The horse’s brain is the control centre of his body. This complex organ receives sensory information from billions of nerves, before processing it and coordinating appropriate responses through the release of chemicals and the control of muscle contraction.
With the spinal cord, the brain forms the central nervous system (CNS), one part of this sophisticated network. The other part is the peripheral nervous system, which is formed of the nerves that connect the CNS to the rest of the body.
Brain facts and figures
The equine brain weighs 400-700g, amounting to around 0.1% of total bodyweight. This ratio is much smaller than in humans: our brains constitute 2% of our bodyweight, which suggests a greater cognitive ability or intelligence in people.
As in other mammals, the horse’s brain is divided into three sections: the forebrain, midbrain and hindbrain. The organ is bathed in cerebrospinal fluid (CSF) and surrounded by three protective membranes, known as meninges.
Horses have evolved a large forebrain, or cerebral cortex, where information is stored as memory, allowing learnt behaviours to be remembered. The cerebral cortex is divided into the frontal, parietal, temporal and occipital lobes. The frontal lobe is the motor cortex, where actions and movement are planned. The parietal lobe is where sensory information is perceived and is involved in mental activity (thinking), while the temporal and occipital lobes process sound and vision respectively.
Another part of the horse’s forebrain is the thalamus, which includes the pituitary gland, vital in hormone production. Dysfunction of the pituitary gland, termed pars pituitary intermedia dysfunction (PPID, or Cushing’s disease) results in lack of regulation of the production of several hormones and an increased concentration of circulating chemicals called glucocorticoids, leading to abnormal hair growth, increased thirst, sweating, laminitis and muscle loss.
The midbrain and hindbrain are involved in subconscious activities such as balance, breathing control, blood pressure regulation and feeding reflexes. Many emotional responses, including excitement, anger and pain, are also thought to originate from this part of the brain.
Illness and injury
Any disease process that affects the normal function of the equine brain can manifest itself as neurological dysfunction. Clinical signs may vary from mild depression to severe lack of coordination, with the horse unable to stand or suffering seizures. While conditions of the equine brain are rare, many of those known to affect people can, on occasion, also affect horses.
Head trauma is one of the most common causes of brain injury in the horse, most often sustained if the horse rears and falls over backwards, if he is kicked or if he collides with a solid object such as a tree.
Bacterial meningitis can follow traumatic injury or occur secondary to blood-borne spread of infection from elsewhere within the body. Immunosuppressed individuals may be at increased risk of meningitis. Streptococci are the most prevalent bacteria isolated in these cases, as in human meningeal infections.
Narcolepsy, the tendency to suddenly fall asleep, has been observed in a number of horse breeds. This is thought to result from abnormalities of neurotransmitters (chemical signals) within the brain.
Horses very rarely suffer from space-occupying lesions, such as abscesses or brain tumours. If these do occur, they can compress the surrounding brain tissue or obstruct the flow of CSF, leading to an increased intra-cranial pressure.
Abnormal electrical activity leading to excessive discharge of the brain neurons manifests itself as seizure activity, as in humans. Precise clinical signs will depend on the part of the brain affected, ranging from mild changes in consciousness to paddling (flailing) of the limbs and recumbency.
A number of neurotoxins, such as yellow star thistle or fusarium, are known to lead to the degeneration of the equine brain.
Similarly, liver dysfunction can lead to the accumulation of naturally occurring toxins such as ammonia in the body. These toxins are thought to interfere with neurotransmission and cause brain swelling. Infectious agents such as viruses or bacteria can also lead to inflammation of the central nervous system.
The ageing brain
Ageing in humans is often associated with a progressive decline in cognitive function, known as dementia, brought about by Alzheimer’s disease. The cause is unknown, although genetic and environmental factors are thought to play a role. Whether such changes occur in the brains of older horses is yet to be determined.
Very little is known about the effect of age on equine brain function. This probably reflects our inability to appreciate subtle changes in cerebral activity, such as memory loss, in these animals.
Early treatment is vital
Horses with brain problems can make a full recovery, depending on the underlying cause and the severity of the damage.
Early treatment is associated with a better prognosis. Once the acute swelling of the brain has subsided, the response to initial treatment can be better appreciated.
Numerous conditions can affect equine brain functions, although fortunately they occur rarely when compared to other problems such as colic or orthopaedic injuries.
The adaptation of advanced imaging modalities, such as MRI, has greatly enhanced our ability to assess horses with suspected brain lesions. Complex three-dimensional structures of the head can be assessed in greater detail, providing more information regarding soft tissue structures, such as the brain, than other conventional imaging techniques.
Case study: ‘He was bashing his head on the wall’
When her warmblood gelding Korenbloem Royal Rebel (Reg) started walking circles and falling over, Lottie Flinn feared the worst.
“He had come in from the field with a stiff neck and we thought he’d pulled a muscle,” she says. “But when he began bashing his head on his stable walls, we knew it was something neurological.”
Reg was rushed to hospital, where basic neurological tests pointed to a brain problem. A lumbar puncture confirmed a diagnosis of bacterial meningitis.
“The lumbar puncture takes cerebrospinal fluid to identify bacteria,” says Lottie, who was told that the chance of recovery was about 2%. “Reg was given antibiotics and steroids, but was disorientated and kept collapsing. At one point, he wasn’t expected to survive the night.”
Amazingly, Reg pulled through. After five weeks in hospital he began a lengthy recuperation at home, returning to dressage at prix st george level the following year — with no apparent after-effects. The condition is incredibly rare.
“Fewer than 0.2% of adult horses with neurological diseases have bacterial meningitis,” says Rosie Naylor, explaining that entry of bacteria to the central nervous system results in a marked inflammatory response. “The signs are thought to be a result of this inflammatory reaction and subsequent neuronal (nerve cell) damage. Changes in blood flow can also lead to increased pressure and swelling within the brain.
“The two main sources of infection are the spread of bacteria in the blood from other sites of infection within the body, such as the sinuses, or penetrating wounds, such as fractures of the skull, that become contaminated.”
Ref Horse & Hound; 11 August 2016