Glycogen Storage Disease Type IV is caused by a branching enzyme deficiency that causes an inability to metabolize glucose properly, resulting in an accumulation of an abnormal form of glycogen in the muscles and liver. This disorder leads to the enlargement or malfunction of different vital organs in the body and can also lead to muscular degeneration. Symptoms may become evident after about five to seven months of age, but the disorder more commonly manifests within the womb, causing stillbirth or death soon after birth, primarily from hypoglycemia.

Glycogen storage disease is a rare, inherited metabolic disease that can affect both people and animals. In people and most animals, the condition falls into one of three types depending on the type of enzyme deficiency. The fourth classification pertains only to cats and so is commonly referred to as Glycogen Storage Disease Type IV, or GSD IV, but can also be known as glycogenosis. Glycogen molecules normally act as carbohydrate storage and convert to glucose whenever the body is in need of energy. An accumulation of an abnormal form of glycogen, unfortunately, has fatal consequences. It is almost entirely found in Norwegian Forest cats, a rare breed of domestic cats. Inbreeding of these cats to create an American line appears to be the original cause of the disorder. Many of these lines have stayed in Europe, but the disorder is becoming more prevalent in the United States. 

Should the kitten survive birth and the neonatal period, symptoms will typically begin to appear sometime between five and seven months of age. These symptoms are:

The one cause of GSD IV was originally due to inbreeding. This led the genetic line of Norwegian Forest cats (and eventually select other, closely related cats who bred with Norwegian Forest cats) to have a deficiency of a glycogen branching enzyme that aids in glycogen metabolism.

If you have a Norwegian Forest cat, you should have it tested for GSD IV immediately. There is a very high chance that your cat is either directly affected or is a carrier. Being a carrier seems to be more common.

There are several tests your veterinarian can perform to make a definitive diagnosis for GSD IV. These include an enzyme analysis, blood tests for glycogen and creatine kinase levels, an electrocardiogram (ECG) to test for heart irregularities, and a urine analysis. 

A genetic test for GSD IV has been developed in order to help identify the condition in a kitten and to see if a seemingly healthy cat is an early carrier. Your veterinarian may or may not have access to this test. If he or she does have access, it is the best test that can be conducted for a definitive diagnosis.

If your cat is determined to have GSD IV, the main condition to be treated is hypoglycemia, which is a deficiency of glucose in the blood (low blood sugar). Close monitoring and a change to a high carbohydrate diet will be the first to be addressed. 

Unfortunately, there is no cure for GSD IV and death is certain. If death did not occur during or soon after birth, it will occur sometime between 10 and 14 months. Your cat may first physically deteriorate, lapse into coma, and then experience heart failure, however. You may want to consider euthanasia before this happens to prevent your cat from suffering. 

If your cat is determined to be a carrier, you need to have your cat neutered or spayed right away in order to prevent genetically spreading the disorder. Your cat’s offspring also need to be tested so that you and your veterinarian can proactively manage the symptoms and determine a plan for when death takes place. 

Since GSD IV is an inherited disorder of Norwegian Forest cats, contact your veterinarian right away to schedule testing for it. It is likely that cats of this breed are affected in some way.

Being a carrier of GSD IV is very possible, so the prevention of spreading it to your cat’s offspring is essential. If your cat is found to be a carrier, it is extremely important that it, or its parents, are not bred. You should instead neuter or spay every cat who is affected immediately. 

If you are considering buying a Norwegian Forest kitten from a breeder, be sure to choose a reputable breeder and ask very specific questions regarding the health of the breeding pair including whether they and their parents have been tested for Glycogen Storage Disease Type IV. Routine testing of Norwegian Forest cats may help to prevent the spread of this disorder since it is very possible that it could become easily established if any affected cat should mate with a healthy cat-- of either the Norwegian Forest breed or a different breed.