Glycogen Storage Disease (Discovered in the Norwegian Forest Cat) is caused by a deficiency of a glycogen branching enzyme (GBE). The disease leads to excessive glycogen accumulation in muscle cells, liver cells, and neurons and impairment of endogenous glucose production. Abnormal function of these cells leads to severe organ dysfunctions. Most affected kittens die at birth or soon after birth due to profound glucose deficiency. Kittens surviving the postnatal period seem clinically normal until roughly five months of age, when progressive neuromuscular and cardiac degeneration occurs. The condition leads to severe organ dysfunctions before one year of age with skeletal muscle atrophy, tetraparesis, severe cardiac problems, persistent hyperthermia, tremors, and seizures.

Most kittens do not survive the immediate postnatal period. Ensuring adequate glucose intake is essential for affected kittens. The kittens that survive the first few days of life tend to have progression of clinical signs after five months of age, and affected cats are usually euthanized on welfare grounds by one year of age.

There are many responsibilities to consider when breeding cats. Regardless of test results it is important that your cat is in good general health and that you are in a position to care for the kittens if new responsible owners are not found. For first time or novice breeders, advice can be found at most cat registry websites.

This disease is autosomal recessive meaning that two copies of the mutation are needed for disease signs to be shown. A carrier cat with one copy of the GSD mutation can be safely bred with a clear cat with no copies of the GSD mutation. About half of the kittens will have one copy (carriers) and half will have no copies of the GSD mutation. Kittens in a litter which is expected to contain carriers should be tested prior to breeding. Carrier to carrier matings are not advised as the resulting litter may contain affected kittens. Please note: It is possible that disease signs similar to the ones caused by the GSD mutation could develop due to a different genetic or clinical cause.