Cats are wired to hide pain and illness. It's a survival instinct carried over from their wild ancestors, where showing weakness meant becoming prey. A cat who looks fine may not be fine. By the time a cat is obviously sick, they've often been sick for a while.
Knowing what to watch for is one of the most useful things you can do for your cat's health.
Go to the vet today
Difficulty breathing. Open-mouth breathing, rapid shallow breaths, or breathing with visible effort is an emergency in cats. Don't wait.
Straining to urinate with little or no output. A blocked bladder is a life-threatening emergency, especially in male cats. If your cat is going to the litter box repeatedly and nothing is coming out, get to a vet immediately.
Sudden inability to use the back legs. This can indicate a blood clot and requires emergency care.
Seizures. Any seizure in a cat warrants an emergency vet visit.
Suspected poisoning. If your cat may have ingested something toxic, call your vet or the ASPCA Animal Poison Control Center at 888-426-4435.
Call your vet within 24 hours
Not eating for more than 24 hours. Cats can develop hepatic lipidosis (fatty liver disease) quickly when they stop eating. Don't wait this one out.
Vomiting more than once or twice. Occasional vomiting can be normal. Repeated vomiting, especially with lethargy or loss of appetite, is not.
Sudden changes in litter box habits. Going outside the box, blood in the urine or stool, diarrhea lasting more than a day, or straining without the emergency signs above all warrant a call.
Significant changes in water intake. Drinking much more or much less than usual can signal kidney disease, diabetes, or hyperthyroidism.
Sudden weight loss. If you can feel your cat's spine and ribs more easily than you used to, something has changed and it's worth finding out what.
Watch and monitor
Hiding more than usual. A cat who suddenly wants to be alone and is unusually quiet is often telling you something.
Changes in grooming. A previously fastidious cat who stops grooming, or a cat who suddenly starts over-grooming one area, is worth watching closely.
Squinting or pawing at one eye. Eye issues in cats can worsen quickly. A vet visit within a day or two is smart.
If you're not sure whether something needs a vet visit, call your vet's office and describe what you're seeing. Most will tell you over the phone whether it warrants coming in. When in doubt, go.