There is nothing as horrible as having a wonderful, healthy horse suddenly become ill. Often there are few precursor signs and symptoms to signal that the horse may be developing cardiac or respiratory problems that, when left untreated, can be fatal.
Horses in general are highly unlikely to have sudden heart failure that leads to immediate death. What we know as cardiac failure in humans is rare in horses, rather they tend to have poorer functioning of the heart muscle resulting in fatigue, inability to exercise or work and lethargy and lack of energy and enthusiasm. Horses, by nature, are very curious and active animals, so any changes in their interest in the environment around them or changes from active, energetic horses to tired, inactive horses are usually signs of a serious health condition. These conditions tend to become more significant as the horse ages, and may even be mistaken for old age.
The most common heart problem in horses is atrial fibrillation. This is a condition that causes irregular heartbeat, known as an arrhythmia. The exact cause of an atrial fibrillation is due to the nervous impulses not being carried properly through the heart, resulting in the muscles of the heart not completely relaxing and contracting, actually reducing the ability of the heart to pump blood. The irregular arrhythmia does not have a pattern when you listen to the heart, rather it seems to be beating randomly, without a set beat like a normal heart. An electrocardiogram can actually show the heart beats and will provide a definite diagnosis. The condition will get progressively worse as the heart muscle continues to work overtime, resulting in a poor overall prognosis for the condition. Horses can be treated with a medication called quinidine if they can be monitored around the clock in an equine clinic or treatment center, have had the condition for less than four months and are otherwise normal horses without any other cardiac problems. Quinidine can be a highly toxic drug so only a vet that is familiar with the procedure, usually an equine specialists, should supervise the treatment.
Horses may also have infections and other diseases that affect the heart, just as infections and disease can affect the respiratory system. Horses may have problems with the upper or lower parts of the respiratory tract, each having different signs and symptoms. Discharges from the nose, coughing, labored breathing, fever, lack of appetite and energy are all signs of respiratory problems. In addition to viral infections and bacteria, horses can also have physical deformities of the upper respiratory tract in the nose and throat that can affect the intake and expulsion of air. Typically these conditions will be congenital, or present at birth, but they may become more noticeable as the horse matures and grows. Surgical treatment for obstructions of the upper respiratory tract are usually very successful, as are antibiotic treatments of the upper and lower respiratory tracts. If horses have severe infections that have lead to fluid accumulating on the lungs this fluid can be drained through the use of shunts, giving the tissue time to heal.