Sudden Behaviour Changes in Dogs and Cats: Medical Causes and Warning Signs

Sudden Behaviour Changes in Dogs and Cats: Medical Causes and Warning Signs

By Dr Duncan Houston

A change in behaviour is sometimes the first visible sign that a dog or cat is unwell. Pain, neurological disease, hormonal disorders, nausea, urinary disease and medication effects can all alter how a pet sleeps, moves, interacts or responds to handling.

Do not assume that a pet has suddenly become stubborn, spiteful or aggressive. Behaviour provides information, but the underlying cause may be physical, emotional or a combination of both.

Quick Answer

Arrange a veterinary assessment when your pet develops a new, persistent or unexplained behavioural change, particularly if it appears suddenly or is accompanied by pain, appetite changes, weight loss, altered drinking, house soiling, confusion or aggression.

Seek emergency care for seizures lasting five minutes or longer, repeated seizures, collapse, difficulty breathing, head pressing, suspected poisoning, sudden inability to walk or unsuccessful attempts to urinate.

Behavioural Changes That May Indicate Illness

Potential warning signs include:

• Sleeping considerably more or less than usual

• Night-time pacing, vocalising or restlessness

• Hiding or withdrawing from the family

• Reduced interest in walks, play or social contact

• Reluctance to jump, climb stairs or use furniture

• Irritability when approached or touched

• Sudden snapping, growling or aggression

• Urinating or defecating outside the normal area

• Increased vocalisation

• Excessive licking, chewing or overgrooming

• Repetitive pacing, circling or tail chasing

• Staring into space or appearing disorientated

• Chasing invisible objects or snapping at the air

• Increased sensitivity to sounds or movement

• Changes in appetite, thirst or food-related behaviour

• Difficulty settling or repeatedly changing position

The significance depends on what is normal for that individual pet. A subtle change noticed by someone who knows the animal well can be more important than one dramatic sign viewed in isolation.

How Urgent Is the Behavioural Change?

Situation	Examples	Recommended action
Emergency	Prolonged seizure, repeated seizures, collapse, head pressing, breathing difficulty, paralysis, suspected poisoning or inability to urinate	Seek emergency veterinary care now
Same day	Sudden confusion, severe pain, abrupt aggression, persistent circling, marked weakness, repeated vomiting or rapid deterioration	Contact a veterinarian today
Prompt appointment	New house soiling, night waking, reduced activity, touch sensitivity, appetite changes or repetitive behaviours	Arrange an examination within the next few days
Monitor briefly	A mild, isolated change with an obvious temporary explanation and no physical signs	Record the change and seek advice if it persists, returns or worsens

If you are uncertain, it is safer to telephone a veterinary clinic and describe exactly what you are seeing.

Medical Conditions That Can Change Behaviour

Pain

Pain is one of the most important medical causes of behavioural change.

A painful animal may:

• Avoid being picked up or touched

• Growl when approached while resting

• Stop jumping or climbing stairs

• Refuse walks

• Hide more frequently

• Become restless at night

• Lick or chew a specific area

• Guard beds, furniture or resting places

• Withdraw from other animals

• React aggressively during grooming

Common sources include arthritis, spinal pain, dental disease, ear infections, injuries, abdominal discomfort and urinary disease.

Cats commonly hide pain. A cat with arthritis may simply stop using a favourite windowsill, hesitate before jumping or begin toileting beside the litter tray because climbing into it has become uncomfortable. International Cat Care notes that chronic pain can cause defensive and fear-related behaviour in cats. Read more about medical causes of feline behaviour problems.

Dental and Oral Disease

Dental pain may cause:

• Reduced interest in hard food

• Chewing on one side

• Dropping food

• Pawing at the mouth

• Avoiding facial contact

• Irritability when the head is touched

• Sudden food guarding

• Reduced grooming in cats

Pets can continue eating despite substantial dental disease, so a normal appetite does not completely exclude oral pain.

