Why Do Horses Bleed?

Imagine that you work at the transportation department in a big city. Your job is to monitor the key highways and help the maximum number of people use them to get to their destinations, safely, in a timely fashion.

It is rush hour, and you expect a high volume of travelers along the major arteries. As more and more cars get on the highways, the number of people in transit keeps going up. But in the back of your mind, you know that if the number of cars on the road goes beyond a certain threshold, the key arteries will become congested, creating long delays, even stopping the flow of traffic completely. You are worried because you know this also raises the probability of fatal accidents.

The highway analogy helps to describe the very delicate balance that racehorse trainers and equine veterinarians have to manage—a tipping point between high aerobic performance on one hand and congested blood flow causing bleeds on the other. In this analogy, the highways are the horses’ blood vessels, the cars are red blood cells, and the travelers are oxygen molecules on route to the muscles and organ systems that are working hard during a race.

Horses are natural blood dopers in that their spleens store extra red blood cells and dump them into circulation during exercise. The increase in red blood cell volume boosts oxygen delivery and aerobic performance but only up to a certain threshold, which is different for each horse. Once this optimum threshold is breached, cardiac output and aerobic performance begin to decline due to the increase of blood viscosity [1].

Exercise-induced pulmonary hemorrhage (EIPH or “bleeds”) is a condition that is very difficult to treat because it is part of the natural biology of the horse. In a paper published at the Veterinary Clinics of North America: Equine Practice, scientists at the New Bolton Center of the University of Pennsylvania wrote: “At this time, there is no treatment that is considered a panacea, and the currently allowed treatments have not proven to be effective in preventing EIPH.” [2]

Current evidence suggests that increased blood viscosity of exercising horses contributes to blood flow resistance and high blood pressure in the lungs, causing capillaries in the lungs to rupture [3]. Hemorrhaging blood accumulates in the interstitial spaces and alveoli, which can be detected using endoscopy. A spike in blood viscosity that goes beyond a horse’s optimum threshold level during intense exercise is the key factor that increases capillary wall stress and causes capillary vessels to burst.

Even though the horse’s blood vessels expand during exercise, which decreases flow resistance, high blood viscosity triggers increases in friction that can also damage the capillaries of the lungs. Like sandpaper, thick and sticky blood is abrasive to the vessel wall. The composition of red blood cells in the blood can surpass 60% (Hct > 60) in many horses, causing sharp spikes in blood viscosity.

Blood viscosity is a key factor for preventing bleeds. The Equine Blood Viscosity Test provides scientific data that can be used monitor and control treatments that not only stop horses from bleeding but also prevents bleeds before they happen.

Click for additional scientific abstract

1. Boning D, Maassen N, Pries A. The hematocrit paradox--how does blood doping really work? Int J Sports Med 2011; 32:242-6.
2. Birks EK, Durando MM, McBride S. Exercise-induced pulmonary hemorrhage. Vet Clin North Am Equine Pract 2003; 19:87-100.
3. Boucher JH, Connes P. Horses: Ideal hemorheological models of human exercise pathophysiology. International Congress of Biorheology and International Conference on Hemorheology. Penn State University, State College, PA. 10 July 2008. Scientific Program Symposium.