Mucopolysaccharidosis (MPS) is a rare, inherited metabolic storage disorder affecting cats. The disease is characterized by the body’s inability to properly break down certain complex sugar molecules. The resulting buildup of these substances within cells and tissues leads to progressive damage throughout the cat’s body.
The Metabolic Basis of Feline Mucopolysaccharidosis
MPS belongs to a group of conditions known as lysosomal storage diseases. Lysosomes are small compartments within cells responsible for waste disposal and the breakdown of certain molecules. In MPS, the body fails to produce or utilize specific lysosomal enzymes necessary for this digestive process.
The complex sugars that cannot be broken down are called glycosaminoglycans (GAGs). GAGs are long chains of sugar molecules found in connective tissues, cartilage, and bones. When the necessary enzymes are missing, GAGs accumulate inside the cells, causing them to swell and malfunction. This accumulation leads to widespread tissue and organ damage.
Several types of MPS have been identified in cats, with the most common being MPS I, MPS VI, and MPS VII. MPS I is defined by a deficiency of the alpha-L-iduronidase enzyme, while MPS VI involves a defect in the arylsulfatase B enzyme. A deficiency in the beta-glucuronidase enzyme characterizes MPS VII. Each specific enzyme failure causes the accumulation of different types of GAGs, leading to the physical signs observed in affected cats.
Genetic Basis and Inheritance Patterns
Feline mucopolysaccharidosis is classified as an autosomal recessive genetic disorder. This means the condition is passed down through genes located on non-sex chromosomes. A cat must inherit two copies of the defective gene—one from each parent—to develop the disease. The specific gene mutations affect the production or function of the required lysosomal enzymes, such as the IDUA gene for MPS I or the ARSB gene for MPS VI.
Cats that inherit only one copy of the mutated gene are considered carriers. Carriers do not show signs of the disorder but can pass the defective gene to their offspring. When two carrier cats breed, there is a 25% chance for each kitten to inherit both copies of the mutated gene and be affected by MPS.
Genetic screening is an important tool for managing this disease within breeding populations. Breeds such as Siamese, domestic shorthair, and Maine Coon have been identified as carrying specific MPS types. Identifying carriers allows breeders to make informed decisions to prevent the disease’s spread.
Recognizing the Clinical Signs in Cats
The accumulation of GAGs causes physical abnormalities that often become noticeable early in a kitten’s life, sometimes as early as six to eight weeks of age. One of the most common signs is retarded growth, leading to overall smaller body size and skeletal abnormalities. This often manifests as dwarfism, irregular bone structure, and stiff, painful joints that limit mobility.
Skeletal deformities often include a wide cervical spine and a general coarseness of the body frame. Affected cats may display facial dysmorphia, characterized by a broad face, a flattened nose bridge, and a shortened muzzle. These changes result directly from GAG storage in the bone and connective tissues of the face and skull.
Ocular problems are frequently observed, particularly a clouding or opacity of the cornea. This is caused by the deposition of GAGs within the layers of the eye, which can impair vision. Systemic involvement can include organomegaly, which is the enlargement of internal organs such as the liver or spleen.
Cats may also develop heart valve thickening, which can lead to cardiac issues and respiratory distress. Neurological signs, such as impaired motor functions and difficulty walking, can occur in more severe forms of the disease. These result from GAG accumulation in the nervous system.
Diagnostic Procedures and Long-Term Supportive Care
Diagnosing MPS involves clinical assessment and specialized laboratory tests. The process begins with a physical examination to identify characteristic signs like facial changes, joint stiffness, and growth delays. This is followed by a specialized urinalysis to screen for elevated GAG levels, as the undegraded molecules are often excreted in the urine.
A definitive diagnosis requires measuring specific lysosomal enzyme activity in blood samples or tissue, known as an enzyme assay. A significantly reduced or absent enzyme level confirms the diagnosis and identifies the specific MPS type. Genetic testing, which analyzes the cat’s DNA for known mutations, can also confirm the diagnosis. Imaging techniques like X-rays evaluate the extent of skeletal and joint abnormalities, revealing decreased bone density or irregular structure.
There is currently no cure for feline MPS, so long-term management focuses entirely on supportive care to maximize the cat’s quality of life. Pain management is a primary concern, involving medication to address discomfort associated with degenerative joint disease. Mobility assistance, such as physiotherapy or environmental modifications, helps maintain movement for cats with joint stiffness.
Supportive care also includes regular monitoring for potential respiratory or cardiac issues arising from GAG accumulation. Nutritional support and soft food may be necessary as the cat develops eating difficulties related to facial changes. Advanced treatments like Enzyme Replacement Therapy (ERT) and Bone Marrow Transplantation (BMT) have been studied experimentally. These treatments aim to provide the missing enzyme but are highly specialized and often performed in a research setting.