If you've recently noticed an increased resistance to exercise and a tendency to lay around on the part of your dog, then he or she might very well be the victim of a phosphofructokinase deficiency.
Phosphofructokinase deficiency is a disease that inhibits the body from metabolizing glucose into usable energy, leading to the aforementioned lack of enthusiasm and pep that can seem to strike without warning. In advanced stages, the disease can even lead to the onset of more advanced muscular diseases and destroy red blood cells, which can lead to anemia. [...]
English Springer Spaniels are generally healthy dogs, but within the breed, there are some common health problems. The breed is prone to hip dysplasia, progressive retinal apathy, retinal dysplasia and phosphofructokinase deficiency. Owners should contact a veterinarian if symptoms present themselves. [...]