Horses have a very large heart with a low resting heart rate. They have big lungs and have evolved to run, with an elevated foot. Horses essentially walk on their tiptoes, and their entire musculoskeletal structure has developed to accommodate this motion.
When health problems appear in a performance horse, they often show up as a change of attitude.
"If a horse that always liked to perform doesn't want to perform, I like to give them the benefit of the doubt," says Montgomery.
When a horse begins showing signs of poor performance there are three major body systems to be considered: respiratory, cardiovascular and musculoskeletal.
Upper airway obstructions are caused by malfunctioning structures that lead to airflow obstruction. One common condition of the upper airway is known as roaring, or laryngeal hemiplegia. It can cause difficulties in active horses and can manifest as noises – "roaring" — during exercise.
The lower respiratory tract is at risk for viral diseases and bacteria that can affect the lungs. Performance horses are especially vulnerable. Common viral diseases are rhinopneumonitis (equine herpesvirus) and equine influenza. Performance horses have increased risk of exposure to these diseases and should be vaccinated.
Several risk factors are associated with the spread of viral diseases:
- Horse shows where lots of animals mingle
- Many animals of different age or immune states
- Transportation often causes stress in horses
- Stress acts as an immunosuppressive, which may cause the horse to pick up a virus
Preventive measures include:
- Quarantine horses coming or returning to the farm
- Don't share tack between new arrivals and those already on the farm
- Prevent nose-to-nose contact
- During transport, horses should be able to lower their heads, which helps clear the airway of dust and other particles
- If a disease outbreak occurs, control animal and human traffic to minimize spread
There are no specific treatments for viruses, but Montgomery recommends a low-dust environment and time to heal. If the animal has a low fever, they may go off their feed.
"Really, the most important thing is rest. I can't stress that enough, because once the clinical signs go away, the lungs are not completely healed," she says.
Viral respiratory infections can lead to complications such as bacterial pneumonia or inflammatory airway disease. Researchers are also investigating the connection between viral respiratory disease and recurrent airway obstruction (RAO or heaves) – a chronic, asthma-like condition.
Horses that perform at maximum lung capacity — such as racehorses —are also at risk for developing a condition called exercise-induced pulmonary haemorrhage. It's thought that damage to veins within the lungs results in pulmonary bleeding.
High-performance horses often develop specific arrhythmias because of their low resting heart rates. Arrhythmias may appear during a regular physical examination or during a mandatory veterinary check:
- Atrioventricular block or dropped beat. The horse's heart beats at a normal rhythm with a pause every three or four beats at a regular interval. This is common in fit horses – as well as very fit human athletes – and should go away with exercise.
- Atrial fibrillation. This arrhythmia can occur without an underlying heart disease. The atria will start to fibrillate, or contract irregularly. In high performing horses, this can present as poor performance or exercise intolerance. This condition can be caused by low potassium. If an endurance horse is sweating a lot or on a diuretic, they may be at risk.
Murmurs result from turbulent blood flow through the heart, often caused by leakage in the heart valves. Small leaks can be clinically insignificant but still show up during an exam. Age can cause changes in the aortic valve, but unless there are performance concerns, it doesn't usually require further investigation.
Murmurs can also appear as a symptom of other problems such as dehydration or a low red blood cell count. These "functional murmurs" usually go away when the underlying problem is addressed.
Over time the horses' body has evolved to run more effectively. Montgomery compares the body of a horse to a suspension bridge – the front limbs are attached only through muscles and function like a sling as the body is propelled forward.
The horse carries about 60 per cent of its body weight on the front limbs and uses the hind legs for power. The force of this impact can affect its legs and feet. Because the horse is evolved to run, its legs are made up of tendons and ligaments instead of muscle.
The horses' hind legs directly connect to the back muscles, which means problems with the hind limbs sometimes present as back pain. Because the horse will often shift its weight around to compensate for pain, this can create other problems and it can be difficult to isolate the source.
Certain issues also correlate to specific equine sports. Horses required to collect or shorten, their gait – such as barrel racers or dressage horses — often suffer from sore hocks. Similarly, horses involved in driving and show jumping where a high head carriage is normal are prone to developing back pain.
In racehorses, speed and fatigue can result in limb injuries. Back problems can also arise from several other sources: an improperly seated rider, a poorly fitted saddle or prolonged exercise on one lead. In racehorses, surface texture can also lead to musculoskeletal injuries. With a hard surface, the force of impact on landing increases. Soft surface can result in an increased strain of soft tissue structures.
Tying-up syndrome is another common disease in horses that can be related to exercise or an underlying disease. Signs include stiffness, firm and painful muscles, sweating, an elevated heart rate and dark brown urine. These signs should be considered an emergency situation, as the horse is in a lot of pain and the breakdown of the muscle – muscle necrosis – can affect the health of the kidneys.
Jeanette Stewart of Rockglen, Sask., is a communications officer at the Western College of Veterinary Medicine. Reprinted with permission from the Canadian Horse Journal (horsejournals.com).