Pulmonary Hemorrhage and Sudden Death in the Horse

Submitted by Lisa Borgia

Aug 26, 2009 - Dr A Kent Allen DVM explains what happens to horses when a hemorrhage occurs, and if there is anything that can be done to prevent it.

Sudden death can occur in a horse that exercises at speed. The good news is that this catastrophic situation is exceedingly rare in competition horses. Horses that fall under the classification of "competition horse that works at speed" include the event horse, the thoroughbred racehorse, steeplechase racehorses and foxhunters. Here are several situations which can be identified as a cause of sudden death, and some tactics to help avoid such an outcome.

Bleeding or exercise induced pulmonary hemorrhage (EIPH):

These episodes can be quite mild, with a trickle of blood seen that is not clinically relevant. They may also be severe, requiring rest and a significant workup to determine the underlying cause of the bleeding.

The worst expression of this is an internal rupture of a pulmonary vessel deep in the lung that presents as sudden death during exercise. This can happen with profuse bleeding from the nose in a vessel near the trachea or a deep vessel that can fill the lungs or chest cavity with little or no blood seen in the nose.

The best approach to address a horse that has had nasal bleeding or exercise intolerance is a thorough examination to determine the severity of the problem.  The most common method of examination is performed with a fiber optic endoscope passed into the nasal cavities to visualize the upper and lower airways. This exam is often followed by a broncho-alveolar lavage (BAL). A BAL is a way of flushing cells out of the lower airways and examining them. A sample of mucus may be collected from the lower airways for culture and examination.

X-rays of the chest cavity may also be indicated. A careful exam performed by listening to the airways with a stethoscope is always indicated. These exams may be performed by a veterinarian familiar with exercising horses, or a veterinary medicine specialist.

Overheating or hyperthermia

This is a problem that usually occurs in hot, humid conditions.

If the horse has another problem, such as anhidrosis (where they cannot sweat properly), they can become overheated with work even in mild climates. It is important to regularly monitor the horse's temperature during training. Take the rectal temperature immediately after a hard workout to see how your horse is handling heat and exercise.

After a hard cross-country school it is not uncommon to see temperatures in the 102° to 104° F range. If it is a cool day the rectal temperature should fall to 98° to 100° F (normal) within 10 to 20 minutes. If it is hot and humid temperatures may rise to the 105° F range. If this is the case, horses will need help cooling down by applying water or water and ice over their neck, shoulder and hind quarters.  Any time the temperature reaches 107° F, it is a life-threatening emergency and the horse needs immediate vigorous cooling.

Taking a temperature occasionally during training allows an evaluation as to how the horse is shedding heat. This provides a measure of fitness that is often more useful than pulse and respiration.

Cardiovascular irregularities

Sudden death with no warning is quite rare in the horse, but it does exist. This problem is usually characterized by a heart murmur or arrhythmia which can be heard by careful auscultation of the heart with a stethoscope.

When these abnormalities are found at regular checkups, they should be followed up with echocardiography (ultrasound of the heart) and an electrical exam of the heart rhythm (ECG). These should be done by specialists in cardiology or internal medicine. The severity of the heart problem and future ability for the horse to continue as an event horse should be determined before the horse returns to work.

The incidence of heart-related sudden death with no warning signs is extremely rare, and usually related to sudden arrhythmia of the heart ventricles or parasite migration through the heart vessels or muscle.

So what can be done to prevent the sudden death of event horses?

1. Careful examination of the heart and lungs with a stethoscope at your twice-a-year health and soundness checkup.
2. Monitor the rectal temperature during training and during competition. There are also ID microchips that can measure muscle temperature with an appropriate reader. These are new, and the correlation with rectal or core temperature is not yet determined.
3. Examination of the event horse's respiratory systems if they bleed or if they exhibit exercise intolerance, or a negative change in fitness.
4. Evaluate the soundness of the horses twice a year. Lameness puts additional stress on all of these systems.
   We cannot reduce the risk of sudden death and severe injury in our horses to zero, but we can take measures to minimize such occurrences.

Dr. Kent Allen received his DVM degree from the University of Missouri in 1979. He has served as the United States Equestrian Team (USET) veterinarian, and as vice-chairman of the International Equestrian Federation (FEI) Veterinary Committee. He chairs the USA Equestrian Veterinary and Drug and Medications Committees, and is the contact veterinarian for the FEI and the USEF in the United States, answering medication questions for veterinarians and competitors around the nation and the world. He is the vice chair of the FEI Medication Advisory Group.