Skin Disease, Ear Disease and Allergies

Persistent itching can disrupt sleep and make a pet irritable or unable to settle.

Watch for:

• Repetitive licking or chewing

• Tail chasing

• Scratching

• Head shaking

• Rubbing the face

• Sensitivity around the ears

• Hair loss or damaged skin

• Reduced tolerance of handling

Repetitive licking can begin with an allergy, infection, parasite, joint problem or painful injury. Even if it later becomes habitual, the initiating medical cause still requires investigation.

Gastrointestinal Disease and Nausea

Nausea and abdominal discomfort may look like anxiety or unusual behaviour.

Possible signs include:

• Lip licking

• Repeated swallowing

• Drooling

• Eating grass

• Restlessness

• Hiding

• Stretching or adopting unusual positions

• Refusing food

• Guarding the abdomen

• Snapping when picked up

• Toileting accidents

• Licking floors, walls or furniture

Some repetitive air snapping or “fly biting” episodes have been associated with gastrointestinal disease, although focal seizures, eye problems and compulsive disorders are also possible. A video of the episode can be extremely helpful. VCA discusses the possible causes of fly-biting episodes.

Urinary and Kidney Disease

A previously house-trained pet may urinate indoors because they cannot wait, cannot reach the toilet area or associate toileting with pain.

Possible causes include:

• Urinary tract inflammation or infection

• Bladder stones

• Urinary incontinence

• Kidney disease

• Diabetes mellitus

• Increased urine production caused by medication

• Mobility problems affecting access to the toilet area

Repeatedly entering the litter tray, crying, passing only drops of urine or producing no urine is an emergency, particularly in male cats. A urinary obstruction can become fatal rapidly.

Neurological Disease

Conditions affecting the brain, spinal cord or nerves can cause dramatic behavioural changes.

Potential signs include:

• Circling

• Head pressing

• Sudden disorientation

• Staring episodes

• Fly snapping

• Unexplained vocalisation

• Tremors

• Loss of balance

• Abnormal sleep patterns

• Sudden aggression

• Seizures

• Failure to recognise familiar people

• Getting trapped behind furniture or in corners

Neurological differentials include seizures, inflammatory brain disease, infection, trauma, congenital abnormalities, toxins and brain tumours.

Not every repetitive behaviour is neurological. Videos, a detailed history and a physical examination help determine which investigations are appropriate.

Cognitive Dysfunction

Dogs and cats can develop age-related cognitive decline. Common signs are sometimes summarised using the categories:

• Disorientation: Appearing lost or becoming trapped

• Interaction changes: Becoming withdrawn, clingy or irritable

• Sleep changes: Waking, pacing or vocalising at night

• House soiling: Forgetting established toilet routines

• Activity changes: Repetitive pacing or reduced exploration

• Anxiety: Developing new fears or difficulty coping alone

• Learning and memory changes: Forgetting familiar routines

Cognitive dysfunction is a diagnosis of exclusion. Arthritis, kidney disease, hyperthyroidism, hypertension, sensory loss, urinary disease and brain disorders can produce similar signs. International Cat Care highlights several medical conditions that can resemble cognitive dysfunction in older cats. Read its guidance on feline cognitive dysfunction.

Vision and Hearing Loss

Sensory changes may cause a pet to:

• Startle when approached

• Bark or vocalise more

• Become reluctant to walk in unfamiliar places

• Appear less responsive

• React defensively when touched unexpectedly

• Struggle in dim lighting

• Develop new fears

Older pets may compensate remarkably well, making gradual sensory loss difficult to recognise.

Approach within the pet’s remaining field of vision, avoid startling them while asleep and keep furniture and resources in predictable locations.

Hormonal and Metabolic Disorders

Several internal diseases can influence energy, sleep and responsiveness.

Hyperthyroidism in cats

Hyperthyroidism most commonly affects middle-aged and older cats. It may cause:

• Weight loss despite an increased appetite

• Restlessness

• Night-time vocalisation

• Irritability

• Increased activity

• Increased thirst and urination

• Vomiting or diarrhoea

VCA’s feline hyperthyroidism guide describes restlessness, increased vocalisation and irritability among the recognised signs.

