Myasthenia Gravis in Dogs

What Is Myasthenia Gravis (MG) in Dogs?

Myasthenia gravis (MG) is an autoimmune disease in dogs that causes varying degrees of muscle weakness.

MG interferes with your dog’s nerves and the way these nerves should communicate with muscles to provide normal body movements and normal muscle contractions.

Nerves transmit signals to the muscles. However, with MG, they don’t make it to the muscle properly.

The immune system makes antibodies that destroy and block the receptor (called acetylcholine receptors)  for the nerve signal. Because of this failure, nerves can’t do their job, which is to contact muscles for body movement.

In addition to muscle weakness, this condition can also affect a dog’s mental state because it can be quite debilitating. If your dog is exhibiting any changes in their physical or mental condition, they should be seen by their veterinarian as soon as possible.

Symptoms of Myasthenia Gravis (MG) in Dogs

There are three forms of MG in dogs:

• Generalized MG. This form affects 57% of dogs with acquired MG and symptoms may include:
  • Exercise-induced stiffness that resolves with rest
  • Tremors
  • Weakness, both extreme and progressive
  • Excessive fatigue
  • Megaesophagus
• Focal MG. Symptoms may include:
  • Weakness in the muscles of the face, throat, and esophagus
  • Change in bark
  • Weakness in a specific part of the body
• Fulminant MG. Least common form of MG; symptoms may include:
  • Sudden, decreased muscle tone
  • Megaesophagus
  • Rapid progression of paralysis in muscles that control a dog’s breathing

If you believe your dog is experiencing any of the above symptoms, contact an emergency veterinarian immediately.

Causes of Myasthenia Gravis (MG) in Dogs

Myasthenia gravis can be both inherited (genetic) or acquired. Acquired MG is more common than inherited MG in dogs.

MG does not just occur in dogs and cats; it also can affect humans. The neuromuscular disease is caused by a deficiency in the acetylcholine receptors found on the surface of muscle cells.

This deficiency causes the signals between nerves and muscles to be interrupted, leading to muscle weakness in different areas of the body. The muscle weakness then leads to extreme fatigue within the dog.  

The following breeds are predisposed to inherit congenital MG, meaning they’re born with the condition:

• Jack Russell Terriers

• English Springer Spaniels

• Smooth Haired Fox Terriers

• Smooth Haired Miniature Dachshunds

Dogs with inherited MG are born with fewer receptors and tend to be diagnosed when they are less than 1 year old.

Acquired MG is more often seen in older dogs, including:

• Newfoundlands

• Great Danes

• Golden Retrievers

• Akitas

• Scottish Terriers

Acquired MG can occur because of environmental, infectious, or hormonal influences.

These influences affect the immune system negatively and cause antibodies to be produced against the receptors, thus preventing muscle movement.

This immune response is typically seen in dogs with chest tumors (thymoma) and can be associated with hypothyroidism.

How Veterinarians Diagnose Myasthenia Gravis (MG) in Dogs

If your dog is showing any changes in their physical or mental ability, they should be seen by their vet as soon as possible.

It’s also a good idea to take a video of your dog’s behaviors to show your vet in case your pup can’t show the signs at their appointment, such as exercise-induced symptoms.

Your vet will perform a complete physical exam and analyze your pup’s bloodwork, including thyroid hormone levels. A diagnosis of MG in dogs is made when your vet takes a blood sample to measure antibodies against acetylcholine receptors.

Elevated blood levels can confirm an MG diagnosis and the results are also used to monitor your dog’s response to treatment. Since this blood test needs to be sent to a special laboratory, it may take a few days to a few weeks to get the results.

If MG is suspected by your vet based on the pup’s clinical signs and/or the breed of dog, a Tensilon test can be ordered while waiting for the blood test results. For this test, your dog will get an IV injection.

This test will diagnose a pup’s MG. When a dog is given this injection and they do have MG, you will see an immediate improvement of weakness and other symptoms. Unfortunately, this injection and relief of symptoms only lasts for a few minutes.

Additionally, chest X-rays will be performed to check for megaesophagus, aspiration pneumonia, or cancer. Aspiration pneumonia occurs secondary to regurgitation and muscle weakness in the throat.

Treatment of Myasthenia Gravis (MG) in Dogs

MG in dogs cannot be cured, but it can be treated and successfully managed. Treatment will depend on the type of MG diagnosed in your dog and the symptoms associated with their type of MG.

If your dog has been diagnosed with MG, the most common medication used for treatment is pyridostigmine bromide.

This medication will improve your pup’s muscle function by increasing the amount of time acetylcholine stays in the receptors on the muscle cells to coordinate movement.

Prednisone or other steroid medications can also be used because they help suppress the immune system, thus suppressing the response that is causing MG. If your dog is diagnosed with associated hypothyroidism, levothyroxine sodium will most likely be prescribed for the rest of their life to manage the condition.

Dogs with megaesophagus need to be fed their meals in a very special manner. They must be fed in an elevated food bowl, off the ground or in a special “Bailey chair” to keep them upright.

This chair helps to to avoid inhalation of their food, which could result in aspiration pneumonia. It is recommended that pups with this condition be fed small meals more frequently during the day instead of two large meals. Dogs with megaesophagus should also be kept standing upright on their hind legs or in the special chair for 20 to 30 minutes after eating.

If the dog has a thymoma tumor, surgery may be an option to remove if they are healthy enough for the operation. 

Recovery and Management of Myasthenia Gravis (MG) in Dogs

A diagnosis of MG is typically serious, and the prognosis varies depending on your dog’s clinical signs, the type of MG they have, and any underlying causes.

If your dog’s MG is caught and treated early, remission can be reached after several months, but some dogs do need lifelong treatment.

Dogs that have aspiration pneumonia have a less favorable prognosis, especially if they have breathing complications or chest tumors.

Prevention of Myasthenia Gravis (MG) in Dogs

There are no known ways to prevent myasthenia gravis in dogs at this time.