
New aggressive behavior

Some cats are more aggressive than others. However, their owners usually learn to judge how their cat will behave in a particular situation. For instance, some cats will tolerate having their belly rubbed when sprawled on their backs. Others will not – and clamp onto the intruding hand in a feline version of the Venus fly trap. Sudden changes to behavior can indicate a poorly cat.
Decreased appetite

Cats may lose their appetite for several reasons. Stress is one possibility. Perhaps there’s a new pet in the house, a new baby, or maybe you – and the cat – have recently moved house. A feline pheromone spray can help manage a cat’s stress, while it gets used to its new circumstances. Dental disease is another possible cause of decreased appetite, and requires veterinary attention.
Changes to play routine

Some cats are naturally more playful than others. However, if a previously playful animal becomes withdrawn and disinclined to play, this may be a sign that the cat is ill or injured. Like many animals, cats frequently hide signs of illness or injury, which can require you, as owner, to play detective. If in doubt, consult your veterinarian and book a check-up for your pet.
Diarrhea

If your cat doesn’t use a litter box, it may be a while before you notice the diarrhea. Remember, too, as a relatively small animal, diarrhea can quickly cause dehydration in a cat. This may be due to common feline contagious diseases – though less likely if your pet has had all relevant shots. Eating inappropriate foodstuffs or poisonous plants is a leading cause.
Scratching

Of course, it’s natural for a cat to scratch itself occasionally. However, excessive scratching is usually a sign that something is wrong. Occasionally, it’s a behavioral response to external stresses but, more usually, it’s a result of a flea infestation, skin infection or seasonal allergies. Food intolerances are another possibility. Note that, when checking for fleas, many cats are such efficient groomers that it’s difficult or impossible for their owners to spot fleas or their eggs.
Changes to sleeping routine

Adult cats naturally sleep for around 16 hours, with much of that sleeping taking place during the daytime. Any changes to your cat’s sleep routine need attention. As with other animals, an ill or injured cat is likely to sleep more and to nap at unusual times. At the other end of the spectrum, stress, anxiety or illness can stop your cat from sleeping.
Excessive grooming

Cats are conscientious and reliable groomers. However, excessive grooming increases the risk of hair balls in the stomach and can make the skin red, raw and vulnerable to infection. Potential causes include fleas, mites, ticks, scabies, and food-related or seasonal allergies. With care and patience, you or vet can usually work out if one of these is the problem.
Hiding

Many cats naturally take themselves out of the way when visitors come round, especially if they bring a dog. However, most cats don’t hide when in the company of their owners. If your cat starts hiding away, it’s definitely worth checking to see if it’s unwell or hurt. Your suspicions should be heightened if your cat is also eating or drinking less, not grooming, or looks visibly ill.
Vocalization changes

A cat that suddenly becomes noisier is fairly clearly attempting to tell its owner something. Perhaps they’re stressed, anxious, hungry or unwell. However, one that becomes quieter than usual may also have a problem. One possibility that you might not have thought of is that your cat is depressed. Common reasons for this include the arrival of a new kitten or sudden confinement indoors.
Becoming less or more sociable

Some cats are more sociable than others – something that includes how they feel about other cats as well as their human companions. If you notice changes to how much or little company your cat wants, there’s likely to be a reason. They could be feeling unwell or anxious about something that’s changed within the household. To find the source, look out for other signs, such as a decreased appetite.
Hunched posture

Cats often arch their backs but the change to posture is usually brief and the cause is often very clear. However, a cat that routinely adopts a hunched position, perhaps with tail curled beneath the body, is more likely to be unwell. In this case, the hunched posture is a response to pain. Any cat that suddenly adopts a hunched posture and cries out when handled requires veterinary attention.
Changes to gait

A cat exhibiting changes to how it moves could, of course, just be stalking something. However, a pronounced and prolonged change to gait, especially if it involves a dragging leg, is a sign that something is wrong. This could mean that the cat in injured or that it’s suffered a stroke. Arthritis is another possibility, as is a cognitive problem.
Head tilt

The most common cause of a new head tilt in a cat is an inner ear infection. Just as it is in people, an inner ear infection in a cat is very painful and requires prompt treatment. You may also notice a smelly discharge from the affected ear or that the cat has difficulty eating, is scratching at their ears and, in some cases, that the animal’s face appears to droop on one side.
Change to tail position

