Dogs that are dragging their back legs are usually suffering from a form of paralysis, which is related to the nervous system, the muscular system and the spinal system. Dogs that are dragging their back legs should be brought to their veterinarian immediately because if the problem is not addressed quickly, the dog can become fully paralyzed.
In paralysis situations, "the sooner the dog receives treatment, the better the outcome" writes the University of Purdue veterinary school. Many times, the dog will need to see a specialist for the best results.
The reason for the dragging of the back legs is usually related to trauma, such as ruptured vertebral disks, degenerative myelopathy or a car accident. Other causes include polymyositis, polyneuritis, distemper and embolus as well as cancer and tumors.
In the trauma cases, the dog's brain is unable to tell its muscles and nerves to move the legs. The communication between the brain and the body has been shut off. If the paralysis only affects the back legs then that means the communication is still working between the brain and the front legs. Paralysis can occur throughout the entire body and it is also possible for the brain to be unable to communicate with the lungs in order to breathe. All diagnosis and treatment will depend on the condition writes PetMD.