Sudden Aggression in Dogs

This post is from the Tucson Dog Magazine September-October 2017 issue.

Do you have dogs that once lived happily together then suddenly began fighting? Do you have a dog that abruptly began nipping at visitors in the home? If your dog is suddenly snarling, growling, fighting or biting people or other dogs, you need to seek professional help. Who should you call?

Your Veterinarian

Your veterinarian should be the first call you make. Sudden changes in behavior can be associated with either known or undiagnosed health issues. In humans, a protein known as the pro-inflammatory cytokine Tumor Necrosis Factor is elevated during times of illness and is associated with hostility and other sickness behaviors. Dogs have similar sickness behaviors as humans, but dogs cannot use words to tell us they don’t feel well. A change in your dog’s behavior could be related to any number of health issues including arthritis, hip dysplasia, dental issues, thyroid dysfunction, allergies, ear infections, digestive issues, yeast infections, cognitive decline, including hearing and vision loss, and Valley Fever. According to the Valley Fever Center for Excellence, dogs raised from birth in Pima or Maricopa county in Arizona have a 28% chance of being infected with the Valley Fever fungus by two years of age. Early signs of valley fever include coughing, fever, weight loss, and lack of appetite and lack of energy. Untreated valley fever can result in lameness, seizures, or body lesions. Sudden changes in appetite, energy level, or social interactions are good indicators your dog might not feel well.

A Trainer or Behaviorist

If your dog is diagnosed with a medical condition, his behavior may return to normal once the medical issue is resolved or managed sufficiently. Sometimes a behavior pattern develops due to the incidents of aggression, and the behavior may continue. It is not uncommon for relationships between dogs in the home to be damaged due to fights that occurred or for dogs to continue to avoid being touched in certain spots, for example the ears if they had an ear infection. If the behavior persists and a medical reason is no longer or never was a potential factor, it’s time to contact a professional dog trainer or veterinary behaviorist who uses science-based, positive methods. And be aware, correcting or punishing a dog for aggressive behavior can worsen the behavior.

Other Reasons for Aggression

Aggression is normal behavior in dogs and is an important component in canine communication. Unless there is a genetic aspect to your dog’s behavior (which usually exposes itself very early in a dog’s life), most behavior can be modified to some degree to restore some harmony in your dog’s life. Still, other reasons may explain your dog’s aggressive behavior.

Social Development Periods

Puppies undergo a critical socialization period in the first several months of life. Puppies that have not been exposed to novel stimuli, other dogs, and people are at a higher risk for developing anxiety and aggression as adults. One puppy class is not enough. If you attend puppy class and then isolate your dog from the world, your puppy is at a higher risk later in life. If you can repeat your puppy class or advance on to the next level, do so. Try to have your dog in a group dog training class every few months until 2 years of age.


Testosterone can cause a dog to react more quickly to stimuli with longer duration and intensity. Neutering male dogs may help to reduce this risk. Data from one study showed 62% of canines reduced dog-dog aggressive behavior after neutering. Estrogen and Progesterone can also impact behavior during heat cycles in female dogs that are not spayed. Talk to your veterinarian about the best age to spay or neuter your dog.

Learned Behavior

Aggression is a part of communication. When your dog uses less intense canine language to stop another dog or person from engaging him and it doesn’t work, he escalates his communication to be heard. Barking, lunging, growling, and biting are effective ways to communicate how a dog is feeling. If he barks and lunges at the UPS man and he goes away, from the dog’s perspective he made that happen.

Stress and Fear

Stress and fear are the biggest underlying causes for many aggression cases. Dogs who undergo chronic stress or have several stressors stacked on top of one another have a difficult time coping. When stress is chronic, a physiological change takes place altering the brain and behavior patterns. A “normal” response to stress for dogs is to avoid conflict, but a dog who is chronically stressed or stress stacked may go into defensive mode. Fear and stress are often seen together. Fear also triggers a physiological response. When afraid, a dog can either freeze up, flee, or fight, usually in that order. When the first does not work, the brain escalates the response. This type of reaction is physiological and not a learned behavior. Dogs think in terms of what is safe or not safe. They decide what is fearful or stressful, not us. If you are struggling with aggressive behavior in your dog, please consult with your veterinarian and a professional dog trainer or veterinary behaviorist.

Kim Silver CPDT-KA, KPACTP is owner of Building Bonds, a positive, reward based training and behavior consulting business in Tucson, AZ. Kim offers private training and group classes for puppies and adults.

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