinarians usually diagnose CMO through:

• Physical examination

• X-rays or CT scans

• Evaluation of jaw swelling and pain

• Breed and age history

Radiographs often reveal symmetrical thickening of the jawbones and skull. Because symptoms can resemble infections or tumors, imaging is important for confirming the condition.

Treatment and Management

There is currently no cure for CMO, but treatment focuses on controlling pain and maintaining quality of life while the puppy matures. Common treatments include:

• Anti-inflammatory medications

• Pain relief

• Soft-food diets

• Supportive care

Many mild to moderate cases improve naturally as growth slows after the first year of life. However, severe cases can lead to permanent jaw problems or difficulty eating.

The Importance of Responsible Breeding

Because CMO is believed to be inherited, responsible breeders work carefully to avoid producing affected puppies. Dogs known to have CMO, or dogs closely related to affected animals, are generally not recommended for breeding. Genetic research into the disease continues, and DNA testing may help reduce its occurrence in the future.

Living With a Westie With CMO

Although the diagnosis can be frightening, many Westies with mild forms of CMO go on to live happy and active lives. Early diagnosis, careful veterinary management, and supportive care can greatly improve comfort during the difficult growth phase.

For owners, patience and attentive care are essential. Soft meals, regular veterinary checkups, and monitoring for pain can make a major difference in helping a young Westie navigate this challenging condition.

include information on testing and results

CMO Testing and Results in West Highland White Terriers

Craniomandibular Osteopathy (CMO), commonly called “Westie Jaw,” is an inherited bone disorder that affects young West Highland White Terriers. Because the disease has a strong genetic component, modern DNA testing has become an important tool for breeders and owners seeking to reduce the incidence of the condition in the breed.

Understanding the Genetic Basis of CMO

Research has shown that CMO in Westies is linked to inherited genetic mutations that affect bone growth and development. The condition is generally considered an autosomal recessive disorder, meaning a puppy must inherit a copy of the defective gene from both parents to develop the disease. (animalabs.com)

This inheritance pattern creates three possible genetic outcomes in tested dogs:

• Clear (Normal) — the dog does not carry the mutation

• Carrier — the dog carries one copy of the mutation but usually shows no symptoms

• Affected — the dog carries two copies and is at risk of developing CMO

Understanding these results helps breeders make informed mating decisions.

How CMO Testing Is Performed

CMO testing is typically done through a simple DNA sample collected from either:

• A cheek swab

• A blood sample

The sample is then analyzed by a veterinary genetics laboratory for known CMO-associated mutations. Several canine genetic laboratories now offer breed-specific CMO testing panels for West Highland White Terriers. (animalgenetics.com)

Testing can be performed at any age because the DNA itself does not change during the dog’s lifetime.

Interpreting Test Results

Clear / Normal

A clear dog has tested negative for the mutation associated with CMO.

Result meaning:

• The dog is unlikely to develop inherited CMO

• The dog cannot pass the mutation to offspring

These dogs are ideal candidates for breeding programs focused on reducing disease prevalence.

Carrier

Carrier dogs possess one copy of the mutation.

Result meaning:

• Usually clinically normal

• Can pass the mutation to puppies

• Should only be bred carefully to clear dogs

A carrier bred to a clear dog should not produce affected puppies, although some offspring may also become carriers.

Affected

Affected dogs carry two copies of the mutation.

Result meaning:

• High risk of developing clinical CMO

• Can pass the mutation to all offspring

• Generally not recommended for breeding

Affected puppies may begin showing symptoms between three and eight months of age, including jaw pain, swelling, fever, and difficulty eating.

Why Testing Matters

Before DNA testing became widely available, breeders relied heavily on pedigree analysis and observation of affected bloodlines. Because carriers appear healthy, the disease could unknowingly persist within breeding programs for generations.

Genetic testing now allows breeders to:

• Reduce the number of affected puppies

• Preserve genetic diversity responsibly

• Avoid unnecessary elimination of carrier dogs

• Make evidence-based breeding decisions

Completely removing every carrier from breeding programs immediately can sometimes reduce genetic diversity in a relatively small breed population. For this reason, many breed specialists recommend gradual reduction strategies using carefully planned matings. (thekennelclub.org.uk)

Limitations of Testing

Although DNA testing is extremely valuable, it is important to understand that:

• Not all jaw disorders are CMO

• Rare mutations may not yet be fully understood

• Clinical symptoms should always be evaluated by a veterinarian

A dog with jaw swelling still requires proper diagnostic examination, including X-rays or CT imaging, even if genetic test results are available.

The Future of CMO Prevention

Advances in canine genetics continue to improve understanding of inherited diseases in West Highland White Terriers. Increased use of health testing, responsible breeding practices, and open communication among breeders have already helped reduce the incidence of CMO in many bloodlines.

For owners and breeders alike, DNA testing represents one of the most effective tools available for protecting the long-term health of the Westie breed.