By Dial A Vet Veterinary Team
Last updated: 2 November 2026
A dog that tires after a short walk, walks stiffly or seems sore all over can be hard to read. Owners often put it down to age, arthritis or a pulled muscle. Sometimes the cause is polymyositis, a condition in which the immune system attacks the dog's own muscles. It can affect the muscles used to swallow and breathe as well as the legs, so some dogs develop regurgitation or breathing changes that need prompt care. The good news is that most dogs respond well to treatment.
Key takeaways
- Polymyositis is an immune-mediated inflammation of many muscles at once, mainly in the legs and body.
- It mostly affects adult dogs. Large breeds, Boxers, Newfoundlands and Vizslas appear to be overrepresented.
- Common signs are weakness that gets worse with exercise, muscle pain or swelling, a shifting lameness, fever and weight loss.
- Some dogs regurgitate food because the oesophagus is affected, which raises the risk of aspiration pneumonia.
- Diagnosis usually involves blood tests, muscle testing and a muscle biopsy.
- Treatment suppresses the immune system. The outlook is generally favourable, although relapses can occur.
What is polymyositis?
"Poly" means many and "myositis" means muscle inflammation. The Merck Veterinary Manual describes polymyositis as an immune-mediated, inflammatory disease of the limb muscles that can come on suddenly or develop slowly over time.
In most dogs the trigger is never found. Merck notes that it can occur alongside other immune-mediated diseases, such as myasthenia gravis, lupus and polyarthritis, and has been linked to certain antibiotics. Infections such as Neospora and Toxoplasma can also inflame muscles, so vets usually rule these out before settling on an immune-mediated cause.
Polymyositis is different from masticatory muscle myositis, which targets only the chewing muscles of the head and makes it painful for a dog to open its mouth.
Which dogs are at risk?
Any breed can develop polymyositis, but Merck notes that large breeds, Newfoundlands and Boxers appear to be overrepresented, and breed-related forms are described in Newfoundlands, Boxers and Vizslas. It mainly affects adult dogs and is rare in cats.
Signs of polymyositis
Merck lists these signs:
- Lethargy
- Fever
- Generalised weakness that often worsens with exercise
- Weight loss
- Shifting leg lameness, where the sore leg seems to change
- Muscle pain
- Muscle swelling
- Regurgitation
- A change in voice or bark
- Muscle wasting in longer-standing cases, which can show over the head and body
If the muscles of the throat or voice box are involved, a dog may struggle to swallow or breathe.
Regurgitation looks different from vomiting. Food comes back up passively, often soon after eating, without the heaving and retching of vomiting. It happens because inflamed muscle in the oesophagus can lead to megaoesophagus, a widened, floppy food pipe that does not push food to the stomach properly.
When to see a vet
Book a vet visit within a day or two if your dog:
- Tires much faster than usual on walks
- Seems sore, stiff or reluctant to move
- Has a lameness that moves from leg to leg
- Is losing weight or muscle
Go to a vet urgently if your dog:
- Is regurgitating food or water
- Has a cough, fever or fast breathing, especially after regurgitating, which can signal aspiration pneumonia
- Has noisy breathing, struggles to breathe or cannot swallow
- Collapses or cannot stand
How vets diagnose it
Merck explains that diagnosis usually starts with the clinical signs. Blood tests often show raised levels of muscle enzymes such as creatine kinase. Electromyography, a test of the electrical activity in the muscles, can show abnormal activity, and a muscle biopsy is the ideal way to confirm the diagnosis.
Your vet may also suggest chest x-rays to look for megaoesophagus or pneumonia, and tests for infections and other immune diseases that can cause similar signs. Some of these tests are done at a referral or specialist neurology centre.
Treatment
Treatment aims to calm the immune attack on the muscles. Merck notes that corticosteroids are the treatment of choice, given at immune-suppressing levels until the disease is in remission and then slowly reduced. Dogs with severe or relapsing disease may need additional immune-suppressing medicines. Your vet will decide the medicines and plan, and will monitor your dog for side effects.
Dogs with megaoesophagus need extra care at mealtimes. The Merck Veterinary Manual describes feeding with the upper body raised, for example with the front legs on a platform, and keeping the dog upright for at least 15 minutes afterwards so gravity can help food reach the stomach. Small, frequent meals often help, and your vet can advise on the best food texture for your dog. Dogs with aspiration pneumonia may need hospital treatment.
Never stop or change steroid medicines suddenly without your vet's advice.
Outlook
Merck describes the outlook for dogs with polymyositis as favourable, although relapses can occur. Regular rechecks help your vet adjust treatment and catch a relapse early.
Merck also notes that some Boxers have developed a type of cancer called a round cell tumour in the months after polymyositis, so your vet may suggest closer follow-up if you have a Boxer.
FAQ
Is polymyositis painful for dogs?
It can be. Inflamed muscles are often sore and swollen, and some dogs are reluctant to move or be touched.
Can polymyositis be cured?
Many dogs go into remission with treatment and do well. Some need longer-term medicine, and the disease can return, so ongoing monitoring is important.
Is polymyositis contagious?
No. Immune-mediated polymyositis does not spread to other pets or people. Infections that cause similar signs are tested for separately.
How is it different from arthritis?
Arthritis affects the joints, while polymyositis affects the muscles. Both can cause stiffness and lameness, which is why a vet examination and testing are needed to tell them apart.
Talk with a vet
If your dog has become weak, stiff or is bringing food back up, Dial A Vet's real vets and vet nurses can talk it through with you on a video consult and help you decide how soon your dog needs to be examined in person. Dial A Vet consults cover dogs and cats. Related reading: vomiting versus regurgitation and neosporosis in dogs. Book a video consult with Dial A Vet.
References
- Merck Veterinary Manual. Polymyositis in Dogs and Cats.
- VCA Animal Hospitals. Masticatory Myositis in Dogs.
- Merck Veterinary Manual. Disorders of the Esophagus in Dogs.
- Merck Veterinary Manual. Neosporosis in Dogs.
Disclaimer: Educational only. This page does not replace an examination or advice from your own veterinarian. If you are worried about your pet right now, contact your local vet or nearest emergency animal hospital.