What is Cat “Rust”? Causes & What to Look Out For

by beaconpet
What is Cat “Rust”? Causes & What to Look Out For

Join me as we explore the fascinating phenomenon of cat “rust.” Black cats, once associated with bad luck, have gained popularity thanks to social media cat groups. These dark beauties, often referred to as “voids,” have mesmerized cat lovers worldwide. However, as you’ve probably noticed, your black cat may have a reddish-brown hue on their fur. Don’t worry, it’s not rust! In this article, we’ll delve into what cat “rust” actually is, its causes, and when it may indicate a more serious health concern. So, let’s unravel the mystery of cat “rust” with BEA CON PET now!

What is Cat Rust?

Cat rust refers to the phenomenon where a black cat’s fur develops a reddish-brown color, resembling rust. While it can be alarming to see your black cat’s coat changing color, cat rust is relatively common and usually harmless. It is important, however, to understand the causes of cat rust and be aware of any potential underlying health concerns.

What is Cat Rust?

Causes of Cat Rust

Sun Exposure

The most common cause of cat rust is sun exposure. Just as prolonged sun exposure can lighten human hair, it can also cause the dark fur of black cats to turn brown. This is a natural and harmless occurrence, and you may notice that the rusting is more noticeable when your cat spends a lot of time in direct sunlight. The sunlight bleaches the fur, resulting in the rust-like coloration.

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Tyrosine Deficiency

Another cause of cat rust is a tyrosine deficiency. Tyrosine is a non-essential amino acid that is responsible for the production of eumelanin, the black pigment in hair. A deficiency in tyrosine can lead to a change in the color of a black cat’s coat. This deficiency is typically caused by a lack of proper nutrition, specifically a lack of animal protein sources in the cat’s diet. Feeding your cat a high-quality commercial diet formulated for their nutritional needs can help address this deficiency and prevent further rusting of the coat.

Sun Exposure

Sun exposure is the primary cause of cat rust. When a black cat’s fur is exposed to sunlight for extended periods, the UV rays from the sun can cause the pigment in the fur to break down, resulting in a change in color. This is similar to how human hair can lighten or bleach in the sun. While cat rust caused by sun exposure is harmless, it is important to limit your cat’s sun exposure, especially during peak sun hours, to protect their skin and fur. Providing shade and a safe indoor space for your cat can help prevent excessive rusting.


Tyrosine Deficiency

A tyrosine deficiency can also contribute to cat rust. Tyrosine is essential for the production of eumelanin, which gives black fur its color. If a cat’s diet lacks sufficient tyrosine or high-quality animal protein sources, it can lead to a deficiency and a change in the coat color. Feeding your cat a balanced and nutritious diet that includes animal protein can help prevent tyrosine deficiency. High-quality commercial cat foods formulated for the specific needs of cats can provide the necessary nutrients to maintain a healthy coat color.

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Bigger Concerns With Cat Rust

While most cases of cat rust are harmless and can be attributed to sun exposure or a tyrosine deficiency, there are instances where rusting can indicate more serious underlying health issues. It is essential to be aware of these potential concerns and lookout for other symptoms that may accompany cat rust:


Cat rusting, along with weight loss, increased appetite, restlessness, and hyperactivity, can be a sign of hyperthyroidism. This condition occurs when the thyroid gland produces an excessive amount of thyroid hormone. If you notice significant rusting in your cat’s coat and any of these accompanying symptoms, it is crucial to consult a veterinarian for further evaluation and appropriate treatment.

Kidney Disease

Rusting can also be a symptom of kidney disease in cats. Other signs of kidney disease may include increased thirst and urination, loss of appetite, weight loss, and a general decline in energy levels. Kidney disease is a serious condition that requires veterinary attention. If you suspect your cat may have kidney disease, consult with your veterinarian for a proper diagnosis and appropriate treatment options.

Rusting can also be a symptom of kidney disease in cats

Liver Disease

In some cases, cat rusting can indicate underlying liver disease. Liver disease can cause a wide range of symptoms, including yellowing of the eyes and skin (jaundice), poor appetite, vomiting, weight loss, and lethargy. If you notice rusting in your cat’s coat along with any of these symptoms, it is essential to seek veterinary care to diagnose and address the liver condition.


Cat rust, the change in color of a black cat’s coat to a reddish-brown hue, is a common occurrence that is usually harmless. It can be caused by sun exposure or a deficiency in tyrosine, a nutrient important for maintaining the black pigment in fur. While these are the most common causes, it is important to be mindful of any accompanying symptoms or changes in behavior that may indicate more serious underlying health issues, such as hyperthyroidism, kidney disease, or liver disease. Regular veterinary check-ups and a balanced, nutritious diet can help prevent and address cat rust and ensure the overall health and well-being of your feline companion.

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Cat rust is usually harmless

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