The nervous system is the part of the body responsible for the coordination of the actions and behaviours. It does this by using a complex network of nerves and cells responsible to transmit messages between the brain, the spinal cord and the other parts of the body. The cells responsible for this function are called neurons and have properties that allow them to transmit very rapid and precise signals to the other cells in the body.
Nervous systems are present in most multicellular animals, with the exception of the sponges and very small “bloblike” organisms. Nonetheless, it varies a lot in complexity between the animals. Just like humans, animals are born with a fixed number of neurons. One characteristic of the neurons is their inability to divide or multiply, resulting in progressive loss as the animal gets old. This is usually the result of natural ageing processes, but in some cases, neurons can be destroyed at a faster rate than normal resulting in neurodegenerative diseases. These are debilitating and incurable pathologies characterized by a progressive and irreversible destruction of the neurons.
With the exception of the elephant, humans have the largest number of neurons in their body (approximately 100 billion), followed by gorillas (33 billion) and chimpanzees (22 billion). Cats and dogs have approximately 800 and 160 million neurons, respectively.
Recently, the incidence of neurodegenerative diseases has been rising both in humans as well as in domestic animals. This is mainly the result of the new scientific advances in medicine and subsequent increase in life expectancy. But not all neurodegenerative diseases occur in the elderly. Some are hereditary and can affect predominantly young animals and children.
At present, there is no cure for these diseases and the only therapies available are designed to delay cell death and slow down disease progression. Consequently, early detection of the symptoms is of paramount importance to prevent disease progression. Nonetheless, the diagnosis becomes more difficult in animals, given that the symptoms are less obvious than in humans.
Many neurodegenerative diseases are very similar between humans and animals. For instance, dogs and cats can often suffer from a type of dementia equivalent to the Alzheimer’s disease in humans called “cognitive dysfunction syndrome”. Just like in humans, this pathology tends to affect predominantly elderly animals and is a result of the accumulation of a toxic protein (beta-amyloid) in the brain. With the time, this neurotoxin form deposits that will cause changes in the brain and slowdown the mental function, resulting in alterations in behaviour and in the daily routines. The animals often lose memory and experience walking problems, incontinence, anxiety, aggressiveness, changes in sleeping patterns and disorientation in familiar places. Cats can also suffer from excessive night-time vocalization. Currently, it is not possible to stop or regress the progression of this disease and the treatments are designed solely to reduce the animal symptoms.
Equally, animals can suffer from a disease similar to Parkinson’s in the humans, characterized by tremors, difficulty in walking and balancing, muscle stiffness and slowness. Whist in humans this disease is more frequent in older people, in animals, this condition tends to affect mainly young animals. This is because in the animals, this is often a result of a hereditary condition, caused by a mutation in an important protein called dopamine. Dopamine is responsible to transmit the nervous signals, so once mutated, causes problems with the animal’s movement. Since all mammals produce dopamine, this condition can affect many species such as dogs, cats, horses, rabbits and monkeys. However, it is rare in older animals, because the majority of the species don’t live long enough to reach old age.
Finally, pets can also suffer from pathologies similar to the “multiple sclerosis” and “amyotrophic lateral sclerosis” in humans. In dogs, this is called “degenerative myelopathy” and is a genetic and progressive disease strongly associated with a mutation in a protein. Once mutated, the protein starts destroying the nerve cells, causing weakening of the hind limbs that can progress to total loss of limb function and incontinence. This pathology is most frequent in adult and large dogs, particularly breeds of Rotweiller, German Shepherd, Siberian Husky, Collie and Labrador, but it is rare in cats.
It is of paramount importance that the owners are aware and attentive to changes in behaviour and symptoms in their pets, to help dealing effectively with the problems that may arise from these pathologies, allowing a happier and healthier life.
Ricardo Ribas, PhD
Veterinary doctor, doctorate in veterinary sciences and researcher in the area of oncology