Signs Your Cat Is Sick: Behaviour and Physical Red Flags to Watch

The clearest sign a cat is sick is a change from its normal: a cat that feels unwell will usually eat a little differently, drink more or less, change its litter habits, groom less, or simply do less, often long before it looks obviously ill. Because cats are masters at masking illness, learning to read these quiet changes is the single most useful health skill an owner can have.

I have lived with cats my whole life, and the lesson that has stuck is that they almost never announce when something is wrong. The most useful thing I do is keep a loose mental note of each cat’s normal, so that when one of them shifts, I notice fast. This article is a companion to our complete cat health guide, focused specifically on how to read the signs.

Why cats hide illness

Cats hide illness because they are wired to. The domestic cat descends from a solitary desert hunter that was both predator and prey, and an animal that shows weakness in the wild becomes a target, so concealing pain or sickness was a survival advantage. International Cat Care notes that this instinct persists in pet cats, which is why a genuinely unwell cat will often just withdraw and go quiet rather than cry out. So many feline illnesses are caught late not because the cat had no symptoms, but because the symptoms were deliberately subtle.

The five things to watch: eating, drinking, litter, grooming, activity

The most reliable early warning system is to track five everyday behaviours: eating, drinking, the litter box, grooming, and activity. A change in any one of them is your first clue. A sick cat will alter at least one before it shows anything more dramatic, and the change is usually a reduction: less food, less play, less grooming. The shift can be small, so the baseline matters more than any single observation.

The first time one of my cats went off colour, the only thing I could point to was that she had stopped sitting on the windowsill in the mornings. No vomiting, no obvious pain, just a small absence. It turned out she had an early urinary problem, and that tiny change was the only flag. I now treat “she’s just not herself” as real information rather than a feeling.

Changes in eating and drinking

A change in appetite or thirst is one of the most important signals, in either direction. Eating less, or stopping entirely, is always significant: a cat that has not eaten at all for 24 hours should be seen, because cats can develop a serious liver condition (hepatic lipidosis) when they stop eating. Eating more while losing weight is a classic pattern in hyperthyroidism, one of the most common conditions in older cats. Drinking noticeably more is just as telling and is a hallmark of chronic kidney disease (very common in older cats) and of diabetes. If you find yourself refilling the water bowl more often, or the litter is suddenly heavier, mention it; see our guide to getting a cat to drink more water for what normal looks like.

Changes in the litter box

The litter box is a daily health report, so any change in how often, how much, or how comfortably your cat goes deserves attention. Straining to urinate with little or nothing produced is a genuine emergency, especially in male cats, because a blockage can become life-threatening within hours. Going much more often, very dilute or very strong urine, blood, or accidents outside the box can all point to urinary disease, kidney disease, or diabetes. On the other end, straining without producing stool, or diarrhoea that does not settle, both warrant a call. Our guides to urinary problems in cats and diarrhoea in cats cover these in detail.

Changes in grooming, coat, and weight

A cat’s coat is a visible mirror of how it feels, so a scruffy, greasy, or matted coat often means the cat has stopped grooming because it feels unwell or is in pain. Cats are normally fastidious self-groomers, and reduced grooming is common in arthritis, dental disease, and many systemic illnesses. The opposite can also signal trouble: over-grooming one patch until it thins or goes bald often points to pain, itch, or stress. Weight matters too. Gradual weight loss is one of the most reliable early signs of illness in cats, and because it happens slowly under a coat it is easy to miss at home; running your hands over your cat’s ribs and spine every week or two catches it sooner than the eye does.

Changes in behaviour and activity

A drop in normal activity, or any new hiding, is one of the most common ways a sick cat tells you something is wrong. A cat that suddenly sleeps in unusual places, stops jumping to its favourite high spot, hides under the bed, or no longer greets you may be unwell or in pain rather than just relaxing. Arthritis is a good example: it is under-recognised, yet shows up mostly as a quiet reluctance to jump or play rather than an obvious limp. Changes in temperament also count: new aggression when touched, unusual clinginess, or a normally chatty cat going silent. If your cat’s behaviour shifts, our guide to understanding cat body language can help you read what it is telling you.

Red flags: when to act immediately

Some signs are not “watch and wait” but “go now”. Contact a vet immediately, day or night, if your cat shows any of these:

• Difficulty breathing or open-mouth breathing
• Straining to urinate with little or nothing produced (a life-threatening emergency, especially in male cats)
• Collapse, extreme weakness, or seizures
• Repeated vomiting, or vomiting with obvious pain
• Heavy bleeding, or a known trauma such as a fall or road accident
• A suspected poisoning, including lilies, which are highly toxic to cats and can cause fatal kidney failure

For everything subtler, the rule of thumb is simple: a change that lasts more than a day or two, or any change at all in a senior cat, is worth a call. Our guide to when to take a cat to the vet walks through the judgement calls in more detail. You know your cat’s baseline better than anyone, so trust the feeling that something is off; your vet would always rather hear from you early than late.

This guide is general information, not a diagnosis. Anything that worries you about your own cat should be checked by your own vet, who can examine your cat and knows its history.