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Hiding -

If your normally sociable cat starts to shun your company, it could be an indication that something's amiss.

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Feeling vulnerable -

When a cat feels unwell, it will seek out a dark, quiet place in order to be alone. This is instinctive. A less responsive cat is more vulnerable to predators and will look for somewhere to hide.

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Lethargic -

Similarly, lethargy can be a sign of a problem. While they frequently nap, a cat that lies prone for longer than usual periods, or displays a lack of energy with no inclination to play, is probably telling you that all is not well.

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Over-grooming -

If your cat is excessively grooming, in other words beyond what's required for normal coat care and scent distribution, check for signs of skin irritation or hair loss.

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Skin problems -

Skin problems may be a sign of allergies, external parasites, or another form of dermatitis. Either way, it's very likely to be painful and extremely uncomfortable for your pet.

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Under-grooming -

Conversely, cats that aren't feeling very well can also under-groom, leaving a messy or greasy coat, mats of fur, or clumps of loose hair.

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Changes in toilet habits -

Both diarrhea and constipation are indicators of disease in cats. While diarrhea will clearly manifest itself, constipation is more difficult to diagnose. Also, increased amounts of urine can indicate your cat's inability to properly conserve water, a potential sign of a urinary tract issue or kidney problem. On the other extreme, if your cat is struggling to urinate, seek immediate veterinary attention as this constitutes a medical emergency.

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Litter box refusal -

Any changes in the way your cat uses its cat litter box should also be noted. The refusal by your pet to use the box properly and instead urinate or defecate on the floor is a sure sign that things aren't right.

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Loss of appetite -

Any change in your cat's appetite or thirst should be carefully monitored. Loss of appetite can be normal for cats, especially given their reputation as notoriously fussy eaters. But if they cease eating altogether, that's cause for concern. Always check the teeth: cats with dental problems will naturally shy away from food.

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Increased appetite -

Likewise, increased appetite should also be of concern. Cats with metabolic issues, for instance hyperthyroidism or diabetes mellitus, can display a hearty appetite and increased thirst. Overeating shouldn't be ignored, as it can of course lead to obesity.

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Increased thirst -

Is the water bowl emptying faster than you can fill it? As previously mentioned, increased thirst in a cat can indicate endocrine disorders—hyperthyroid disease or diabetes. As a rule of thumb, cats with liver or kidney problems often lose their appetite, but drink like a fish.

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Weight changes -

Weight changes in cats should always be taken seriously, whether it's gain or loss. A cat that starts to lose weight could be suffering from an underlying health problem. Left unchecked it can lead to a weak and malnourished feline. Weight gain is usually more of a long-term problem, and is often successfully addressed by a change in diet.

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Changes in breathing -

If your cat develops breathing difficulties such as shallow breathing, rapid breathing, wheezing, or panting, treat the condition as an emergency.

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Respiratory disorders -

Cats can suffer from a number of different respiratory diseases, including asthma, pneumonia, lung cancer, and lung disease.

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Other breathing problem indicators -

Besides audio indicators, cats experiencing breathing problems will often have their head and neck extended, have their mouth open, and may be unable to sleep in a normal position.

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Bad breath -

Bad breath is a sign of dental problems. A mild case of halitosis usually indicates that it's time to have your cat's teeth checked. Any worse and it could be an indicator of something more serious.

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Oral issues -

Your cat is not immune to oral infections. The mouth harbors a host of bacteria that can migrate to the rest of the body if any cuts, lesions, or sores are left untreated. And the smellier the breath, the more serious the issue.

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Discharge from eyes and nose -

Runny eyes and nose in a cat usually indicates feline upper respiratory disease, their version of the common cold. Symptoms include sneezing, nasal discharge, and coughing. The condition can be quite debilitating, and in some cases be contagious to other cats in your home. If problems persist, seek advice from your vet.

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Hunched-up posture -

A normally lithe and graceful-moving cat that suddenly adopts a lingering hunched-up posture may well be experiencing abdominal pain. They automatically tuck in their belly in a protective attitude. If they continue to do so, a trip to the vet is in order.

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Increased vocalization -

A cat in pain or discomfort will often vocalize more vociferously to indicate its distress. Generally if a cat meows for more than 36 hours then it's time to pay it some serious attention. It may be that it's simply bored and seeking a bit of fuss. However, the cat could also be in pain or displaying a behavioral trait. And remember: a purring cat doesn't always mean the animal is healthy and content.

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Personality changes -

As a cat gets older, a modest change in its personality is to be expected–less playful perhaps and more content to sit in the sun all day. However, if a normally happy-go-lucky cat suddenly becomes aggressive or unfriendly, then deeper more psychological issues may be at play. At this stage it's wise to seek the professional opinion of a vet.

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Frightened -

Similarly, if your once cheerful and confident cat is suddenly acting timid and afraid, it's time to seek the professional opinion of a vet or animal behavioralist.

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Stress -

Symptoms of a stressed cat include a change in its usual patterns of behavior and routines, toileting outside the litter box, and scratching excessively on furniture.

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Limited mobility -

Felines are naturally active and curious. But if your cat is displaying difficulty jumping up to its favorite spot on the shelf, or shuffling its paws up and down the stairs, it could be as a result of an injury or a signal of arthritis. Either way, get it checked out as soon as possible.

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Swelling -

Never ignore swelling or a lump on your cat's body. That innocuous bump under the skin may be a wound that's turned into an abscess. Worse still, it could be a tumor. Don't waste time getting the situation evaluated.

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Ear problems -

Any debris or discharge from the ear should be treated as potentially serious. An ear infection is hugely uncomfortable for cats to deal with, and if it worsens it can cause considerable pain. And while we're on the subject, it's worth noting that inherited congenital deafness in cats is seen almost exclusively in white-coated felines.

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Deafness in cats -

As well as not responding to a name, or the noise of everyday objects, a deaf cat is prone to loud and excessive vocalization and will often display a needy urge to be cuddled and fussed over.

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Emergency situations -

If your cat suffers serious trauma, take it immediately to the nearest vet. Likewise, if it suddenly becomes completely immobile, falls unconscious, or is unresponsive, an emergency trip to the veterinarian is required.

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Care for your cat -

Other conditions that merit an emergency response include seizure, dizziness and disorientation, severe pain, high temperature, and any profuse bleeding.

Sources: (VCA) (International Cat Care) (Cats Protection)

See also: Why do cats hiss, and what are they trying to tell us?

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