Also known as Yellow Fat Disease or Pansteatitis, steatitis a painful condition characterised by a marked inflammation and yellow coloration of the adipose (fat) tissue. The discolouration of the fatty tissue is caused by deposits of large amounts of ceroid pigment. The condition is caused by feeding a diet high in unsaturated fatty acids and deficient in vitamin E. Oily fish, especially red tuna, but also sardines and white fish are the most common cause of this condition.
Vitamin E acts as an antioxidant and found in foods, these antioxidants inhibit oxidation. Oxidation is a reaction which produces unstable chemicals known as free radicals which damage cells. Unfortunately, tuna is an inadequate source of Vitamin E. Therefore the overabundance of unsaturated fatty acids (which also oxidize and destroy Vitamin E), combined with the deficiency of vitamin E causes damage to body fat, which results in the painful inflammatory response. 
The condition can occur in cats of any age, however, it is typically seen in younger cats who are obese.
What are the symptoms of steatitis?
How is steatitis diagnosed?
Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you. In some cases, your veterinarian will be able to diagnose steatitis based on symptoms and a history of your cat's diet.
He may need to run some diagnostic tests including:
Complete blood count and biochemical profile which may reveal an elevated white blood cell and neutrophil count.
Biopsy of the subcutaneous tissue. The sample will show a yellow/orange appearance and firm with a nodular appearance.
How is steatitis treated?
If left untreated steatitis can result in death. So it is important to seek veterinary attention immediately so they can begin the cat on the following treatments:
- Elimination of fish from the diet immediately.
- Supplementation with Vitamin E in the form of ?-tocopherol.
- Force feeding if necessary.
- Some veterinarians recommend the use of corticosteroids to reduce inflammation, pain, and fever.
The prognosis for cats who are diagnosed early is very good. Complete recovery will take three to four weeks. During this time, your cat may be reluctant to be touched, this will gradually improve.
Cats should always be fed a well-balanced diet.
Can I feed my cat human grade tuna?
The short answer is yes, but feeding tuna should be restricted to special treats only, and only fed extremely infrequently.
Tuna is quite addictive to cats and they can quickly develop a taste for tuna and refuse any other foods. The term "tuna junkie" has been used to describe such cats.
Raw fish also contains the enzyme thiaminase which destroys thiamine (Vitamin B1), resulting in thiamine deficiency. Cooking destroys thiaminase, thus protecting thiamine.
What's the difference between human grade tuna and tuna flavoured cat food?
Human grade tuna is just that, tuna whereas tuna flavoured cat food is not 100% tuna, and usually contains other meats and nutrients which are necessary for the cat's diet.
Even cat food containing fish should only be given occasionally and not all the time. Hyperthyroidism has been linked to feeding tinned food, particularly fish flavours. So it is always better to err on the side of caution and make it a sometimes food only.
*1 The Cornell Book of Cats.