Epilepsy, neuromuscular disorders and a host of other conditions can cause muscle spasms or tremors in Jack Russell terriers. Myotonia congenita, an inherited neuromuscular disorder affecting skeletal muscles, postpones muscle relaxation following rigorous exercise in several breeds, including the Jack Russell terrier.
Neuromuscular conditions involve neurons and muscle fibers, and the most prominent clinical symptom is muscle weakness, according to "Veterinary Neurology and Neurosurgery Journal." Epileptic seizures, brain tumors and encephalitis produce muscle spasms, as well.
Jack Russell terriers with myotonia congenita experience muscle spasms and muscle rigidity following exercise. “Stiffness is usually present after rest and characteristically disappears after exercise ('warming-out' phenomena),” according to the "Veterinary Neurology and Neurosurgery Journal." Increased muscle development, or hypertrophy, of the tongue, neck and proximal limb muscles usually coincides with myotonia congenita, states "Veterinary Neurology and Neurosurgery Journal."
Canine epilepsy causes seizures. Just before a seizure strikes, Jack Russells often experience anxiety and muscle spasms, which can worsen during the seizure. Anti-epileptic medication, including Phenobarbital, as well as surgery, can manage some forms of epilepsy very well, according to VetInfo.com.
Brain Tumors and Encephalitis
Brain tumors and encephalitis in Jack Russell terriers can cause muscle spasms. Mosquitoes cause viral encephalitis, an inflammation of the brain, and muscle spasms
Applying heat to areas where muscles are in spasm provides some pain relief as it relaxes the affected muscles. A 20-minute application of moist heat penetrates the muscles more deeply, but care must be taken to avoid burning the Jack Russell terrier accidentally.