There is a group of inherited genetic diseases known as muscular dystrophies (MD), which gradually weaken the muscles, making walking increasingly difficult. The most common form of muscular dystrophy in humans is Duchenne muscular dystrophy (DMD). Dogs are known to exhibit a similar type.
A number of breeds have been reported so far, including the Golden Retriever, Rottweiler, German Shorthaired Pointer, Irish Terrier, Groenendaeler Shepherd, Japanese Spitz, Samoyed, Miniature Schnauzer, Brittany Spaniel, Rat Terrier, Old English Sheepdog, Cavalier King Charles Spaniel, Weimaraner, Alaskan Malamute, Pembroke Welsh Corgi, Labrador Retriever, Boston Terrier, Cocker Spaniel, and Chihuahua.
MD is unfortunately incurable, and as the disease progresses, it can lead to loss of quality of life or life-threatening complications.
What causes muscular dystrophies?
MDs are genetic disorders. Genes are composed of a biological material known as DNA. Genes are responsible for controlling the production of chemicals (proteins) in the cell. In MD, faulty genes (mutations) interfere with the production of proteins that are necessary for the healthy development of muscles and for the protection of the muscles against damage. As a result of the lack of these muscle proteins, the skeletal muscles gradually degenerate; with some types of MD, the cardiac muscles may also be affected, resulting in heart disease. When one of these genes is defective, muscular dystrophy results.
MD can take many forms. A genetic mutation is responsible for causing each type of MD. It is common for mutations to be inherited, but they can also occur spontaneously.
The most common form of muscular dystrophy is dystrophin deficiency, which is caused by genetic mutations of the dystrophin gene. It is known in humans as Duchenne MD. The dystrophin gene is located on the X-chromosome. Males possess one X chromosome and one Y chromosome, while females possess two X chromosomes. Due to the fact that males have only one copy of each gene on the X chromosome, if one of these genes is mutated, as with dystrophin-deficient MD, they may be affected. Due to the fact that females have two copies of the X chromosome, they are less likely to develop the disease since the normal copy of the chromosome can usually compensate for the defective one. There is a tendency for female dogs to exhibit milder signs than male dogs when they do become affected.
What are the signs of muscular dystrophy?
Most often, the signs appear in the first six months of life and progress rapidly. It is common for affected dogs to exhibit muscle weakness, difficulty standing or walking, difficulty swallowing and excessive drooling. There is a rapid loss of muscle mass in most muscles. Consequently, shortening of muscles (contractures) can further reduce mobility by limiting joint movement in the limbs. In some cases, muscles may appear bulkier than normal (e.g. the tongue and some muscles of the thighs).
MD becomes life-threatening when the heart and breathing muscles are affected. It is also possible to inhale food when swallowing difficulties occur, which can lead to life-threatening pneumonia.
How do I diagnose muscular dystrophy?
In order to diagnose MD, a number of tests may be performed, as well as to rule out other muscle disorders that may also have similar signs (for example, infectious or immune diseases causing muscle inflammation, inherited muscle diseases, hormonal diseases affecting salt levels which affect muscle function).
- Blood enzyme tests: Damaged muscles release enzymes into the blood. Creatine kinase (CK) is one of these enzymes. A traumatic injury, such as a road traffic accident, may result in an elevated blood level of CK. In animals with MD, however, CK blood levels are often very high, reflecting the deterioration of muscle fibres.
- Electromyography: An electrode needle is inserted into the muscle to test muscle function and integrity. Anaesthesia is required for this test. In spite of the fact that this test can confirm the presence of a muscle disease, it cannot determine the nature of the disease.
- Muscle biopsy: The removal and examination of a small sample of muscle tissue. The analysis of this muscle tissue allows confirmation of the diagnosis of MD. An expert neuromuscular laboratory can determine the type of MD.
- DNA (genetic) testing: A blood sample or buccal swab to test the animal DNA is available for some breeds known to be affected by MD.
Can muscular dystrophies be treated?
There is currently no known cure for MD, but there are ways to help manage its signs. As inactivity can worsen the disease, physical therapy is recommended as a means of preventing contractures.
In the research setting, new therapies are being investigated in humans and dogs (stem cells, gene therapy, pharmaceutical agents intended to prevent muscle degeneration and promote muscle regeneration).
Will my dog recover from muscular dystrophy?
Unfortunately, the long-term prognosis of the disease is very poor, since most animals affected will progressively become worse and lose their ability to walk in the future. Inhalation of food can also cause difficulty breathing, heart failure, and pneumonia.