Myositis in dogs – muscle inflammation and jaw problems

Overview

Not every muscle disease is inflammatory

Myopathy is a general term for disease affecting skeletal muscle. Myositis involves inflammatory changes within muscle. Other muscle disease groups include inherited degeneration, metabolic disorders and traumatic injury. These do not automatically require the same treatment as immune-mediated myositis.

Polymyositis may be acute or chronic and progressive. Inflammation often affects the limb muscles. Masticatory myositis particularly targets muscles involved in closing the jaw. It therefore appears different from a disease mainly impairing movement.

With muscle-related weakness, sensation and proprioception may remain normal. Distinguishing nerve, neuromuscular junction and muscle diseases can nevertheless require several investigations.

Causes

Immune-mediated muscle damage

In polymyositis, the dog's immune system causes inflammation in muscle. In masticatory myositis, antibodies target the chewing muscles' specialised 2M muscle fibres. These muscles have a fibre type different from that of the limb muscles. This helps explain why inflammation can be confined to the chewing muscles.

The initial trigger for masticatory myositis usually remains unknown. Polymyositis can also occur alongside other immune diseases. In some dogs, muscle inflammation may be associated with a tumour. This does not mean every dog with myositis has cancer.

Infectious myositis

A muscle may have inflammation caused by a bacterium or another microbe. Clostridial infection may affect one or more muscles in a single limb. A bite or other tissue injury may be responsible. The protozoa Toxoplasma and Neospora can also damage a dog's muscles. Infectious myositis can form part of a more widespread inflammatory disease.

Risk factors

Typical immune-mediated polymyositis is most often diagnosed in adult dogs. Large dogs, particularly Newfoundlands and Boxers, are overrepresented. Breed-associated polymyositis is also recognised in Hungarian Vizslas. Any breed can nevertheless be affected.

Dogs with masticatory myositis are around 3 years old on average. The disease has been diagnosed in puppies as young as 4 months. Both sexes can be affected. Breeds considered predisposed include German Shepherd Dogs, Golden Retrievers, Labrador Retrievers and Dobermanns.

A distinct hereditary polymyositis occurs in Kooikerhondjes. In a 2025 follow-up study of 97 dogs, the median age at onset was 3.5 years. The range was 1.0–10.4 years. The outlook for this breed's disease does not apply directly to every form of muscle inflammation in dogs.

Combining Kooikerhondje PM1 and PM2 test results refines the estimated risk of hereditary polymyositis. A July 2026 calculation estimated a 0.3% disease risk for a PM1 carrier clear of PM2. A dog carrying one copy of each change had an estimated risk of 0.8%. For a PM1 carrier with both PM2 risk copies, the estimate was 1.9%.

A dog with two copies of the PM1 change had a 16.9% risk if clear of PM2. With an additional single PM2 risk copy, the estimate was 19%. With two risk copies of both changes, the estimate rose to 73.1%. These figures concern the breed's particular hereditary disease, rather than myositis in other breeds.

Typical symptoms

Signs depend on which muscles are inflamed. Possible observations include:

• Your dog tires and weakens more quickly than usual during exercise.
• Movement is stiff or lameness shifts between legs.
• Muscles are painful or swollen, or your dog has a fever.
• Muscle bulk decreases and your dog loses weight.
• The voice becomes hoarse or swallowing becomes difficult.
• Your dog regurgitates food because of oesophageal dysfunction.
• Your dog cannot open their mouth, chew or pick up a toy.
• The muscles over the temples or cheeks swell or later become hollowed.

In acute masticatory myositis, the eyes may appear to bulge. In the chronic stage, muscle wasting may be more obvious than swelling. Neospora or Toxoplasma infection in a young dog can cause stiff, overextended hindlimbs.

Complications and consequences

Muscle scarring and nutritional problems

Long-standing masticatory myositis can replace muscle tissue with fibrous tissue. Restricted jaw opening and muscle loss may then be permanent. Severe eating difficulties can lead to weight loss and a need for nutritional support. Polymyositis can involve an enlarged oesophagus and resulting regurgitation. Pharyngeal or laryngeal muscle damage can also impair breathing.

Early treatment of masticatory myositis can allow the mouth to open normally and painlessly again. The prognosis for typical polymyositis can also be favourable, although relapse is possible. Masticatory myositis can relapse as well.

The Kooikerhondje disease has a different outlook

In the 2025 hereditary polymyositis follow-up study, Kooikerhondjes had a median survival of 9.8 months. Twelve dogs were excluded from survival analysis because of atypical treatment or death from a tumour. This result concerns that particular hereditary disease, rather than all myositis. Treatment can improve quality of life. Current treatments cannot, however, cure the disease. Some dogs may nevertheless become almost free of signs.

Diagnosis at the vet

Investigating the location and extent of inflammation

Your vet assesses walking, muscle strength, muscle bulk and the location of pain. Neurological examination helps distinguish muscle-related weakness from nerve or neuromuscular junction disease. Polymyositis signs can resemble myasthenia gravis. A separate antibody test can investigate neuromuscular junction disease where needed.

Blood creatine kinase, or CK, can rise when muscle fibres are damaged. AST may also be raised. CK can nevertheless return to normal during chronic masticatory myositis. A normal muscle enzyme result then does not establish that chewing muscle disease is absent.

