Whereas it is naturally available in foods, it is supplemented in most commercially available cat foods to ensure it meets the minimum required amounts. Also, there are some supplements such as NUSENTIA Cat Vitamins – Spectrin which contain this vital mineral.
Some of the foods rich in selenium include fish, brazil nuts, pork, beef, turkey, chicken, cottage cheese, eggs, brown rice, sunflower seeds, pinto beans, oats, spinach, among others. However, as a true carnivore, plant sources may not be suitable for your felines.
Normally, the selenium in animal and plant material is the form of L-selenomethionine while supplementation can be done with organic ones including selenomethionine or inorganic sources such as sodium selenite or sodium selenate. There has been concern concerning sodium selenite in cat food.
Moreover, there is the selenocysteine (a selenoprotein) which is an amino acid containing this mineral as well as the use of selenium yeast.
Normally, organic sources are preferred since they contain selenomethionine which a cat’s body can use in anabolism to produce protein tissue just as it does with the methionine, notes All About Feed. 
Additionally, organic selenium that comes as L-selenomethionine form boosts Se and antioxidant status (even during less feed intake or stress) and it can build reserves of selenium transferrable to offspring via milk, placenta, and eggs.
Finally, in commercial cat foods, the bioavailability of Se is higher in wet foods as opposed to dry ones standing at 53% and 30% respectively.
AAFCO 2014 recommends cat foods to have a minimum of 0.3mg of selenium per kilogram on dry matter basis or 0.075mg per 1000 kcal ME for growth, reproduction, and adult maintenance.
It is essential for an optimum antioxidant status and immune function. According to Vet’s All Natural, it is involved in “the production of antioxidants, in particular, the antioxidant enzyme glutathione peroxidase.” Therefore, it is closely linked to vitamin E and a deficiency in one can be offset by the other. Also, superoxide dismutase antioxidant requires this mineral.
Normally, antioxidants help to help neutralize free radicals that have a damaging effect (oxidative stress). Vitamin A, vitamin C, and vitamin E, as well as minerals such as copper, manganese, and zinc, have some antioxidant properties.
Secondly, it is one of the deiodinase enzymes components. This enzyme is necessary for activating the thyroid hormone.
Additionally, proteins that contain this mineral offer protection against cancers and there is evidence selenium protecting against mercury toxic effects by inhibiting its absorption, WikiVet.
Selenium deficiency symptoms
According to The Nest, deficiencies have been linked to “heart disease, thyroid issues, heart and skeletal muscle issues, cataracts, cancer, and red blood cell disorders.”
The Pet Lover’s Guide to Natural Healing for Cats & Dogs book available at amazon.com links cracked paws and dry skin in kittens to deficiency.
However, some sources state that there has not been any reported case of deficiency of this trace mineral in cats.
Cats can maintain a higher level of selenium in their blood, up to 10 times more than rats and human beings without any toxicity clinical signs such as decreased hair growth and food intake or serum chemistry changes when compared to dogs fed half the as much.
This implies that these pets can effectively excrete any excess selenium and store less in their liver. Excretion occurs via urine in the form of selenosugar B metabolite.