Diseases Affecting Behaviour in Dogs

Almost all diseases can affect behaviour to a variable degree, in dogs as well as in humans. One major difference is that dogs cannot ‘worry’ about the outcome of serious and life-threatening diseases. Nor do they have ‘insight’ of the potential of later relief to help cope with pain and distress. As an overview, common behaviour alterations in particular diseases are presented below by organ system. The overview begins with effects from diseases in the nervous system and sensory organs.

Nervous system

 Clinical signs from diseases in the nervous system include altered behaviour in one way or another. The possible underlying categories of diseases are often, in clinical neurology, systematized as follows: 
● Vascular diseases (infarcts or haemorrhages affecting the nervous system). 
● Inflammatory/infectious/immunological diseases (e.g. distemper, steroid responsive meningitis-arteritis). 
● Traumatic injuries (e.g. skull fracture). 
● Anomalies (e.g. hydrocephalus). 
● Metabolic/toxic/nutritional disturbances (e.g. hypoglycaemia, strychnine poisoning, thiamine deficiency). 
● Idiopathic conditions. 
● Neoplastic diseases (e.g. primary brain tumours, metastatic tumours in the central nervous system, CNS). 
● Degenerative diseases, disc-associated (e.g. disc prolapse). 
● Degenerative diseases, degeneration of nerve cells (e.g. hereditary ataxia in fox terriers). 

In many of these diseases, in addition to the pure ‘behavioural’ component (e.g. becoming more timid, or aggressive, or extremely restless, or not recognizing the owner any more), signs such as blindness, paralysis, paresis, exaggerated movements, uncoordinated movements, tremor, in whole or part of the body present concomitantly. Pure ‘behavioural’ components as well as seizures are strongly correlated with pathological conditions in the forebrain. Also diseases in the peripheral nervous system alter behaviour, even though the brain cells are unaffected by pathology. For example, a dog with paretic jaw muscles due to an inflammation in the motor nerves to the masticatory muscles shows great difficulty in eating and drinking since it is unable to close its mouth. Another example is a dog that becomes urinary incontinent due to a tumour growth affecting nerves in the pelvic region, innervating the urinary bladder and urethra. 

Meningitis

 One quite common form of meningitis in dogs, steroid-responsive meningitisarteritis, occurs in young adult dogs, and immune-mediated mechanisms are strongly suspected to cause the disease. The disease seems to be more common in some breeds, e.g. boxers and Bernese mountain dogs. Main clinical signs are neck stiffness, spinal hyperestesia, and fever. Clinical signs are promptly relieved by corticosteroid therapy in most cases.

 Brain tumours

 The clinical signs of brain tumours depend on the location of the mass inside the brain. One common sign from tumours growing anywhere in the forebrain is seizures. Tumours in the frontal or temporal lobe often also cause other abnormalities in behaviour, such as aggressiveness, extreme restlessness, confusion or an inability to understand instructions anymore. Another behavioural sign connected with diseases of the forebrain, often a brain tumour, is so-called compulsive walking – an aimless continuous wandering. Brain tumours in the occipital lobe often cause visual field deficits. Tumours in the pituitary gland and/or hypothalamus in addition also may cause autonomic and endocrine signs: polyuria, polydipsia, changes in eating and sleeping pattern, and so forth. Main signs of tumours in the brain stem are gait deficits and cranial nerve signs. In later stages of brain tumours, significant alterations in consciousness should be expected. 

Sensory organs

 Deafness, blindness, inability to smell or taste, inability to perceive pain and/or touch and dizziness due to an inability to perceive influences on the balance organs of the inner ear are all clinical signs, occurring with certain diseases seen in dogs and profoundly altering the behaviour of the affected dog. 

Deafness

 Bilateral deafness for a dog means a dependence on other sensory organs for interactions with the surroundings. As long as the visual system in particular is intact, and the head is turned towards an interacting event, an observer might miss that the dog has any deficit at all. The deficit becomes evident as soon as the dog turns its head away from anyone trying to communicate with it. Unilateral deafness is not always revealed clinically, but is noticeable when the dog can hear but is unable to localize the sound. Deafness in young dogs is most often a congenital disease, proven or suspected to be inherited in different affected breeds. Border collie, cocker spaniel, collie, dalmatian and doberman pinscher are some breeds with reported congenital deafness. When elderly dogs become deaf, a loss of cells in the auditory system should be suspected.

Blindness

 Blindness comes from either diseases in the eyes (e.g. glaucoma, retinal detachment), or diseases affecting the central nervous system visual pathways (e.g. hydrocephalus, hepatic encephalopathy, lead intoxication, inflammatory disorders of the optic nerve). A dog that has slowly and progressively become blind, and is kept in surroundings that have not changed since the disease started, might behave in a surprisingly normal manner. But as soon as the owner moves furniture around in the home, or moves to another place with the dog, the deficit becomes evident. A dog with acute onset of blindness will show obvious behaviour alterations, often manifested as an abnormally cautious behaviour and walking into objects.

Inability to smell 

Inability to smell is occasionally a complaint from owners of hunting dogs. This clinical sign could be due to infestation by Pneymonyssus caninum in the nasal cavities, for example. 

Inability to perceive pain and/or touch

 Inability to perceive pain and/or touch can be related to severe injuries of the spinal cord, dorsal nerve roots or peripheral nerves. These injuries also, in almost all cases, concomitantly lead to paralysis of the affected body part. In addition there are some breed-related, possibly inherited, diseases with loss of function in the sensory nerves as the pathologic condition. Complete loss of pain/skin sensation in dogs can give rise to severe self-mutilation, e.g. eating its own tail or toes. 

Dizziness

 Dizziness, from disturbances in the vestibular system (balance system) is seen quite commonly in dogs. Signs that might be included in the resulting ‘vestibular syndrome’ are head tilt, uncoordinated movements, jerky eye movements and strabismus. Diseases that could be involved include deep ear inflammation, intoxication, thyroid disease and neoplasia.