When a Change in Behavior Points to a Health Problem

A cat that has used the same litter box for eight years and starts peeing on the bath mat is not being spiteful. A dog that has been easy to handle all its life and suddenly growls when you touch its back is not turning bad. When a behavior changes in an adult animal, especially if the change is sudden, a medical cause is one of the first things a vet will look for, and it is often found.

Animals hide discomfort well, and a change in behavior is sometimes the only outward sign of pain or illness. Vets often say that every behavior problem is a medical problem until proven otherwise, and that is why a check-up comes before a training plan. The lists below go symptom first: the change you might notice, then the medical causes a vet would want to rule out. They are not a way to diagnose at home, since several causes look alike and need tests to tell apart.

Dogs

House-trained dog having accidents indoors: urinary tract infection, bladder stones, kidney disease, diabetes or Cushing's disease (the last three often bring increased thirst too), and urinary incontinence, which is common in older spayed females and usually happens while the dog is lying down or asleep.
Snapping or growling when touched or picked up: pain from arthritis, a back or neck problem such as intervertebral disc disease, ear infections, or dental disease. Sudden irritability in a previously tolerant dog points to pain more often than to a change in temperament.
Night waking, pacing or barking at night in an older dog: canine cognitive dysfunction (similar to dementia), which also shows up as getting stuck in corners and forgetting house training. Pain, a need to urinate more often, and failing sight or hearing can produce the same restlessness.
Constant licking or chewing of the paws: allergies (environmental or food), skin infections, parasites, or joint pain in that leg. A raw, constantly licked patch on a leg (a lick granuloma) needs treatment.
Licking floors, walls or furniture obsessively: gastrointestinal problems such as nausea or reflux have been linked to this behavior in research.
Reluctance to climb stairs, jump into the car or go for walks: arthritis, hip or elbow dysplasia, a cruciate ligament injury, or the lethargy and weight gain of hypothyroidism.
New fear of noises or of being handled in an older dog: pain, which some studies have linked to noise sensitivity, and loss of hearing or sight. (Lifelong storm fear is a separate problem.)
Head pressing against a wall, circling or sudden confusion: neurological disease or liver disease affecting the brain. This needs a vet the same day.

Cats

Urinating outside the litter box: urinary tract disease, including bladder stones and feline idiopathic cystitis (a painful bladder inflammation often linked to stress), kidney disease, diabetes or an overactive thyroid producing more urine, and arthritis that makes a high-sided box hard to climb into. A cat that strains in the box and passes little or no urine, especially a male, may have a blocked urethra. That is an emergency, and it can become fatal within a day or two.
Sudden aggression toward people or other pets: pain (dental disease and arthritis are both common and easy to miss in cats) and hyperthyroidism, which can make a cat restless and irritable.
Loud yowling at night in an older cat: hyperthyroidism, high blood pressure (often linked to kidney or thyroid disease), cognitive dysfunction, or loss of hearing or sight.
Hiding, sleeping more and grooming less: almost any illness or pain. A greasy or matted coat along the lower back can mean the cat can no longer twist to reach it, which suggests arthritis.
Over-grooming to the point of bald patches: fleas, food or environmental allergies, and pain underneath the area being licked. Cats with bladder pain sometimes lick their belly bare.
No longer jumping onto counters, beds or windowsills: osteoarthritis, which X-ray studies have found in a large share of older cats, most of whom show no limp.
Eating far more while losing weight, or becoming hyperactive: hyperthyroidism or diabetes.
Breathing with an open mouth or panting: heart or lung disease. Unlike dogs, cats rarely pant, and this needs a vet immediately.

Who to see, and in what order

Start with your regular vet, and go prepared. Note when the change began, how often it happens, and anything else that shifted at the same time, such as drinking, appetite, weight or toilet habits. A short phone video of the behavior is often more useful than a description, because the animal may behave perfectly in the exam room. For an older pet, expect blood and urine tests and possibly X-rays.

If the vet finds a medical problem, treating it may resolve the behavior entirely, or only partly. A cat that learned to associate the litter box with painful urination, for example, may keep avoiding it after the cystitis has cleared. That leftover behavior is where specialists come in.

A board-certified veterinary behaviorist is a veterinarian who has completed an additional residency in behavior and passed the exam of the American College of Veterinary Behaviorists. They use the letters DACVB. Because they are vets, they can examine the animal, look for medical causes your regular vet may not have tested for, and prescribe medication alongside a behavior plan. They are the right choice for serious aggression, severe anxiety and cases where medical and behavioral causes overlap. There are relatively few of them, so waiting lists can be long.

A certified applied animal behaviorist (CAAB) holds a doctorate, or for the associate title a master's degree, in animal behavior and is certified by the Animal Behavior Society. They can design a detailed behavior modification program, but they are not vets and cannot prescribe medication, so they work alongside your vet when drugs might help.

Trainers and behavior consultants are the most widely available option, and the right fit once medical causes have been addressed. Dog training is not regulated in the US, so anyone can use the title. Independent certification gives you some assurance: the Certification Council for Professional Dog Trainers (CCPDT) tests trainers and requires continuing education, and the International Association of Animal Behavior Consultants (IAABC) certifies consultants who work with dogs, cats and other species. Look for reward-based methods and someone who asks whether the animal has seen a vet.

Whoever you see, avoid punishment while you wait. Punishing a dog for growling can teach it to skip the warning and bite, and punishing a cat for missing the box adds stress, which can make cystitis worse.

Start a simple log the day you notice the change: date, time, what happened just before, and how long it lasted. Two weeks of entries, plus a video or two, will give the vet or behaviorist far more to work with than memory.