Electromyography, or EMG, can reveal abnormal spontaneous electrical activity in muscle. MRI or CT helps locate inflamed muscles and choose a biopsy site. A muscle biopsy can confirm inflammatory changes and distinguish them from other muscle diseases.

A jaw hanging open is a different finding from a jaw the dog cannot open. Inability to close the jaw may be associated with jaw paralysis. This distinction is also investigated before labelling the problem masticatory myositis.

A chewing muscle antibody test

Masticatory myositis is investigated using a blood 2M antibody test. The aim is to collect the sample before immune-suppressing medication begins. Corticosteroid treatment can cause a false negative antibody result. Antibodies are not found in around 15% of dogs with masticatory myositis.

A negative result may be followed by a chewing muscle biopsy. A biopsy also shows how much scarring has already formed in the muscle. The antibody titre does not establish an individual dog's recovery prognosis. In a rare overlapping form, both limb and chewing muscles can become inflamed and 2M antibodies may be present.

Infection and genetic testing

Infectious myositis is confirmed by identifying the likely organism. Tests may include biopsy, culture, PCR or repeat antibody measurement. A genetic test is available for Kooikerhondjes to assess predisposition to their breed's polymyositis. Predisposition is not the same as a diagnosis of muscle inflammation causing signs.

Treatment

Treatment targets the cause of inflammation

Your vet prescribes care according to the identified disease form and your dog's clinical condition. Immune-mediated polymyositis and masticatory myositis are usually treated with a glucocorticoid. Severe or relapsing disease may require additional medication suppressing the immune response. Pain relief may be needed, particularly early in the disease.

Infectious inflammation requires treatment targeting its cause. Suppressing the immune system alone is therefore not a shared home solution for every type of myositis. Treatment response is monitored through the dog's ability to function and appropriate follow-up investigations.

Maintaining nutrition in masticatory myositis

Soft food mixed with water may make eating easier when the mouth opens only slightly. In severe cases, a feeding tube can maintain nutrition. Severe acute disease may also require intravenous nutrition.

The jaw is not forced open for exercise, even under anaesthesia. This carries a risk of treatment-induced fracture. In acute masticatory myositis, treating inflammation may restore jaw movement without separate stretching. If muscle has been replaced by fibrous tissue, physiotherapy does not restore its normal structure.

Distinguishing treatment effects from disease signs

Corticosteroids can increase appetite, drinking and urination. Long-term glucocorticoid treatment can also cause muscle wasting. Reduced muscle bulk therefore does not always reflect worsening inflammation alone. Relapsed masticatory myositis can be harder to treat than the original episode.

Prevention

The trigger for masticatory myositis is often unknown, so there is no precise general preventive measure. Breed-specific genetic information can be used in Kooikerhondje breeding to assess polymyositis risk. Predisposition does not, however, mean that every dog carrying a risk variant becomes ill.

Examining bites and other tissue injuries is important because bacterial myositis can be associated with trauma. Early recognition and treatment of masticatory myositis improve the chance of preserving normal jaw movement. Movement already lost through chronic scarring may remain restricted.

Important for dog owners

Monitor whether your dog can open their mouth, chew and swallow without obvious pain. Also record walking stamina, stair use, getting up, and changes in muscle bulk and weight. Weakness from polymyositis may worsen during exercise.

Do not force a painful jaw to stretch. Forced opening can cause a fracture. Follow your dog's feeding and follow-up plan. A marked increase in appetite may also be associated with medication.

New swallowing or breathing difficulties need prompt assessment. Tell the treating vet if mouth opening decreases again or movement deteriorates during treatment. Inflammation can relapse even after a good initial response. Keep test results so that later assessments can establish whether testing concerned localised chewing muscle disease or a more widespread condition.

Frequently asked questions

Is myositis the same as a muscle strain?

Myositis involves inflammatory changes in muscle. Traumatic muscle injury belongs to a different muscle disease group. Polymyositis can be immune-mediated and widespread. Masticatory myositis particularly targets the chewing muscles' specialised fibre type. Pain alone therefore does not establish the cause of damage.

Does a normal CK result rule out masticatory myositis?

CK can rise in acute masticatory myositis and return to normal in the chronic stage. Investigations may therefore also require 2M antibodies and a muscle biopsy. The biopsy assesses muscle scarring as well as inflammation. The blood value and jaw function can therefore reflect different aspects of the disease.

Can a negative 2M antibody test be wrong?

Around 15% of dogs with masticatory myositis have no detectable 2M antibodies. Previous corticosteroid treatment can also cause a false negative result. A muscle biopsy can then help clarify the diagnosis. The antibody titre also does not determine an individual dog's prognosis.

Can you stretch a stiff jaw open by hand?

The jaw is not forced open because of the risk of fracture. In acute disease, treating inflammation can restore a normal or near-normal range of movement. Physiotherapy does not restore function to muscle chronically replaced by fibrous tissue. Home care focuses on prescribed treatment and safe feeding.

Does every form of myositis recover in the same way?

The outlook depends on the form of inflammation. Early treatment of masticatory myositis can restore good jaw function. Movement restriction in extensively scarred muscle may be permanent. The prognosis for hereditary polymyositis in Kooikerhondjes requires separate assessment and is often poor despite appropriate treatment.