You can tell a lot about a cat from how it holds its tail. A cat that’s happy to see you may hold its tail erect above its back. A frightened cat fluffs up its tail while an angry one might twitch its tail. However, a sick or injured cat often curls its tail beneath its body. Postural changes are an important clue in gauging whether your cat is ill or injured.
Disinterest in grooming

Cats are such keen groomers that any change to this is an important clue that all is not well. Your cat may be older and struggling to manoeuvre itself into the positions necessary for grooming. It could also be injured and find moving difficult or painful. Another possibility is that it’s unwell – and grooming frequently becomes a low priority for a sick cat.
Not eating

A cat that isn’t eating at all is a real worry. Total loss of appetite may signal an acute onset illness. It can also indicate that the cat has a hairball in its stomach, which is making it feel nauseous and bloated. Hair balls, which can result from normal grooming activities, are normally vomited back up. If they’re not, the cat requires veterinary attention.
Not drinking

A cat that isn’t taking in any liquid is a veterinary emergency. In such a small animal, dehydration can set in quickly and with potentially fatal consequences. Possible causes for a cat who isn’t drinking include diabetes, kidney disease, and heatstroke. As only a veterinarian can tell for sure, it’s important to seek advice as soon as possible.
Constipation

Constipation in cats is normally a short-term problem. However, if it does not resolve, it can become obstipation (painful defecation) and even megacolon, in which the colon is so stretched with cement-like feces that it’s unable to pass them. Megacolon does not always respond to treatment, making it important for you seek veterinary advice soon. Possible causes include stress, bowel obstruction, and diabetes.
Increased urine output

Increasing urinary output is particularly common in ageing cats, although most causes can occur in animals of any age. A urinary tract infection is a common reason. Diabetes and kidney disease are two other possibilities. However, don’t forget to check the potential side-effects of any medication your cat is taking. Many common veterinary drugs frequently have a mild diuretic effect.
Decreased urine output

It might not be immediately obvious that there’s a problem but contact your veterinarian if you notice that your cat isn’t urinating as frequently as usual. Many potential causes can be painful for your cat. They include urinary tract infections, which, if left untreated, can spread to the bladder and kidneys. Cystitis and kidney stones are two other possibilities that need veterinary attention.
Licking genitals

Cats will naturally lick their genitals as part of their regular daily grooming routine. However, if you notice your cat paying more attention than usual to their genitals, this could indicate a problem. Typically, this is a urinary tract infection, and the cat is licking its genitals in an attempt to soothe the associate pain. Unfortunately, the licking aggravates the problem.
Breathing difficulties

Changes to your cat’s breathing are almost always a cause for concern. Inhaled foreign bodies, trauma, asthma, heart failure, and anemia are just some of the possible reasons for labored or other breathing difficulties in felines. Even if you suspect you know the cause (an inhaled foreign body, for instance), your cat is almost certainly going to require attention.
Weight changes

Whether your cat is putting on too much weight or losing it, you need to know why. Unintended weight loss can be a simple consequence of lifestyle changes: perhaps you have a fussy eater on your hands, who doesn’t like a new brand of food. However, it can also be a sign of anxiety, organic disease or intestinal parasites.
Lethargy

Lethargy in cats is unusual. Even an older animal, whose no longer as active as previously, is likely to take a close interest in their surroundings. A lethargic cat, on the other hand, is both more inactive than usual and less alert. It could mean the cat is injured (or recovering from an injury) or that it is unwell in some other way.
Avoiding the litter box

Inappropriate elimination – of feces, urine, or both – is annoying, especially if it’s taking place in the house. Some cats will do this due to stress. Consequently, finding and resolving the cause may resolve the problem. Others may adopt the behavior if their litter box isn’t cleaned sufficiently often. And a third group may be unwell, perhaps with a UTI.
Too active at night

Cats are crepuscular, which means that they’re naturally most active at dawn and dusk. This might be normal behavior for that particular cat, especially if it’s sleeping in the day. However, excessive nighttime activity can also result from changes to sleeping patterns caused by anxiety, old age or health problems. Don’t just accept your cat’s new nocturnal activities; try to find out what’s driving them.
Chewing at skin

Although grooming is normal and desirable behavior in a cat, it doesn’t usually extend to chewing at the skin. Instead, it’s a possible sign that the cat may be allergic to something in its environment. It could also signify that the cat is suffering from a skin infection or infestation. Scabies, fleas, ringworm, ticks and mites are all possibilities that require ruling out.
Arched back

