Craniomandibular Osteopathy in Dogs: Causes, Symptoms & Care

Craniomandibular osteopathy (CMO) is a hereditary bone disorder that primarily affects certain terrier breeds, sometimes referred to as “lion jaw.” It causes abnormal bone growth mainly in the mandibles (jaw bones), temporomandibular joints, and occasionally other areas of the skull or even the limbs.

This abnormal growth results in noticeable changes in the facial structure, considerable discomfort, and difficulties with eating, drinking, or opening the mouth, often accompanied by excessive drooling. Thankfully, this condition is self-limiting and usually improves as the dog matures, so many adult dogs no longer show symptoms.

Because CMO is genetic, it can be passed from parents to puppies. Responsible breeders in the United States take advantage of DNA health testing to screen their breeding dogs, minimizing the chances of passing this condition on to future litters.

This guide outlines the breeds most vulnerable to CMO, explains its inheritance, describes common symptoms, and highlights the importance of DNA testing to help breeders and prospective buyers make informed decisions.

What Is Craniomandibular Osteopathy?

CMO is a developmental bone disease impacting young, growing dogs, typically between 3 and 8 months old. It is characterized by excessive, irregular bone growth around the jaw and skull, particularly the mandibles and temporomandibular joints. In some cases, abnormal bone changes may affect the bones near the ears or, less commonly, the leg bones.

The condition creates painful swelling on both sides of the face, which may not always be symmetrical. Puppies with CMO often struggle to open their mouths, eat, or drink normally and tend to drool excessively while showing signs of lethargy. Feverish episodes can also occur in cycles lasting one to two weeks.

Most often, signs diminish as the dog reaches adulthood, so adult dogs affected by CMO may be difficult to detect unless health screening is performed early.

Breeds Most At Risk

In the U.S., craniomandibular osteopathy mainly affects terrier breeds, especially the Cairn Terrier puppies, West Highland White Terrier puppies, and Scottish Terrier puppies. Due to the limited genetic diversity in pedigrees of these breeds, inherited conditions like CMO tend to spread more easily within these populations.

It’s important to understand that CMO is not contagious; only dogs that inherit the problematic gene from their parents develop the disorder.

How Is CMO Passed Down?

CMO follows an autosomal dominant inheritance pattern with incomplete penetrance. This means dogs can carry one or two copies of the mutated gene. Dogs with two copies usually have a higher likelihood of showing symptoms, but some with just one copy can also be affected, while others with two copies remain symptom-free carriers.

Carrier dogs that have one mutated gene and one normal gene typically do not exhibit symptoms but can pass the gene to around half their puppies. Because of this inheritance complexity, breeders must carefully consider health screening results to avoid increasing the frequency of CMO in the breeding population.

Identifying Symptoms in Puppies

Early symptoms generally appear between 4 and 8 months of age. These include uneven or symmetrical jaw swelling, painful or restricted mouth opening, excessive drooling, and trouble eating or drinking.

Puppies affected may also display withdrawal, lethargy, and recurring fever episodes. Prompt veterinary care is necessary for effective diagnosis and to ensure the puppy’s comfort and proper management.

Health Screening and Responsible Breeding

DNA testing plays a crucial role for American breeders committed to preventing CMO, especially in at-risk terrier breeds. Using blood samples or cheek swabs, laboratories can determine if a dog is clear, a carrier, or affected by the gene mutation.

Breeders use these test results to avoid breeding two carriers or affected dogs together. Puppy buyers should ask for health test documentation and transparency from breeders, supporting the purchase of healthier puppies with a lower risk of inherited diseases.

Responsible breeding and informed buying decisions help maintain the health of terrier breeds and contribute to the overall welfare of dogs in the United States.

Caring for Dogs with CMO

While CMO often resolves as dogs mature, puppies showing symptoms may need supportive care. This includes pain relief through medications prescribed by a veterinarian—such as nonsteroidal anti-inflammatory drugs (NSAIDs) like carprofen or meloxicam, corticosteroids, or opioids like tramadol—as well as feeding soft foods to minimize mouth discomfort.

Regular veterinary appointments and monitoring are essential to maintain the dog’s quality of life during the active stages of the disease.

Summary

Craniomandibular osteopathy is a painful, inherited bone disorder that primarily affects young terrier breeds in the United States, causing abnormal growth in the jaw bones that typically improves with age.

Recognizing the genetic component and symptoms aids in early diagnosis and care. DNA testing supports responsible breeding, reducing the chance of passing this condition to future generations, resulting in healthier dogs and improved welfare.

If you're considering a terrier puppy, always inquire about CMO health screening and choose reputable breeders dedicated to ethical practices.