Study the Studies: Examining HISA Research on Lasix and EIPH in Racehorses

(from the Summer 2024 issue of The Horsemen’s Journal)
By Clara Fenger, DVM, PHD, DACVIM; Kimberly Brewer, DVM; and Thomas Tobin, MRCVS, PHD, DABT

Horses are arguably the most well-adapted elite athletes of the animal kingdom. One of the most efficient biomechanical specimens, horses originally evolved this efficiency to elude predation, but as they were domesticated, their highly efficient system was adapted through selective breeding to develop speed and agility, originally to aid in military ventures but also to contest speed events.

Among these adapted features is the ability of highly trained equine athletes to move as much as 250 liters (66 gallons) of blood per minute at maximal exercise through their muscles and lungs to deliver oxygen and remove carbon dioxide. This high throughput of blood is so fast that the horse does not fully oxygenate the blood, so horses are actually hypoxic, or low in blood oxygen, during maximal exercise, a feature unique to horses in the animal kingdom.

The Thoroughbred is a close-to-perfect animal for racing, flawlessly adapted to this assignment. While the horse is as well-equipped as several thousand years of selection allows, the incredible capacity to move blood through its lungs can work to its disadvantage.

EIPH in Horses

In a horse’s respiratory system, critical events occur at the interface between the airborne oxygen in the horse’s lungs and the red blood cell traversing the lungs’ capillaries.

The membrane between the air in the lungs and the red blood cell in the pulmonary capillaries is delicately thin, allowing rapid and efficient transfer of oxygen to the horse’s blood, one of the exquisite adaptations of the horse to racing performance. The horse’s pulmonary system is designed on the absolute edge of physical integrity because the more delicate the membranes between the red blood cell and the alveolar oxygen, the more rapidly and efficiently the transfer of oxygen to the bloodstream can occur.

One outcome of this delicate balance is that, during maximal effort, the pressure of 250 liters of blood per minute passing through the thin capillary walls results in some extent of bleeding into the lungs, or exercise-induced pulmonary hemorrhage (EIPH). Horses in all disciplines suffer EIPH, including any sport in which speed is important and even draft horses in pulling competitions. EIPH events can range from barely noticeable to impaired performance and include significant risk of injury or death for horses and their riders.

The horse’s white blood cells can clear small amounts of blood from the airways, but larger amounts can accumulate in the airways and be moved up the trachea, where it is observed by tracheobronchoscopy, more commonly known as scoping.

Scoping is routinely performed after exercise to evaluate the presence and severity of EIPH, which is scored from 1 to 4 with a score of 1 indicating minimal specks of blood and 4 being extensive blood in the trachea. Post-race tracheal EIPH scores of 3 or 4 are associated with reduced racing performance.

EIPH and Furosemide

At some time in the 1960s, American horsemen became aware that pre-race administration of furosemide had a protective effect on the horse during racing events. What is particularly interesting is that the American racing community became aware of this protective effect well before the tracheal manifestations of EIPH were identified and understood by veterinarians. This presumably happened when horsemen became aware that their horses performed more formful overall when furosemide was administered pre-race.

The outcome of this observation was that, from the 1970s on, furosemide became increasingly authorized for pre-race administration in racehorses in the U.S.

The regulatory concern with pre-race furosemide administration was the potential masking of doping agents through urinary dilution. Furosemide produces significant urinary dilution, and in the early 1970s, equine drug testing was entirely dependent on urine testing.

Several research institutions took up this matter and were able to demonstrate that any drug masking of furosemide by urinary dilution was essentially over 2.5 hours after administration, leading to a three- to four-hour pre-race rule for furosemide administration in racehorses. These findings removed a significant barrier to the regulatory acceptance of furosemide for use in horse racing in the U.S., and the last major American racing jurisdiction to approve the use of furosemide in racehorses was New York, which legalized its race-day use on September 1, 1995.

Furosemide and the Horseracing Integrity and Safety Act

The Horseracing Integrity and Safety Act includes a provision for the prohibition of furosemide within 48 hours of a race three years after the program’s effective date, unless a unanimous vote of the Horseracing Integrity and Safety Authority (HISA) board decides that it should continue. The decision is supposed to be made after careful review of a study commissioned by HISA, as shown in the text of the Act:

Section 1206. HORSERACING ANTI-DOPING AND MEDICATION CONTROL PROGRAM. (e)(1) IN GENERAL.—Not later than the program effective date, the Authority [Horseracing Integrity and Safety Authority] shall convene an advisory committee comprised of horse racing anti-doping and medication control industry experts, including a member designated by the anti-doping and medication control enforcement agency, to conduct a study on the use of furosemide on horses during the 48-hour period before the start of a race, including the effect of furosemide on equine health and the integrity of competition and any other matter the Authority considers appropriate.

The purpose, it would appear, is to determine once and for all whether furosemide is detrimental or beneficial to the racehorse. Rather than conduct such a study themselves, the HISA committee funded a series of scientific studies on the matter of the pre-race use of furosemide in racehorses.

