Polymyositis is a relatively common disease in dogs that causes muscle weakness and fatigue. There are 3 main causes of polymyositis in dogs: infectious agents, immune-mediated and paraneoplastic causes. While there is no cure for polymyositis, there are several treatments that can help alleviate symptoms and improve the dog's quality of life. If you think your dog may have polymyositis, please see your veterinarian as soon as possible. Early diagnosis and treatment are essential for managing this condition.
What is polymyositis in dogs?
Polymyositis is an inflammation of the striated skeletal muscle that involves more than one muscle group. It can be a primary disorder or develop secondary to other conditions. The 3 main causes are infectious agents (bacterial, rickettsial, viral, protozoa, and parasitic organisms), immune-mediated and paraneoplastic.
Polymyositis can affect any age, gender or breed of dog. Some types of polymyositis are more common in certain breeds of dogs. Polymyositis can include dermatomyositis which occurs in shelties and collies and masticatory myositis which occurs in large breed dogs.
What are the causes of polymyositis in dogs?
The exact cause of polymyositis is unknown. However, 3 main causes have been identified which include infectious agents, immune-mediated and paraneoplastic causes.
Infectious agents: These are the most common type of polymyositis and can be caused by several different bacteria, viruses, protozoa or parasites. Examples include Neospora and Toxoplasma which are protozoal organisms and can infect muscles.
Immune-mediated: This type of polymyositis occurs when the body's immune system attacks and damages healthy muscle tissue. Polymyositis has been linked to systemic lupus erythematosus and dermatomyositis, as well as other autoimmune diseases. Extraocular myositis and masticatory myositis are two distinct, local types of immune-mediated polymyositis in dogs.
Paraneoplastic: This type of polymyositis is caused by cancerous tumours that release substances into the body that damage muscle tissue. An example of paraneoplastic causes of polymyositis in dogs is lymphoma as it can affect the muscular system.
What are the signs and symptoms of polymyositis in dogs?
The signs and symptoms of polymyositis will depend on the underlying cause. Dogs affected by polymyositis experience lameness and stiffness that comes and goes, as well as swelling in their joints. Eventually, the muscles around these joints will start to atrophy. Some common signs may be seen in dogs with polymyositis which includes:
- Generalized weakness that worsens with exercise
- Muscle swelling
- Anorexia (lack of appetite)
- Exercise intolerance
- Muscle wasting
If the muscles of a dog's head are affected, they may drool excessively, have difficulty swallowing, and a change in their bark may be noticed. If the esophagus is involved, regurgitation may occur. If your dog is showing any of these signs, please see your veterinarian as soon as possible.
How is polymyositis diagnosed in dogs?
A diagnosis of polymyositis can be difficult as several different conditions can cause similar signs. Your veterinarian will perform a comprehensive history and physical examination.
This will be followed by a series of diagnostic tests which may include:
Complete blood count (CBC): This will help to rule out other conditions such as infections that can cause similar signs.
Biochemical profile: This will assess your dog's organ function and check for any abnormalities. Within the biochemical profile, the CK (creatinine kinase) level can be checked which will evaluate any muscle damage that may be present.
Muscle biopsy: A small sample of muscle tissue will be taken and examined under the microscope. This is the best way to confirm a diagnosis of polymyositis.
What is the treatment for polymyositis in dogs?
Specific therapy depends on the underlying cause of polymyositis. For immune-mediated polymyositis, immunosuppressive therapy is indicated. Common immunosuppressants include prednisone, azathioprine and cyclosporin. Until muscular discomfort subsides in response to the primary therapy, analgesics might be given. Physiotherapy can also be beneficial for some dogs.
For polymyositis caused by infectious agents, treatment will be focused on the specific organism that is causing the condition. For example, if the polymyositis is caused by Neospora or Toxoplasma, treatment with antiprotozoal medications will be necessary.
You will need to have physical exams and get your dog's CK (creatine kinase) levels checked regularly to see how well the treatment is working. If your dog is on immunosuppressive therapy, your veterinarian will recommend having regular blood tests to check for any potential side effects from the medication.
What is the prognosis of polymyositis in dogs?
The chances for a good prognosis vary depending on the cause of the disease and how well it can be treated. The more severe the disease is when it's found, the worse the prognosis is likely to be. Muscle atrophy (wasting) that's present at diagnosis is especially serious because it usually can't be reversed.
Polymyositis in dogs is a condition that affects the muscular system. There are several different causes, but the most common is an inflammatory response to infection or cancer. Symptoms include generalized weakness, muscle wasting, and difficulty tolerating exercise. Treatment depends on the underlying cause and may involve immunosuppressive therapy, antibiotics or antiprotozoal medications. The prognosis varies depending on the severity of the condition but can be quite good with early diagnosis and treatment. If you have any questions or are concerned about polymyositis in your dog, please contact your veterinarian.