What is vitamin A toxicity?
Vitamin A is an essential vitamin for cats, dogs and humans. Deficiency in vitamin A can lead to serious conditions, especially blindness. However, too much vitamin A or hypervitaminosis A can lead to serious toxicity. While somewhat uncommon in North America, vitamin A toxicity is sometimes diagnosed in cats that are fed primarily table scraps.
What causes vitamin A toxicity?
"Vitamin A toxicity is usually caused by eating too much organ meat..."
Vitamin A toxicity is usually caused by eating too much organ meat, especially liver, which contains high levels of vitamin A, or by supplements containing high levels of cod liver oil. There seems to be considerable variability in how susceptible individual cats are to this problem. Some cats can eat large quantities of vitamin A and never develop problems while other cats seem much more prone to develop the problem. This condition is extremely unlikely to develop if you feed your cat a high-quality commercial cat diet produced by a reputable manufacturer.
What are the clinical signs of vitamin A toxicity?
It takes a long time for the clinical signs associated with vitamin A toxicity to develop; symptoms do not usually appear until the cat is at least middle-aged. The most common problem is a form of arthritis in which new bone develops around joints. This leads to stiffness and immobility of the joints; in some cases, the joint may become totally fused. A common site of new bone formation is between the vertebrae of the neck, which interferes with movement of the neck and results in pain and discomfort. Affected cats often have difficulty grooming themselves and the coat may become very unkempt and dirty. In extreme cases, the cat is unable to move its neck and has difficulty eating. The elbow, wrist, shoulders and hip joints are also often involved. This can lead to lameness and pain. Owners may notice that the cat hisses or cries out when picked up. Many cats begin to withdraw from contact and spend much of their time hiding.
How is vitamin A toxicity diagnosed?
Diagnosis is based on a history of being fed foods high in vitamin A, clinical signs and radiographic evidence of new bone formation. Your veterinarian may recommend blood and urine tests to rule out other causes of these clinical signs.
How is vitamin A toxicity treated?
Once cats have developed signs of vitamin A toxicity, there are usually high levels of vitamin A stored in the cat's liver, which will last indefinitely.
"The damage to the joints and the new bone formation are irreversible..."
Fortunately, many cats will improve if the diet is changed and any vitamin A supplementation is stopped. The damage to the joints and the new bone formation are irreversible changes that do not improve with treatment. In severe cases, it may be beneficial to remove some of the new bone surgically, but this is not always helpful. Your veterinarian will discuss if this is a possibility based on your cat's specific condition. In very severe cases, there is often no treatment possible other than pain relief.