Hyperthyroidism is uncommon in dogs and should not be treated as a routine explanation for canine hyperactivity.

Hypothyroidism in dogs

Hypothyroidism may cause reduced energy, weight gain, cold intolerance and skin or coat changes. Some affected dogs appear mentally dull or less interested in activity.

Hypothyroidism should not be diagnosed from behaviour alone or from a single mildly decreased total T4 result. Other illnesses and medications can lower thyroid hormone measurements.

Diabetes and Cushing’s disease

Increased thirst, urination and hunger can produce:

• Night waking

• Food-seeking

• Indoor accidents

• Restlessness

• Changes in social behaviour

These changes are consequences of altered physical needs rather than deliberate misbehaviour.

Low blood glucose and electrolyte abnormalities

Hypoglycaemia, low calcium and significant electrolyte disturbances may cause:

• Weakness

• Restlessness

• Tremors

• Confusion

• Collapse

• Seizures

These signs require urgent veterinary attention.

Liver Disease and Hepatic Encephalopathy

Severe liver dysfunction or an abnormal blood vessel bypassing the liver can allow toxins to affect the brain.

Hepatic encephalopathy may cause:

• Disorientation

• Staring into space

• Circling

• Head pressing

• Poor coordination

• Excessive drooling

• Abnormal vocalisation

• Aggression

• Collapse or seizures

Signs may fluctuate and can sometimes become more obvious after eating. Head pressing, severe disorientation or seizures require emergency care. The Merck Veterinary Manual outlines the neurological and behavioural signs associated with hepatic encephalopathy.

Medication Effects

Medications can influence appetite, sleep, activity and responsiveness.

Examples include:

• Corticosteroids causing increased hunger, thirst, panting or restlessness

• Sedatives and pain medications causing sleepiness or poor coordination

• Some seizure medications causing temporary sedation

• Appetite stimulants causing vocalisation, agitation or increased activity

• Medication combinations contributing to confusion or serotonin toxicity

• Incorrect dosing or accidental ingestion causing neurological signs

Do not stop corticosteroids, seizure medication or behavioural medication abruptly unless a veterinarian specifically instructs you to do so.

Contact the prescribing clinic with:

• The medication name

• Strength and dose

• Time of administration

• Time the behaviour began

• Other medications or supplements being used

• A video of the behaviour, if available

Toxins

Sudden agitation, tremors, confusion or unusual behaviour may result from poisoning.

Potential exposures include:

• Cannabis

• Human antidepressants or stimulants

• Nicotine products

• Caffeine

• Chocolate

• Rodenticides

• Insecticides

• Illicit drugs

• Human sleep medication

• Toxic plants or mushrooms

Do not induce vomiting unless instructed by a veterinarian or poison service. Bring the packaging or photograph the suspected substance.

Medical or Behavioural?

A veterinary examination may not identify a medical condition in every case. Environmental and emotional factors can also cause genuine behavioural change.

Possible non-medical contributors include:

• A new person or animal in the home

• Moving house

• Construction or unfamiliar noises

• Reduced exercise or enrichment

• Conflict between household animals

• Changes in the owner’s working hours

• Punishment or frightening training methods

• Separation-related distress

• Insufficient access to food, water, litter trays or resting areas

• A frightening experience

Medical and behavioural causes frequently overlap. Arthritis may make a dog more defensive, while household stress may further reduce their tolerance. Treating only one part of the problem may produce incomplete improvement.

What Will the Veterinarian Investigate?

The investigation depends on the pet’s age, species, medical history and specific behavioural change.

