Spinal Muscular Atrophy in Maine Coon Cats (SMA)
SMA is an inherited disorder affecting the skeletal muscles of the trunk and limbs. Loss of neurons in the first few months of life leads to muscle weakness and atrophy that first becomes apparent at 3-4 months of age. Affected kittens develop an odd gait with a sway of the hindquarters and stand with the hocks nearly touching. They may also stand with toes out in the front. By 5-6 months of age they are too weak in the hindquarters to readily jump up on furniture and often have a clumsy landing when jumping down. The long hair Maine Coon may hide it, but careful feeling of the limbs will reveal reduced muscle mass. Affected kittens are not in pain, they eat and play avidly, they are not incontinent, and most live very comfortably as indoor cats for many years. Known affected kittens have occurred in breeding programs across the United States, and, in retrospect, likely carriers have been exported widely.
Onset of clinical signs is observed between 15 and 17 weeks of age. Initial abnormalities are hind limb weakness and a fine generalized tremor. Affected kittens lose the ability to jump strongly by five months and walk thereafter with a sway of the hindquarters. Abnormal sensitivity to touch over the back, exercise intolerance, and labored breathing are variably observed. After an initial period of rapid loss of function, the progression of the disorder slows or plateaus with variable muscle atrophy, weakness and mobility.
Mode of inheritance
This disorder is inherited as a simple autosomal recessive trait. For a kitten to have the SMA disorder, it must receive the mutated copy (allele) of the disease gene from both parents, and male and female kittens are equally affected. The parents of affected kittens show no outward signs of disease, but they are obligate carriers.