The quality of any research depends first and foremost on how it is designed. In medicine, the quality of evidence is ranked. Weak evidence is gleaned from studies in laboratory animals and cell cultures. Moderate-quality evidence comes from studies that involve case reviews and from observational studies in which the investigators are unable to control which animals receive which treatments and simply record what they see.

Strong evidence includes those studies that randomly assign animals to blinded or masked treatment groups, so no underlying bias can determine which group an animal goes into, and neither animal trainers nor investigators can interject their own opinions about treatments. For example, a randomized controlled study for Lasix (furosemide) would have the researcher randomly assign horses into a Lasix or no-Lasix group, with no regard for whether the horse has previously experienced EIPH. Neither the investigator nor the horse trainer would know which horses received Lasix and which received a placebo. This would prevent bias from people with known bleeders putting those horses into the Lasix group, which would skew the results toward a higher percentage of EIPH episodes in the Lasix group.

The absolute strongest evidence in research is from meta-analysis, in which multiple studies are analyzed as a group for overall conclusions.

In veterinary medicine, randomized clinical trials are uncommon because of the ethical considerations associated with potentially depriving an animal of necessary medication to be in a placebo group. Usually, these studies involve experimental animals because of the ethical considerations when using client-owned animals. Despite this rarity, one subject that has been investigated with strong scientific evidence is the use of furosemide in racehorses. The most recent of those studies was the 2009 landmark Hinchcliff paper that demonstrated unequivocally that pre-race furosemide decreases the incidence and severity of EIPH in Thoroughbreds.

Studies Funded by HISA

The first study chosen by HISA to fund with money taken from the industry falls into the moderate level of evidence category:

Examining Associations Between Furosemide Treatment & Racehorse Health and Welfare
Principal Investigator: Amanda Waller, BSc, PhD, research scientist, Center for Clinical and Translational Research, Nationwide Children’s Hospital

This study will examine the effects of race-day furosemide treatment on the health and welfare of Thoroughbreds as well as their long-term racing performance. An analysis will be conducted to assess the association between pre-race furosemide administration and fatal injury while also comparing the performance metrics—including lifetime earnings, career length, lifetime starts, starts per year, placings and average speed figures—of horses that raced exclusively on furosemide as 2-year-olds and horses that did not receive furosemide for any races during their 2-year-old year.

This is a completely observational study, in which the investigators simply record what happens or what happened. The study claims to investigate whether race-day administration of furosemide affects lifelong performance metrics and fatal injuries.

Since furosemide for works (breezes) is not banned in any type or class of horse in the U.S. or any other racing jurisdiction, any horse competing without furosemide is likely to have received almost as much furosemide as any horse competing with furosemide. Further, this study aims to look at horses that raced exclusively with furosemide as 2-year-olds and compare that group to horses that raced exclusively without furosemide. Since the implementation of HISA, no 2-year-olds in HISA-regulated jurisdictions are permitted to race with furosemide. Therefore, this study must simply use historical data dating to when some 2-year-olds were able to race with furosemide. Using historical data only, the researchers will have no access to any records, such as the HISA medication database, showing whether the same horses received furosemide at times other than races. This is an ambitious undertaking of mining huge amounts of data to achieve evidence that is moderately strong at best. It also lacks what is perhaps the most relevant piece of information, namely whether the same horses received furosemide for breezing.

If this study uses all data before HISA implementation, it also misses a major consideration. Before the 2-year-old furosemide ban, the reason certain 2-year-old horses might be administered furosemide would be based on individual decisions of the horse’s connections. In some cases, the horse might have already bled in a work, skewing the findings with the furosemide group having a greater propensity to bleed. This lack of control over the experimental design is precisely why this type of study does not meet the standard of strong evidence.

The second study funded by HISA appears to fall into the stronger evidence category, a controlled study with experimental animals:

Effects of Repeated Furosemide Administration on Electrolyte Homeostasis and Bone Density in Healthy Adult Exercising Thoroughbreds
Principal Investigator: SallyAnne L. DeNotta, DVM, PhD, DACVIM, clinical assistant professor, Large Animal Medicine, University of Florida College of Veterinary Medicine

This study will examine the effects of repeated furosemide administration on electrolyte homeostasis, parathyroid response and urinary electrolyte excretion in exercising adult Thoroughbreds. The study also will examine the effects of repeated administration on bone density and strength using minimally invasive methods of measurement, including DEXA scan and OsteoProbe.

Very little besides the titles of the HISA-funded studies have been released to the public, and little about study design can be discerned from this title. Presumably the subjects in this study are research horses since similar studies have been conducted at the University of Florida using exercised Thoroughbreds. The advantage of this type of study is the ability to control variables to limit confounding factors. The disadvantages include the inability of a laboratory environment to realistically mimic race training.

