The Warning Signs of Cancer in Dogs and Cats (and the Quieter Feline Ones)

Veterinary cancer specialists have used much the same list of warning signs for decades, and most veterinary cancer charities and university cancer centres publish a version of it. Colorado State University's cancer centre, one of the best known in the world, lists ten (CSU, 2021). They're worth knowing not because each one means cancer, but because each one means "this is a body change worth checking".

• A lump or swelling that lasts or keeps growing. Stroking your pet is genuinely the best way to find these. A swelling that persists, grows, or changes is one to have looked at.
• A sore that won't heal. Most scrapes and scabs heal in a week or two. One that lingers, or keeps coming back in the same spot, deserves a closer look.
• Weight loss you didn't plan. If your pet is losing weight without a diet change, that's a signal, not a bonus.
• Going off their food. A lasting drop in appetite isn't normal, and can point to pain, nausea or a mass somewhere, including in the mouth.
• Bleeding or any unusual discharge. Blood, pus or odd fluid from anywhere, including persistent vomiting or diarrhoea, should be checked.
• A bad smell that wasn't there before. A new, offensive odour from the mouth, ears, or rear end can sometimes point to a tumour in those areas.
• Trouble eating or swallowing. Dropping food, drooling, or struggling to swallow can flag a problem in the mouth or throat.
• A cough or laboured breathing. A persistent dry cough in an older pet is worth chest X-rays; difficulty breathing always warrants prompt attention.
• Limping that has no obvious cause. Unexplained lameness is a recognised warning sign, and CSU notes it can point to cancer, especially of the bone (CSU, 2021). It's worth taking seriously in a large or giant-breed dog, the body type in which bone cancer is most common.
• Tiring easily, or trouble going to the toilet. Losing stamina, or straining to wee or poo, can be early hints that something isn't right.

If you're noticing one of these, you haven't found a diagnosis. You've found a reason to book an appointment.

Here's the part that matters most, and that the dog-shaped version of this list tends to skip. Cats are far harder to read than dogs, and it's not because they feel less. It's instinct.

Cats are small predators who are also potential prey, and in the wild any obvious sign of weakness marks an animal out as an easy target. So cats have evolved to hide illness and pain, and that instinct doesn't switch off just because your cat lives indoors on a heated bed. As VCA Animal Hospitals puts it, "cats have evolved to hide signs of illness and pain" (VCA). They will often carry on looking more or less normal until they're really quite unwell.

That means the dramatic, easy-to-spot signs frequently don't show up in cats. Instead, the clues are quiet, and they're easy to explain away as your cat "just slowing down" or "getting older". The ones worth taking seriously are:

• Slow, subtle weight loss. This is often the single most important one in cats, and it's sneaky. A cat can lose a worrying amount of weight so gradually that you only feel it when you run your hands down their ribs and spine and realise they're sharper than they were (VCA). Weighing your cat now and again catches what your eyes miss.
• Eating a bit less, or snubbing favourites. A cat who picks at food, leaves meals, or turns her nose up at a previously loved treat may be feeling nauseous or sore, not fussy (PetMD).
• Hiding more, and joining in less. A cat who's started retreating under the bed, sleeping in odd quiet spots, or no longer coming to greet you is telling you something. Withdrawal is one of the commonest ways a cat shows she feels unwell (VCA).
• A scruffier coat. Cats who feel rough often stop grooming properly, so a coat that's gone greasy, matted or unkempt, especially in a cat who was always immaculate, is a real sign rather than just laziness (VCA).
• Less jumping, less play, more sleep. Not leaping onto the windowsill any more, or sleeping even more than a cat's already-impressive baseline, can be quiet markers of pain or illness.

None of these is dramatic. That's exactly why they get missed, and exactly why they're worth taking seriously in a cat. Because cats hide illness so well, regular weigh-ins and a yearly check-up earn their keep, and they matter even more as your cat gets into the senior years. Our senior pet hub goes into screening in more depth.

This is the bit to hold on to, because it's easy to read a list like this and convince yourself of the worst. Not one of these signs means cancer on its own. Every single one has common, far less frightening explanations.

Weight loss can be an overactive thyroid, kidney disease or diabetes. A cough can be a chest infection or heart disease. Going off food can be a sore tooth, an upset stomach, or simply a hot day. Lumps are usually benign; the majority of skin lumps turn out to be harmless (Lap of Love). Vomiting and diarrhoea are among the most common things vets see, and the overwhelming majority have nothing to do with cancer.

So the message of this page isn't "panic". It's "get it checked". A change that's new, that's lasting more than a week or two, or that's slowly getting worse is worth a vet's opinion, whatever turns out to be behind it. Most of the time you'll get reassurance, or a simple problem with a simple fix. And on the rarer occasions when it is something serious, you'll have caught it at the point where you have the most options.

Most of these signs can wait the few days it takes to get a routine appointment. A handful shouldn't.

Get seen promptly, the same day if you can, for difficulty breathing, a swollen or painful belly, collapse or sudden weakness, pale gums, ongoing bleeding, or a pet who's stopped eating and drinking altogether. These aren't necessarily cancer either, but they're signs of a pet who needs help now, not at the weekend.

There's no mystery to the appointment, and it helps to know what's coming. Your vet will start with a thorough hands-on examination, feeling for lumps and enlarged glands, listening to the chest, checking the mouth, and weighing your pet so there's a number to compare against next time. From there, the path depends on what they find. It might be a quick needle sample of a lump, a blood test, or imaging like X-rays or an ultrasound scan to look inside. Often the first visit ends with reassurance or a straightforward diagnosis that has nothing to do with cancer at all.

What you can do beforehand is be the eyes and ears that count. Jot down what you've noticed and when it started, weigh your cat or dog if you've a way to, and photograph anything visible like a lump or a sore. The history you bring is frequently the most useful thing in the room.

If it's a lump you've found, read what to do about a lump next. If you want to understand what a sample involves, our guide to aspirates and biopsies walks through it. And if a diagnosis has already been made, the guide to what grade, stage and prognosis really mean is the calm next step.