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6+ Medical Causes of Aggressive Behaviors in Dogs

Adrienne is a certified dog trainer and former veterinarian assistant who partners with some of the best veterinarians worldwide.

Hypothyroidism is often seen in Rottweilers.

Hypothyroidism is often seen in Rottweilers.

Medical Conditions in Dogs That Cause Aggression

Unfortunately, many dog owners feel compelled to give up their canine friend as soon as he displays aggressive behaviors—either by giving him away, or worse, surrendering him to a shelter or permanently solving the problem by putting him down.

However, there are cases where aggressive behaviors in dogs can be caused by medical conditions. Whereas a dog owner may perceive an out-of-the-blue, unprovoked aggressive attack as a sign of a serious behavioral problem, it may actually be the direct result of the dog's experience of physical pain.

Veterinarians know this very well. In every veterinarian's office, tucked away in a handy drawer, lies a set of differently sized muzzles ready to be used. They know for a fact that even the most docile dogs may easily turn into vicious and snarling monsters at the sight of a needle or upon being treated for a painful wound.

Yet, while in these cases the pain is visible to the eye and quite obvious, there are sometimes medical conditions in aggressive dogs that cause very subtle, insidious symptoms that can be hardly recognized and detected by even the most attentive dog owners.

Owners of normally well-mannered and well-tempered dogs should suspect a medical condition when their dog acts aggressively out of the blue or in a gradual, yet, steady manner.

Medical conditions should be suspected as well in stray dogs or rescue dogs with an unknown medical history. However, not all owners are willing to give these dogs the benefit of the doubt, either out of ignorance (not knowing that there are actually conditions causing aggressive behaviors) or out of fear (the dog is scary, and they feel that they must get rid of him as soon as possible!).

The truth is, an aggressive dog is indeed a scary dog, especially when the aggression is targeted toward other pets, children and people. Such dogs are perceived as a liability, an animal too dangerous to be handled and is easily converted from man's best to man's worst enemy.

A once much-loved dog has progressively transitioned into a snarling animal ready to bite. While many cases of dog aggression are behavioral in nature, it would be unjust to simply assume so without first attempting to rule out a potential medical issue.

This is why it is imperative to have a dog seen by a veterinarian right before calling a behaviorist or sending the dog to a dog trainer. Indeed, reputable dog behaviorists should ask for proof of a medical exam before seeing the dog. A dog behaviorist will never be able to successfully fix a dog acting aggressively because of an underlying medical problem!

Medical Causes of Dog Aggression

  1. Chronic pain
  2. Hypothyroidism
  3. Seizures
  4. Brain conditions
  5. Low calcium levels in lactating dogs
  6. Other possible causes

Following are some medical conditions known to cause aggressive behaviors in dogs. Of course, not every dog exhibiting aggression necessarily has an underlying medical condition, and even if a medical condition is found, that doesn’t mean that the behavior and the medical condition are related.

1. Presence of Pain

An underlying ear infection may cause a dog to growl or bite upon being touched near the head. Spinal and neck problems may cause aggression when the dog's collar is touched to put on a leash.

Chronic disorders such as arthritis and hip dysplasia may cause grumpy behaviors. For instance, a dog suffering from arthritis may react defensively when laying down and feeling like he cannot remove himself from an irritating situation, such as when being pestered by a child.

Suspect some type of orthopedic pain if your dog shows aggressive behavior after exercising or first thing in the morning (when dogs with arthritis are more stiff) and when there are accompanying tell-tale signs such as limping, trouble jumping in the car, a stiff gait, decreased play, decreased muscle tone on one side and hesitancy to moving around.

A recent study revealed that sudden outbreaks of aggression are often pain-related.

2. Hypothyroidism

This condition affecting the endocrine system of dogs is caused by a low count of thyroid hormones. This condition can be easily detected through a simple blood test. Affected dogs typically develop increased weight gain, hair loss, lethargy, low tolerance for cold, and behavior changes such as anxiety, fear and aggression.

It is quite unusual for a dog to develop aggression as a stand-alone symptom when having this condition; however, running a thyroid test is still important. The condition is easily treated with the long-term administration of thyroid pills which will dramatically improve the dog's condition.

3. Neurological Issues

Dogs may develop a variety of neurological disorders which can cause behavior changes.

