Phosphofructokinase (PFK) is an important enzyme that is found in cells of the body and is involved in the use and regulation of glucose as an energy source. A deficiency of the PFK enzyme in cells interferes with the metabolism of glucose, and the main signs seen in affected dogs relates to the muscles and the blood cells. PFK deficiency is an autosomal recessively inherited defect, and is often present by 2 – 3 months of age. However diagnosis is often delayed, as owners may not notice the more subtle changes, and sometimes affected dogs may go for several years before being diagnosed.
PFK deficiency leads to muscles that fatigue very easily, because they cannot use energy very efficiently. This manifests as muscle weakness, an inability to exercise properly, or muscle cramping. Also the muscle breakdown product myoglobin can be excreted in the urine, causing a brown discolouration of the urine. This myoglobin can also cause kidney damage or acute renal failure in large amounts.
PFK activity in red blood cells is also reduced, leading to an abnormally short lifespan of these cells. This causes a (usually mild) haemolytic anaemia, and often a reticulocytosis can be detected on routine blood screening. Occasionally dogs may present with an acute severe haemolytic crisis, where a large number of red blood cells have been broken down at the same time, causing severe anaemia with shortness of breath, lethargy, weakness, fever and pale gums. Sometimes jaundice may be seen as well (yellowing of the sclera of the eyes and gums).
There is no cure for PFK deficiency, and treatment is supportive only. Affected animals should avoid exercising, which can lead to muscle damage and pain, and the kidneys should be protected from muscle breakdown products (with iv fluids) to protect them from damage if necessary (e.g. after an episode of running/exercise). Occasionally blood transfusions may be required for severe anaemia. General therapy with L-carnitine, riboflavin and coenzyme Q10 has been reported to have some beneficial effects. Dogs may have a relatively normal lifespan if they avoid situations of stress and/or exercise.
After 3 months of age the activity of PFK can be tested in the red blood cells, and there is also a DNA test available for several breeds (the English springer spaniel, the American and English cocker spaniel, the Wachtelhund and the whippet), which allows for accurate screening of breeding stock. This disease has also been reported in mixed breed dogs.