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What Is Cat “Rust”? Causes & What to Look Out For (Vet-Verified Info)

Written by: Crystal Uys

Last Updated on July 2, 2024 by Catster Editorial Team

What Is Cat “Rust”? Causes & What to Look Out For (Vet-Verified Info)


Dr. Luqman Javed Photo


Dr. Luqman Javed

DVM (Veterinarian)

The information is current and up-to-date in accordance with the latest veterinarian research.

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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, more and more people come 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 us as we discuss this interesting phenomenon!

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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 relatively 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 outside of house
Image Credit: Zemiko, Shutterstock

What Causes Cat Rust?

There are two main causes of cat rust. 

Eumelanin Fragility

The pigment which is required to produce black fur is known as eumelanin. This pigment is considered somewhat fragile and this fragility means that the loss of the pigment often results in the coat turning to a rust color. This risk is much higher in cats that frequently spend time in the sun. Excessive exposure to the sunlight isn’t going to result in your cat’s coat rusting. It’s also associated with certain cancers in cats, such as squamous cell carcinoma. 

Tyrosine Deficiency

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. However, there’s a catch. Tyrosine is produced by your cat’s body converting phenylalanine to tyrosine. Phenylalanine is considered an essential amino acid for cats. This means that if your cat is deficient in tyrosine, they need to be assessed by a veterinarian and their diet needs to likely be modified as well. 

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  • Rusting of black fur
  • An abnormal, uncoordinated gait
  • Tail bending forward
  • Ptyalism (excessive saliva production)
  • Vocalizing excessively
  • Hyperactivity
  • Weight loss (in kittens)

Though a change in fur color might be perceived as non-threatening, it’s fairly obvious that the other signs of a tyrosine or phenylalanine deficiency are considerably serious. Therefore, your cat should be looked over by your veterinarian if you see any of the signs mentioned above.



It’s somewhat common for black cats to develop rusting of their black coat. 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 usually attributed to excessive sun exposure or a nutritional deficiency.

If your cat is developing noticeable rusting of their coat in conjunction with notable signs of a medical condition, like an abnormal gait, hyperactivity, and 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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