Culprits may include painful intervertebral disk disease, tumors or cancers, cognitive dysfunction, metabolic issues (such as a result of liver shunt) and seizure disorders.

In regards to seizures, aggressive behaviors can be seen in the post-ictal phase, following a seizure.

Partial seizures in dogs that affect a particular portion of the brain responsible for regulating aggression may be also a contributing factor to aggression and aberrant behaviors.

These types of seizures are often found in certain dog breeds such as Cocker Spaniels, Springer Spaniels and Chesapeake Bay Retrievers.

Dog owners should rule out any underlying medical conditions when addressing sudden behavior changes.

Dog owners should rule out any underlying medical conditions when addressing sudden behavior changes.

4. Brain Conditions

Trauma affecting certain regions of the brain may cause neurological symptoms including aggressive behaviors. These may take place after the dog has sustained brain damage.

Brain cancer found often in senior dogs may be another cause for such behavior changes. Encephalitis, the inflammation of the brain may cause significant aggression in affected dogs. Rabies and distemper are forms of encephalitis.

Hydrocephalus, a congenital condition where the brain's ventricles become enlarged may cause a variety of neurological symptoms, sometimes including aggression.

5. Low Calcium Levels in Lactating (Nursing) Dogs

Some dog owners notice their mellow female dogs start acting aggressively towards their puppies. At times this maternal aggression in dogs may stem from low calcium levels and therefore is worthy of investigation by a vet.

Eclampsia is the medical term for low calcium levels which is often seen in dogs having trouble supporting the calcium demand associated with producing milk. It is often seen in the first three weeks of nursing.

On top of no interest or aggression towards the pups, affected dogs may develop restlessness, a stiff, painful gait, muscle spasms, trouble walking and seizures.

6. Other Possible Causes

There are many more examples of medical conditions causing aggression in dogs. A dog reacting aggressively upon being pet on the head may be suffering from an undiagnosed ear infection.

A hearing-impaired dog may easily startle and bite out of fear when approached without notice. A dog who is blind may also react in uncharacteristic ways. A dog may turn grumpy when dealing with tooth pain.

Rule Out Medical Conditions First

These are just examples of dogs that are dealing with pain and medical conditions that often cannot be detected by the most well-meaning owners. The first step therefore for owners of aggressive dogs is to keep the dog muzzled and set on the vet's examination table. Owners really owe this to man's best friends, especially during these times when dogs need help the most.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2010 Adrienne Farricelli


Adrienne Farricelli (author) on August 13, 2019:

Stankdeb, until dogs can talk, we may never really know why they behave in certain ways. There are several things that could potentially have caused the behavior you are seeing, but we may never know for certain. Is your dog still playing with your husband since the behavior has started? In any case, this needs addressed, and it's best to employ a dog behavior professional for help in such cases. Look for one using force-free behavior modification for safety and correct implementation.

stankdeb on August 12, 2019:

I have a 2 year old maltese that my husband loves and plays wirh all the time. Me on the other hans never hardly play with him but now he has grown a strong aggression towards my husband when he comes near me. He barks growls and get in attack mode. My huasband and i are frustrated because no matter what my husband cant come near me without his aggresive behavior. He plays with my husband a lot and he even seem to want to scratch my knees and bite on my feet when my husbands around but hes niceas ever when hes not home but he still has that aggressive behavior when my husband trys to touch or do anything for me. He started this behavior a while back we laughed it off because he wasn't nearly as aggresive as he is now. Especially since i came home from a one day surgery and need assistance from my husband. We have to lock him upbefore my husband can even help me. Why? is he so protective of me when my husband is the one that does mostly everthing for him.

jtrader on October 04, 2016:

Good to remember people can act out because of these conditions too.

Darlene Sabella from Hello, my name is Toast and Jam, I live in the forest with my dog named Sam ... on March 13, 2010:

Wow, what a fantastic hub, I learned so much and really enjoyed reading this. I have owned dogs all my life and have never owned an aggressive pet. Thumbs up my friend great job

theherbivorehippi from Holly, MI on March 13, 2010:

Great information here! Yes, it is scary of an animal turns aggressive but the majority of time, they have a reason...being in pain or sick or sometimes even a toothache can really interfere with the attitude of animal. It's important that pet owners stop giving up on their animals, take responsibility and get to the root of the problem and find out what is actually wrong with them! Great hub!