Sometimes it seems that researchers have concluded thatthe riddle of exercise-induced pulmonary hemorrhage

(EIPH) in horses has been solved. Most of those who havedeveloped theories on its cause are sticking by their opin-ions. Most racetrack practitioners are satisfied with usingfurosemide to “solve” the problem.

For more than 100 years, racehorse trainers have won-dered what causes so much nose bleeding in racehorses.Before the advent of the endoscope, the source of the bleed-ing was only speculation. It was not known whether thebleeding actually reduced performance. There was very lit-tle thought about how to control it.

Then about 30 years ago, it was discovered that thebleeding actually came from the lungs. The advent of equinesports medicine research saw intense focus on EIPH.Despite 30 years of subsequent research, definite answershave been elusive. Many theories have been advanced as tothe cause, but little scientific proof has emerged to solve theriddle once and for all.

For a long time no one was certain that the bleeding ac-tually reduced the horse’s level of competition, especiallywhen the bleeding came in small amounts. With the endo-scope, it became possible to see blood in the lungs evenwhen that blood never reached the nostrils. The techniqueof bronchial alveolar lavage has brought even more refine-ment to the determination of just how much lung bleedinghas occurred. With this technique, a small amount of fluidis injected into the lungs and then aspirated out to obtainsamples of cellular debris and blood.

It has been difficult to prove how much performance isdiminished because of EIPH. It seems likely that with a sig-nificant portion of the lung filled with blood, there wouldbe a reduced capacity for oxygen exchange to occur.Whether a small amount of bleeding is detrimental to per-formance is still under debate.

Because the cause has not been pinned down, it is im-possible to come up with rational therapy. A continual in-crease in knowledge about equine exercise physiology hasbrought researchers close to understanding the cause. Earlyresearch was based on the assumption that some sort of pul-monary disease process was the initiating cause. Viral in-fections, such as flu, are known to kill or weaken the cellslining the alveoli of the lungs. This could reduce thestrength of the thin membrane between lung blood capillar-ies and the interior of the alveoli, where there is a good sup-ply of oxygen.

The most popular current theory about cause is that thetremendous increase in blood pressure that occurs whenhorses exercise heavily forces some of the blood outthrough the lung capillary walls and through the thin mem-brane that separates them from the alveoli. Several factorscontribute to this increase in blood pressure during exer-cise. In the horse, blood volume increases with exercise.This is not true with humans and most other mammals. Themechanism of splenic contraction to force reserve redblood cells into the circulation increases the horse’s capac-ity to carry oxygen from the lungs to the muscles where itis needed during a fast sprint. Cardiac output increases dra-matically with intense exercise, and this also increases theoxygen-carrying capacity for the horse.

Actually, the massive muscles of the horse require farmore oxygen in a sprint than can possibly be delivered tothem from the lungs. This leads to an arterial hypoxemia.The hypoxemia forces the pulmonary system to workharder to correct the situation, making matters worse. Thebottom line may be that horses are not really made to runmore than just a short sprint.

The effort to reduce hypoxemia during exercise may bea key to therapy that will reduce the occurrence of EIPH.Recently it has been shown scientifically that Flair nasalstrips reduce EIPH somewhat. Nasal strips hold the nasalpassages open wider so that more oxygen can enter thelungs in a shorter period of time. It also has been shown thatobstructions in the upper respiratory tract, a common oc-currence in horses, cause an increase in EIPH. Improve-ment in upper airway obstructions can lead to less EIPH.

What Do We Know Today About EIPH?William E. Jones, DVM, PhD

Changing Times

Copyright 2003, Elsevier Inc.All rights reserved0737-0806/03/2306-0013 $30.00/0doi: 1053/jevs.2003.80

Volume 23, Number 6 283

284 Journal of Equine Veterinary Science May 2003

Furosemide is the standard treatment for EIPH. In somestudies it has been shown that furosemide reduces the capil-lary blood pressure. This may be the mechanism that providessome benefit with use of this drug, although the reduction inblood pressure is not great. There is a great deal of anecdotalevidence that furosemide improves performance in race-horses, whether or not they are bleeders. This is why trainersseek to have their horses classified as “bleeders” in those ju-risdictions where furosemide is allowed only for “bleeders.”

Many other therapies have been used and continue to beused for EIPH. None has been proven effective, with con-trolled studies, in reducing or eliminating EIPH. The list in-cludes conjugated estrogens, coagulants, cromolyn sodium,immune products, and many types of herbal or nutritionalsupplements.

Writing in the respiratory issue of The VeterinaryClinics of North America (Equine Practice), April 2003,Birks et al said, “At this time, there is no treatment that isconsidered a panacea, and the currently allowed treatments

have not proven to be effective in preventing EIPH. Futuredirections for therapeutic intervention may need to includelimiting inflammatory responses to blood remaining withinthe lungs after EIPH.”

Some researchers are not convinced that EIPH has asingle cause. Dr Shaun McKane, who does quite a bit of re-search on EIPH at the University of North Carolina, voicedthis opinion last year in a report to the American College ofVeterinary Internal Medicine.

“A great deal of research and thought has been given toexplain all cases of EIPH through one unifying theory;however, perhaps it is not necessary to explain all cases ofEIPH with one theory,” said Dr McKane. “It is important toremember that EIPH is a clinical sign, not necessarily asingle disease of itself, and as such may have distinctly sep-arate causes in different horses which all result in this com-mon clinical sign. In fact, it may be because there areseveral possible causes for EIPH that there is such a highprevalence and wide range of severities.”