Glycogen Storage Disease Type II in Dogs: Causes & Testing

Glycogen Storage Disease Type II (GSD II), or Pompe disease, is a rare genetic disorder in dogs characterized by a deficiency of the enzyme acid alpha-glucosidase. This enzyme is essential for breaking down glycogen, the stored form of energy found in muscles and the liver. Without it, glycogen accumulates in the cells, especially in the heart and skeletal muscles, leading to severe organ enlargement and failure.

GSD II is an autosomal recessive disease, meaning that a dog must inherit two copies of the defective gene to be affected. Dogs with only one copy are carriers and generally do not show signs but can pass the mutated gene to their offspring.

This guide provides information about GSD II in dogs—a condition seen in breeds such as the Lapphund—for American dog breeders, owners, and veterinarians.

What Is Glycogen Storage Disease Type II (GSD II)?

GSD II occurs due to mutations in the GAA gene, which encodes the enzyme needed to break down glycogen in lysosomes. When this enzyme is absent or deficient, glycogen builds up in cells, particularly in the heart, skeletal muscles, and liver. This leads to progressive muscle weakness, organ dysfunction, and heart failure.

Signs and Symptoms of GSD II in Dogs

Dogs with GSD II often show the following signs:

• Progressive muscle weakness and reduced exercise tolerance.
• Enlarged heart (cardiomegaly) or liver (hepatomegaly).
• Difficulty breathing and rapid fatigue, especially during physical activity.
• Poor growth and weight gain in young dogs.
• In severe cases, congestive heart failure.

Symptoms typically begin in young dogs—sometimes within the first few months of life—and tend to worsen over time. Early-onset cases are often fatal without intervention.

Breeds Primarily Affected

GSD II has been reported in several dog breeds globally, particularly the Finnish and Swedish Lapphunds. In the United States, awareness is growing especially among breeders of Lapphund breeds, but testing is beneficial for all breeds with documented cases or genetic risk.

Genetic Inheritance of GSD II

GSD II follows an autosomal recessive inheritance pattern:

• Clear (homozygous normal): Two normal gene copies; the dog is not affected and will not pass the mutation to offspring.
• Carrier (heterozygous): One normal and one mutated gene copy; the dog is usually unaffected but can pass the mutation to offspring.
• Affected (homozygous affected): Two copies of the mutated gene; the dog will develop the disease.

Breeding outcomes based on parent status:

• Clear × Clear = 100% Clear offspring.
• Clear × Carrier = 50% Clear, 50% Carrier offspring.
• Carrier × Carrier = 25% Clear, 50% Carrier, 25% Affected offspring.
• Clear × Affected = 100% Carrier offspring.
• Carrier × Affected = 50% Carrier, 50% Affected offspring.
• Affected × Affected = 100% Affected offspring.

Genetic Testing for GSD II in the US

In the United States, canine genetic testing for GSD II is available through several veterinary laboratories. Testing typically involves:

1. Collecting a DNA sample—usually via buccal (cheek) swab or blood—by a veterinarian.
2. Sending the sample to an accredited lab such as Embark, Wisdom Panel, or a veterinary school genetic lab.
3. Receiving results that classify the dog as clear, carrier, or affected.

Breeders should test all breeding stock and avoid pairing two carriers or affected dogs to prevent producing affected offspring.

Managing GSD II in Affected Dogs

There is currently no cure for GSD II in dogs. Management focuses on supportive care:

• Monitoring heart and muscle function with regular veterinary check-ups.
• Restricting strenuous exercise to reduce stress on the heart and muscles.
• Dietary adjustments under veterinary guidance to support overall health.
• In some cases, enzyme replacement therapy (ERT) used in human Pompe disease research may be explored, but it is not widely available or approved for dogs in the US.

Responsible Breeding Practices

Given that GSD II is a serious, progressive disorder, responsible breeding is essential to reduce its prevalence. Key practices include:

• Testing all breeding dogs before mating.
• Avoiding pairing of two carriers or any affected dogs.
• Educating buyers and fellow breeders about the genetic risks.
• Reporting cases to breed clubs and health registries such as the OFA (Orthopedic Foundation for Animals) to improve breed-wide health data.

Conclusion

Glycogen Storage Disease Type II is a serious, inherited condition affecting certain dog breeds, particularly the Lapphunds. By understanding the disease's genetic nature and utilizing available DNA testing, American breeders and owners can make informed decisions to protect future generations. Genetic screening is the most effective tool to prevent GSD II in puppies and ensure long-term breed health.