What is Cat “Rust”? What Causes It?

Last Updated on: November 23, 2022 by Crystal Uys

black cat rust color in the sun

Black cats have long been associated with bad luck, but their luck has changed significantly in the last few decades. With the rise of social media cat groups (like ours!), more and more people came together to discuss their love of “voids”, so named for their dark coloration and tendency to look like a cat-shaped void in pictures.

No matter how dark your black cat is, you have likely seen your cat napping in a nice sunbeam and noticed a reddish brown coloration on your cat. Is your cat becoming bleached or changing colors with age? If you’ve noticed this color shift in your black cat, then you’ve seen cat “rust”. Come with me as I discuss this interesting phenomenon!

What is Cat Rust?

Cat rust is given this name because of its tendency to appear as a rust brown color, making it look like your seemingly black cat has rusted. It’s extremely common for black cats to develop rusting, but you may only be able to spot it when your cat is in direct sunlight.

In some instances, you may not be seeing rust. If your cat’s coat is a lighter shade of black or more of a dark grey, then it’s possible that your cat has a pattern to their coat that is not immediately evident but that becomes visible in certain lighting.

black cat on the grass
Image Credit: milesz, Pixabay

What Causes Cat Rust?

The most common cause of cat rust is sun exposure. In the same way that your hair may begin to bleach or lighten after a long time in the sun also explains how some black cats turn brown because of sun exposure. This is completely harmless and shouldn’t be of any concern to you, although your cat’s coat may rust more noticeably if they spend excessive amounts of time in direct sunlight.

A less common cause of rusting in cats is a tyrosine deficiency. Tyrosine is a non-essential amino acid for cats, and it is responsible for the production of eumelanin, or the pigment that results in black hair. A tyrosine deficiency is caused by a nutritional deficiency, but it usually can be remedied by giving your cat a high-quality commercial diet specifically formulated to meet the nutritional needs of cats.

Tyrosine is present in animal protein sources, which are the basis for a healthy diet for cats. A tyrosine deficiency, while not serious on its own, can indicate your cat is not receiving enough protein or appropriate nutrition.

Black cat with a naughty facial expression
Image Credit: milesz, Pixabay

Bigger Concerns with Cat Rust

Although not common, rusting in black cats can also indicate more serious medical conditions, like hyperthyroidism, kidney disease, and liver disease. More often than not, rust will not be the only symptom of any of these disorders. If your cat is experiencing concerning symptoms along with noticeable rust, including weight loss, inappetence, hair loss, lethargy, and yellowing of the eyes and skin, then your cat needs to see their vet as soon as possible to determine what medical problems they may be experiencing.


It’s extremely common for black cats to develop rusting of their black coat, especially as they age. Cats tend to spend a lot of time in the sun, often napping in sunbeams throughout the day. If your cat is turning rusty, it’s probably due to simple bleaching, although it can also indicate a tyrosine deficiency. While this in and of itself isn’t a notable problem, it can indicate your cat’s nutrition is poor and needs to be improved.

Rust can be associated with some serious medical conditions, though. If your cat is developing noticeable rusting of their coat in conjunction with notable symptoms of a medical condition, like weight loss, then your cat needs to be seen by a vet to get them diagnosed and treated quickly.

Featured Image Credit: Gavin Allanwood, Unsplash

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