Type II Muscle Fibre Deficiency in Labrador Retrievers

This is a rare inherited muscle disorder which causes a disturbance of gait in young Labrador Retrievers. Affected dogs have a relative predominance of Type I muscle fibres and deficiency of Type II muscle fibres. Type I muscle fibres are referred to as slow-twitch fibres and they are used when minimal muscluar activity is required. Type II muscle fibres are known as fast-twitch fibres and they are used when a lack of oxygen develops during maximal stimulation (e.g. during prolonged or endurance exercise).

The actual cause of the condition is not known.

What Signs do Affected Dogs Show?

Signs are normally first seen at less than 5 months of age and initially present as an abnormal gait. The first sign seen is an inability of the affected dog to hold its head up, and weakness of all muscles when doing exercise.

The legs become stiff when exercise continues, and become extended causing the dog to take short stops. The hind limbs quite often move in unison, causing the affected dog to 'bunny hop', whilst the front legs may collapse. The signs abate when the dog rests, but reappear as exercise is recommenced.

Affected dog will show decreased muscle mass and overall stunted growth.

Signs are mildly progressive until the dog matures and then usually stabilise, though cold weather can aggrevate the signs.

How can the Condition be Diagnosed?

Suspicion is raised when young Labrador Retrievers demonstrate a stiff tilted gait which worsens with exercise.

Results of blood test show that the creatinine phosphokinase levels are normal which indicates that there is no damage to the muscle tissue. Urine creatinine level is often up to 30 times normal because creatine is formed in the liver and is degraded to creatinine at the site of Type II muscle fibres, which are relatively deficient in affected dogs.

Muscle biopsies taken from large muscle groups of affected dogs show a variation in muscle fibre diameter and a deficiency of Type II muscle fibres. Results of neurological examination and nerve function tests are normal.

How can the Condition be Treated?

There is no known treatment method though Valium may be able to aleviate some signs.