A recently published study in Comparative Exercise Physiology found a relationship between the administration of the medication furosemide, used to prevent exercise-induced pulmonary hemorrhage, and an enzyme that affects the pressure within the blood vessels in a horse’s lungs.

The relationship potentially points toward new avenues to explore regarding the treatment of EIPH in Thoroughbred racehorses.

The study, conducted at Gávea Racecourse in Rio de Janeiro, Brazil, analyzed post-race blood samples from 73 horses over eight race days. Of the 73 horses, 47 had been treated with 250 mg of furosemide before their race and 26 were not medicated.

These samples were then tested for levels of angiotensin converting enzyme (ACE), a potent vasoconstrictor that when active contributes to higher blood pressure. Several studies have affirmed furosemide’s effectiveness in reducing incidences of EIPH, but how the diuretic drug actually works is still unknown. This study showed ACE activity was significantly reduced in the horses that had been treated with furosemide.

“Multiple regression analysis demonstrated that pre-race furosemide significantly influenced ACE activity post-race, while distance raced, temperature, humidity, and hematocrit did not,” the study concluded. “This is a novel finding which might impact on the search for the exact implications of furosemide use, and its effects on physiology and performance of Thoroughbred racehorses utilizing loop diuretics as treatments for EIPH.”

The horses used in this study were already stabled at Gávea and the treated horses were part of the racetrack’s established protocol on managing EIPH. At Gávea, a horse is entitled to pre-race furosemide if an official racetrack veterinarian has documented a bleeding episode through tracheobronchoscopy exam. A registered bleeder can receive furosemide four hours prior to post time and must continue to receive treatment for every race within 90 days from diagnosis. Horses that are younger than 3 1/2 years old are not allowed to receive pre-race furosemide, and any medicated horse is prohibited from competing in a group 1 or group 2 race.

While furosemide has proven to be the most effective method of reducing EIPH, the medication still does not entirely prevent its occurrence. In the Gávea study, 36.2% of the non-medicated horses showed some degree of post-race bleeding compared with 76.9% of the treated horses.

“This study confirms that, although furosemide might reduce EIPH severity after a single bout of exercise, it does not abolish or reduce its occurrence,” wrote the study’s authors. “This conclusion does not argue against the use of furosemide as a treatment for control of EIPH, but indicates the continuing need for better alternatives to limit the progressive and deleterious effects of repeated episodes of EIPH on the lungs of horses, and that further research into the possible role of renin-angiotensin aldosterone system components (like ACE) in developing new treatments is needed.”

The study was published by Dr. Maria Fernanda de Mello Costa, Dr. Fernanda Aparecida Ronchi, Dr. Yoonsuh Jung, Dr. A. Ivanow, Dr. Juliana Braga, Dr. M.T. Ramos, Dr. Dulce Elena Casarini; and Dr. Ronald F. Slocombe.