It may include:

• A complete physical examination

• Orthopaedic and neurological assessment

• Oral, eye and ear examination

• Blood pressure measurement

• Blood and urine testing

• Thyroid testing

• Imaging such as radiographs or ultrasound

• Infectious disease testing

• Advanced brain imaging or spinal fluid analysis

• A structured behavioural assessment

A normal basic blood test does not rule out pain, sensory disease, focal seizures or every neurological condition. Further investigation should be guided by the examination and clinical history.

What Owners Can Do Before the Appointment

Record videos

Film unusual episodes when it is safe to do so. Do not provoke the behaviour to obtain footage.

Keep a diary

Record:

• When the behaviour occurs

• How long it lasts

• What happened immediately beforehand

• Food and medication timing

• Appetite, drinking and toileting

• Whether the pet responds to their name

• How quickly they recover

• Any possible toxin exposure

Prevent injury

Separate animals if conflict has developed. Use gates, closed doors or separate rooms rather than physically intervening in a confrontation.

Avoid punishment

Punishing growling, house soiling, hiding or confusion may increase fear without addressing the cause. New aggression should be managed as a safety and medical concern.

Bring medication information

Include prescribed medication, preventatives, supplements, human medications the pet may have accessed and any recent dose changes.

Common Mistakes

Assuming the pet is being spiteful

Dogs and cats do not urinate indoors, withdraw or become irritable to punish their owners. These behaviours can reflect pain, urgency, fear, cognitive decline or environmental stress.

Treating aggression as a training problem first

Sudden aggression deserves a veterinary assessment. Dental disease, arthritis, fever, sensory loss and neurological disease can all reduce tolerance. VCA notes that pain and other medical conditions may contribute to aggression.

Dismissing changes as normal ageing

Ageing itself is not a diagnosis. Arthritis, kidney disease, hypertension, thyroid disease, sensory loss and cognitive dysfunction may all be treatable or manageable.

Changing medication without advice

Abruptly stopping certain medications can cause withdrawal effects, seizures or recurrence of the condition being treated.

Waiting for dramatic physical symptoms

Behavioural changes may appear before vomiting, collapse or obvious pain. Early assessment often creates more options for diagnosis and treatment.

Preventing Medical Problems From Being Missed

• Arrange regular veterinary examinations

• Monitor weight, appetite, thirst and toileting

• Discuss behavioural changes at routine appointments

• Obtain senior health screening when recommended

• Keep medications securely stored

• Review long-term medications regularly

• Provide accessible beds, litter trays and food stations

• Record recurrent or unusual episodes

• Seek advice when a previously stable behaviour changes

Frequently Asked Questions

Can pain cause aggression?

Yes. A painful pet may react defensively when approached, moved or touched. This is particularly common with dental pain, arthritis, spinal disease, ear infections and injuries.

Can a urinary infection cause house soiling?

Yes, but it is only one possibility. Bladder inflammation, stones, kidney disease, diabetes, incontinence, pain and cognitive decline may produce similar signs.

Does sudden aggression mean a brain tumour?

Not necessarily. Pain, fear, sensory loss, resource guarding and environmental changes are more common explanations. However, neurological disease should be considered when aggression appears alongside seizures, circling, altered awareness or loss of coordination.

Is fly snapping always a seizure?

No. Fly snapping may be associated with focal seizures, gastrointestinal discomfort, eye disease or compulsive behaviour. Record the episode and arrange a veterinary assessment rather than attempting to diagnose it from the behaviour alone.

Can medication change my pet’s personality?

Some medications can cause sedation, agitation, increased hunger, restlessness or disorientation. Contact the prescribing veterinarian if a change begins after starting a medication or altering its dose.

Final Thoughts

Behaviour is part of the clinical picture. A pet who suddenly hides, snaps, paces, vocalises or toilets indoors may be communicating discomfort or illness in the only way they can.

Take new changes seriously, record what you observe and avoid punishment. Identifying the underlying cause may relieve pain, uncover disease earlier and restore the behaviour you recognise.

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If your pet’s behaviour has changed and you are unsure how urgently they need care, ASK A VET™ can help you assess the signs and choose the safest next step.