Dogs are famously known as 'man's best friend', but veterinarians have uncovered a startling truth: your seemingly innocent pet could potentially become aggressive or even dangerous if affected by certain hidden health conditions. From pain-related ailments like arthritis to hormonal disorders such as hypothyroidism, a range of medical issues can significantly increase aggression in dogs, often catching owners off guard.
Understanding the Root Causes of Aggression
Speaking to the Daily Mail, Shauna Walsh, a PDSA Veterinary Nurse, emphasised: 'It can be very upsetting and worrying if your dog shows aggressive behaviour, especially if this feels out of character for them. It's important to remember that aggression is often a sign that something doesn’t feel right for your dog; it doesn’t make them a 'bad dog'.' She added that aggressive behaviour in dogs typically stems from feelings of fear, illness, or discomfort.
Neurological Conditions
Several conditions affecting a dog's brain or nervous system can lead to behavioural changes. For instance, epilepsy may cause some dogs to become more aggressive after a seizure, as noted by vets at Southfields Veterinary Specialists. They advise: 'A very small number of dogs might become aggressive after a seizure. If your dog is aggressive after his/her seizures: STAY AWAY! Make sure they are in a safe room/area and give them space until they are fully recovered.'
Additionally, a sudden spike in aggression could indicate a brain tumour. The team at Apex Vets explains: 'One of the first signs of a brain tumor in dogs is a noticeable shift in behavior. Dogs may display aggression, confusion, or anxiety that was not present before.' In older dogs, aggression might also signal dementia, with cognitive dysfunction making them feel threatened or anxious in familiar situations.
Pain-Related Conditions
Pain is one of the most common triggers for aggression in dogs. Ms Walsh highlights conditions linked to pain, including traumatic injuries like wounds, broken bones, or broken claws, as well as arthritis and dental disease. She advises: 'If your dog is showing any changes in their behaviour, especially if they have started showing aggressive behaviour suddenly, you should contact your vet as soon as possible. Behaviour changes are sometimes one of the earliest signs that a dog may be feeling unwell or in pain.'
Hormonal Imbalances
Similar to humans, hormonal imbalances can increase irritability in dogs. Hypothyroidism, where the thyroid gland produces insufficient hormones, is a common culprit. Jefferson Animal Hospital notes: 'Hypothyroidism in dogs can lead to changes in behavior, including increased irritability and aggression.' Another condition, Cushing's disease, caused by overproduction of cortisol, can lead to increased aggression around food.
Infectious Diseases
Certain infectious diseases can cause sudden aggression. Rabies, for example, can manifest as 'furious rabies', leading to aggressive spikes, while Canine Distemper can invade the brain, causing behavioural changes like sudden aggression or confusion. The PDSA warns that rabies is nearly always fatal, and distemper often starts with symptoms like runny eyes and cough before progressing to neurological issues.
Vision or Hearing Loss
The loss of senses such as sight or hearing can make dogs feel threatened, resulting in aggression. Cataracts, which cloud the lens of the eye, can reduce vision and cause anxiety, while deafness from various causes can lead to similar behavioural shifts.
Medication Side-Effects
Aggression may also be an unwanted side-effect of certain medications. Norma Jeanne Laurette from Canine Correspondence Studies explains: 'Drugs that affect brain chemistry, including corticosteroids, some anticonvulsants, and pain medications, can increase irritability or impulsivity in dogs.'
Early Warning Signs to Monitor
According to Ms Walsh, preventing aggression involves learning what's normal for your dog and watching for subtle signs of discomfort. These include yawning when not tired, turning their head away, licking lips, looking away or showing the whites of their eyes, a tense lowered body, tail tucked underneath, lifting a front paw, or trembling. If missed, these can escalate to more overt signs like baring teeth, growling, snapping, or lunging.
Bryony Francis, Head of Clinical Behaviour at the RSPCA, advises: 'Respond accordingly by removing the cause of the fear – particularly if it’s a child, another person or another pet – or by encouraging the dog to move somewhere they’ll feel safe and calm.'
