Craniomandibular Osteopathy in Dogs: Causes, Symptoms & Care

Craniomandibular osteopathy (CMO) is a hereditary bone condition affecting certain terrier breeds, sometimes called “lion jaw.” It primarily involves abnormal bone growth in the mandibles (jaw bones), the jaw joints, and occasionally other skull parts or legs.

This growth leads to visible facial changes, significant pain, and difficulties with eating, drinking, or opening the mouth, often accompanied by excessive drooling. Fortunately, the condition is self-limiting and tends to improve as the dog matures, meaning many adult dogs no longer show signs.

As a genetic disorder, CMO can be passed from parents to puppies. Responsible breeders use health testing to screen dogs before breeding, reducing the risk of passing this condition to future litters.

This article explores the breeds most at risk, how the disease is inherited, common symptoms, and how DNA testing helps breeders and buyers make informed decisions.

What is Craniomandibular Osteopathy?

CMO is a developmental bone disease affecting young, growing dogs, mainly between 3 to 8 months old. It is characterised by excessive and irregular bone proliferation around the jaw and skull, especially the mandibles and the temporomandibular joints. Sometimes, abnormal bone changes occur in the bones near the ear and, rarely, in the leg bones.

The condition causes painful swelling on both sides of the face, which might be asymmetrical. Puppies with CMO have difficulty opening their mouths, eating, or drinking normally, often accompanied by drooling and lethargy. Fever may also occur in cycles lasting a week or two.

In many cases, symptoms fade as the dog reaches adulthood, making it difficult to identify affected adult dogs unless health screening was performed earlier.

Breeds Most at Risk

CMO mainly affects terrier breeds, including the Cairn Terrier puppies, West Highland White Terrier puppies, and Scottish Terrier puppies. Due to their limited genetic diversity within pedigree lines, hereditary conditions such as CMO can spread more readily among these breeds.

It is important to note that CMO is not contagious between dogs; only those inheriting the faulty gene from their parents can develop the condition.

How is CMO Inherited?

CMO is passed through autosomal dominant inheritance with incomplete penetrance, meaning dogs can carry one or two copies of the mutated gene. Dogs with two copies generally have a higher risk of showing symptoms, but some with one copy can be affected, and some with two copies remain symptom-free carriers.

Carrier dogs, which have one mutated gene and one normal gene, do not usually show signs but can pass the gene to about half of their offspring. This complex genetic pattern means breeding decisions require careful consideration and health screening to avoid increasing the condition’s prevalence.

Recognising Symptoms in Puppies

The first signs usually appear between 4 and 8 months, including swelling of the jaw that may be uneven, painful mouth opening or inability to open the mouth, excessive drooling, and difficulty eating or drinking.

Affected puppies may also appear withdrawn, lethargic, and develop fever episodes in cycles. Prompt veterinary attention is crucial for diagnosis and management to ensure comfort and proper care.

Health Testing and Responsible Breeding

DNA testing is a vital tool for breeders aiming to prevent CMO in terrier breeds. Using a blood sample or a buccal swab, laboratories can identify whether a dog is clear, a carrier, or affected.

Breeders use these results to make informed mating choices, avoiding pairing two carriers or affected dogs. Prospective puppy buyers should seek breeders who provide health test results and transparency about the breeders' practices, thus supporting healthier puppies and reducing the chance of inherited diseases.

Responsible breeding and informed purchasing help maintain the health of terrier breeds and improve overall canine welfare.

Managing and Caring for Affected Dogs

While CMO often resolves as the dog matures, puppies with symptoms may require supportive care, including pain management with veterinary-prescribed medications like NSAIDs or corticosteroids, and feeding soft foods to ease oral discomfort.

Regular veterinary check-ups and monitoring are important to ensure the dog’s quality of life throughout the active phase of the condition.

Conclusion

Craniomandibular osteopathy is a painful, inherited bone disorder mainly affecting young terrier breed dogs, causing abnormal jaw bone growth that often improves with age.

Understanding the genetic nature of CMO and the symptoms enables early diagnosis and management. Health testing empowers breeders and buyers to reduce incidence through responsible breeding decisions, resulting in healthier dogs and improved welfare.

If you are considering bringing home a terrier puppy, always ask about health screening for CMO and buy from reputable breeders committed to responsible practices.