A previous experimental study in 2019 found no effect of furosemide administration on bone density over seven weeks of weekly furosemide administration in non-exercised research horses that were considerably older than most racehorses. The University of Florida proposal would therefore add information to the current state of knowledge by seeing if this lack of impact of furosemide on bone mineral density also is seen in horses closer to the age and exercise status of racehorses. While interesting, decreased bone mineral density has never been demonstrated to contribute to fractures in racehorses.

Interestingly, that 2019 study showed that the weight loss effect of furosemide was attenuated over the seven weeks of the study. In other words, the horses get “used to” furosemide and respond with less weight loss, a recognized phenomenon with this class of diuretic. This effect is from an upregulation of the electrolyte channels in the kidneys and is unrelated to the effect of furosemide in the lungs.

Electrolyte changes in horses administered furosemide also have been previously demonstrated, with prolonged effects on calcium. This effect would be likely to be attenuated if the electrolyte channels are upregulated in horses with weekly administration. If this phenomenon of accommodation of the horse’s kidneys to repeated furosemide administration is confirmed with the University of Florida study, it is likely that the findings will mirror the 2019 study.

Presumably, HISA chose this study on the premise that the fractures identified in racehorses result from abnormal bone density and that furosemide adversely affects bone density. In humans, the literature is mixed on the risk of fracture with chronic furosemide use, with some researchers finding no effect and others finding increased risk. However, it is important to note that these studies evaluate human patients with substantial underlying disease who take furosemide twice daily on an ongoing basis. In the racehorse, at least in the case of proximal sesamoid fractures, there is no difference between bone mineral density of horses that fracture and non-fractured controls. So, even with the stronger tier of evidence used in this study than the others funded by HISA, regardless of what the researchers find, no extrapolation to fracture incidence can be made.

The third study funded by HISA appears to be an observational study:

Does Pre-Race Administration of Furosemide to Thoroughbred Racehorses Prolong Their Racing Careers?
Principal Investigator: Warwick Bayly, BVSc, PhD, DACVIM, professor, Department of Veterinary Clinical Sciences, Washington State University

This study will examine the impact of severe exercise-induced pulmonary hemorrhage on horses’ careers and the health of the racing industry more broadly. In doing so, the study will assess whether regular furosemide treatment is associated with more career starts and greater longevity and the impact of banning furosemide for 2-year-olds on the duration of their careers and number of lifetime starts. The study also will seek to determine the extent to which severe EIPH impacts the number of subsequent race starts, the periods between them and, when applicable, the time between the diagnosis of severe EIPH and retirement.

It is unclear from the study title how these investigators will determine if horses experience severe EIPH. Epistaxis, or bleeding from the nose, is commonly associated with severe EIPH, but horses can bleed from the nose with any grade of EIPH and even without EIPH at all. Further, horses that die of EIPH often have no blood at the nose. Therefore, without actual endoscopy of the horses, there is no way to determine if a horse has suffered severe EIPH.

The projected comparison of career duration and number of lifetime starts between 2-year-olds that could race on furosemide and those that have started under the 2-year-old ban has a number of confounding factors. First, since May 22, 2023, a complete ban on furosemide in all 2-year-old races has been in place. Therefore, to have a group of 2-year-olds that raced with furosemide, the comparison must be made between horses that were 2-year-olds in 2023 and those that were 2-year-olds in 2022 and before. The regulations between those two groups of horses are wildly different and include many other factors that could contribute to career duration and number of lifetime starts.

The limitations of this study are even greater than the first observational study. By necessity, this study must compare horses from different time frames, which would include different regulatory infrastructures. It is hard to imagine how these researchers are going to determine if a horse has experienced a severe bleeding episode. If they rely solely on events identified by regulatory veterinarians and resulting in a period on the vet’s list, many horses will be misclassified as never having endured a severe EIPH event.

Interestingly, these studies in no way question the primary benefits of prerace furosemide administration, namely the protection of the horse against the various manifestations of EIPH.

Perhaps this is because the protective effects of furosemide administered pre-race are well established in clinical experience and the published scientific literature. All three projects will contribute to the base of knowledge regarding furosemide and racing, but not one provides strong evidence that could, on its own, answer the questions required by the Horseracing Integrity and Safety Act to modify the requirement that race-day furosemide be eliminated from U.S. horse racing.

Per the Act, the study must provide evidence that (1) continuing to permit race-day furosemide is warranted, (2) continuing to permit race-day furosemide is in the best interest of horse racing, (3) furosemide has no performance-enhancing effects on individuals, and (4) public confidence in the integrity and safety of horse racing would not be adversely affected by permitting the continued use of race-day furosemide.

The conditions set forth by the Act are vague and subject to personal interpretation by the individual Authority board members, who must vote unanimously to permit the continued use of race-day furosemide. Notably, the Act specifically requires that furosemide must be shown to have no performance-enhancing effect on individuals, and none of the studies that were funded even attempt to answer that question. It simply looks like the Authority provides the appearance that it is adhering to the requirement of the Act with no intention of addressing the four underlying requirements necessary to preserve the use of race-day furosemide in the U.S.

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