An occasional tuna treat given to your cat is probably relatively harmless. It’s true that mercury is often present in tuna, and at low levels, this may not be a problem. But if tuna is fed nearly exclusively, it could potentially pose significant problems.
Nutritional deficiencies are also likely to arise in a cat consuming tuna as a staple part of his or her diet. For example, vitamin E is not present in significant amounts in tuna, which could lead to a deficiency of this vitamin. The resulting disease is called yellow fat disease, or steatitis. This disease causes fever, loss of appetite, and hypersensitivity to touch due to inflammation and necrosis of the fat under the skin. Since tuna is not formulated to meet all of the nutritional needs of a cat, companion felines could also develop other nutrient deficiencies. Most de-boned fish are deficient in calcium, sodium, iron, copper and several vitamins. However, these issues are not likely to be a concern with cats who are getting tuna only as a small, occasional treat.