An arched back could mean your cat is feeling playful – although this is most likely in a kitten. In adult cats it usually signifies defensive behavior, making itself appear larger and more intimidating. However, if an arched back doesn’t return to its normal alignment, it could signal that your pet is in pain. Look out for other associated signs.
Constant meowing

A constantly meowing cat could just be a chatty cat. However, if this level of meowing is unusual for your pet, you’ll need to work out what’s driving the behavior. Stress or fear are both possible causes but if nothing has changed in the cat’s environment, it’s possible that the cat is unwell or in pain. This may require a trip to the veterinarian to assess your pet’s state of health.
Crying in the litter box

A cat that’s always been noisy while using the litter box is unlikely to be anything to worry about. However, if this starts in a previously quiet cat, it might be a cause for concern. They could have a urinary tract infection, now finding urinating painful. Contact your veterinarian if you notice your pet is also apparently trying to eliminate urine or feces but failing to produce anything.
Flicking tail

As a general rule, a flicking tail on a feline means that the cat is annoyed – so watch out! Sometimes, it can have another cause. For instance, if you notice your cat’s tail twitching while it’s resting, your pet could be in pain or feeling unwell. It’s a tricky call as they may twitch their tail when they’re relaxed but, if it appears stiff or tense, it’s possible something’s wrong.
Flattened ears

A cat with flattened ears – sometimes called airplane ears – is a frightened or very stressed cat. Listen to what it’s telling you: it needs space and for you not to try and touch it. You should also work out why your cat is displaying this behavior. Of course, it may be because a dog is barking in a neighboring yard, but it could also be a problem at home for you to resolve.
Eating grass

Cats have several good reasons to eat grass. For instance, it’s a natural laxative and can ease an upset stomach. Many cats also appear to like the taste! However, excessive grass consumption is a cause for concern. Grass is relatively rich in folic acid, which is essential to aid digestion, help produce hemoglobin, and facilitate cell growth, driving cats to eat more grass.
Matted or scruffy coat

Cats are fastidious groomers. However, long-haired cats usually require human help to keep their coats in good condition. Without this assistance, the coat can quickly become matted, painful and even dangerous to the animal’s health. Occasionally, even a short-haired cat will have a matted or scruffy coat – this should be taken seriously as a potential sign of ill-health.
Squinting, watering eyes

You’ll be used to seeing changes in your cat’s eyes. Sometimes these are caused by changing light levels and sometimes they’ll reflect how your cat is feeling. However, squinting, watering eyes may be a sign of something more serious: an eye ulcer. The first sign is often a cat that starts rubbing its head against things. The classic “pink eye” of conjunctivitis usually follows.
Spraying in the house

Although urine spraying is normal behavior in cats, it’s less common in pet cats – and even more rarely seen in neutered animals. However, a very stressed cat, whether it’s neutered or not, may start spraying inside the house. The behavior is instinctive: the cat is marking out its territory, and the act of spraying helps the animal feel more secure in its environment.
Vomiting

Many cats vomit fairly frequently, especially if they’re prone to accumulating fur balls in their stomach. Sudden, severe or repeated vomiting is a different matter. See your veterinarian if your cat has been vomiting for 24 hours. Go sooner if your cat has eaten something inappropriate or if it’s very young, old, has pale gums, is unusually lethargic, or otherwise unwell.
Increased appetite

A cat whose appetite increases but who doesn’t display a corresponding increase in weight may be suffering from hyperthyroidism. An increase in the speed of eating can signal the same problem. If this is the problem, speedy diagnosis is essential. Treatment options include surgery, radioactive iodine therapy, medication, and dietary management.
Pica

Pica means sucking, chewing or ingesting non-edible materials. For cats, this doesn’t, of course, include dead birds or freshly-caught mice. Whatever the cat chooses to suck, chew or eat, the cause of the behavior varies. Sometimes it’s down to stress but it might also be a consequence of a nutritional deficiency.
Sucking or chewing on fabric

Sucking or chewing on fabric – particularly woollen fabric – is a particular type of pica. It’s worth noting that fabric picas are most common in cats with restricted lives, especially those confined indoors. Giving things to do and plenty of interaction can help resolve the problem but do remember that ingested fabric could cause a intestinal blockage.