Bleeders - the facts, fiction and future direction

By Dr. David Marlin

We are now approaching half a century since Bob Cook pioneered the use of the flexible fibreoptic endoscope, which allowed examination of the respiratory tract in the conscious horse. One of the important outcomes of this technique was that it opened the door to the study of ‘bleeding’ or exercise-induced pulmonary haemorrhage (EIPH). But nearly 50 years on the irony is perhaps that whilst we have become good at describing the prevalence of EIPH and some of the factors that appear to increase the severity of EIPH within individual horses, we still lack a clear understanding of the condition and how to manage it. I use the term manage rather than treat or prevent as our knowledge of EIPH must show us that EIPH cannot be stopped entirely; it is a consequence of intense exercise. The other irony is that in the past 50 years, by far the majority of research into the management of EIPH has focussed on the use of the diuretic furosemide. Whilst we have good evidence from controlled studies that furosemide reduces the severity of EIPH on a single occasion, we still lack good evidence to suggest that furosemide is effective when used repeatedly during training and or racing; and there is also evidence to the contrary.

Let’s review some basic facts about EIPH, which should not be contentious.

• EIPH is the appearance of blood in the airways associated with exercise.

• EIPH occurs as a result of moderate to intense exercise. In fact, EIPH has been found after trotting when deep lung wash (bronchoalveolar lavage or BAL) is done after exercise. 

• EIPH most often involves the smallest blood vessels (capillaries) but can sometimes and less commonly be due to the rupture of larger blood vessels.

• The smallest blood vessels are extremely thin. Around 1/100th the thickness of a human hair. But this extremely thin membrane is also what allows racehorses such as thoroughbreds, standardbreds and Arabs to use oxygen at such a high rate and is a major reason for their athleticism. 

• EIPH is a progressive condition. The chance of seeing blood in the trachea after exercise increases with time in racing.

• EIPH is variable over time, even when horses are scoped after the same type of work.

• If you ‘scope a horse after three gallops in a row, you can expect to see blood in the trachea on at least one occasion.

• EIPH damage to the lungs starts at the back and top, and over time moves forward and down and is approximately symmetrical.

• Following EIPH the lung becomes fibrotic (as scar tissue), stiffer and does not work as well. The iron from the blood is combined with protein and stored permanently in the lung tissue where it can cause inflammation.

• High blood pressure within the lung is a contributing factor in EIPH. Horses with higher blood pressure appear to suffer worse EIPH.

• There is also evidence that upper airway resistance and breathing pattern can play a role in EIPH.

• Airway inflammation and poor air quality may increase the severity of EIPH within individual horses.

• Increasing severity of EIPH appears to have an increasing negative effect on performance.

• Visible bleeding (epistaxis) has a very clear and marked negative effect on performance.

In order to make progress in the management of EIPH (i.e., to minimise the severity of EIPH in each individual), there are certain steps that trainers can take based on the information we have to date. 

These include:

• Ensuring good air quality in stables

• Regular respiratory examination and treatment of airway inflammation

• Reduced intensity of training during periods of treatment for moderate to severe airway inflammation 

• Extended periods of rest and light work with a slower return to work for horses following viral infection

• Addressing anything that increases upper airway resistance (e.g., roaring, gurgling)

• Avoiding intense work in cold weather

• Avoiding extremes of going

• Limiting number of training days in race preparation and increasing interval between races

Endoscopy

FUTURE OPPORTUNITIES IN UNDERSTANDING AND MANAGING EIPH

We have to accept EIPH as a normal consequence of intense exercise in horses. Our aim should be to reduce the severity to a minimum in each individual horse. However, there are areas in which we still need a much greater scientific understanding.   

What actually causes the capillaries to leak or rupture?…

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Post-Race Collapse - Prevention & Management

By Dr. David Marlin

Fortunately, incidents of post-race collapse are relatively rare following racing, however if they do occur, it’s important to know what steps can be taken. Common causes of post-race collapse include cardiac arrhythmias, neurologic events, internal bleeding due to large blood vessel rupture, airway obstruction and overheating. All of these are a serious cause for concern and likely to require veterinary support. However, overheating is likely to be one of the most common reasons for post-race collapse, but it is often not recognised as such and can lead to horses not receiving prompt treatment that may ensure a swift and uneventful recovery with no long-lasting injury.

During races, horses get hot because for every unit of energy they use which makes the muscles contract, four times as much energy is produced as heat. The harder and longer the horse works, the more heat it produces. Although horses lose heat by sweating (around 85%) and through breathing (around 15%) during a race, around 90% or more of the heat produced is stored in the muscles. Even so, on a hot day, horses may come in at the end of a race with body temperatures 1-2°C higher than they would for the same race in cool conditions. 

It would not be unusual for horses to finish races with rectal temperatures of 40-41°C. But taking rectal temperature can also mislead us as the temperature inside the working muscles may be much higher; and it can take five minutes for the rectal temperature to reach a peak after a horse pulls up, increasing by another 1-2°C. It’s as we get to rectal temperatures of 42°C that the risk of collapse due to hyperthermia (high body temperature) becomes significant. Let’s look at why high body temperature can lead to collapse.

Firstly, very high body temperature leads to direct and damaging effects on the brain, the nervous system as a whole and the heart, which may lead to collapse. These effects are related to how high the temperature is and how long the horse stays at that elevated body temperature. For example, if a horse was not cooled off following a race, then it may take 5-10 minutes for the onset of collapse. However, post-race collapse on pulling-up and/or returning to the winners enclosure or stables is not uncommon, and this has a different underlying cause.  

During the race, the horse actually reduces blood flow to the skin and chooses instead to send as much as possible to the muscles. This is very different to the situation in people where a significant amount of blood is always sent to the skin to help cooling (thermoregulation). The consequence of blood being directed to the muscles is that the muscle temperatures increase rapidly even over a few minutes of a race. When the horse starts to pull-up, this is reversed and blood is suddenly redirected to the skin. This is most pronounced when the horse comes to a stop. The effect is similar to fainting in people; the flow of blood to the surface causes a fall in blood pressure and effectively the horse faints. 

Clearly, collapse of horses is undesirable and has the potential to cause further injury, so it is important to recognise the risks for post-race collapse with respect to overheating and what to do if the situation arises. One of the common misconceptions of post-race collapse is that this is due to “lack of oxygen”. Whilst this could be true in some cases, this is likely to occur in a very small number of horses and only in those with airway obstruction. From studies on treadmills, for example, we know that within a few seconds of starting to slow down, the low oxygen levels in the blood are immediately reversed and even become higher than they were before exercise. People will often cite the ‘blowing’ of horses after a race as an attempt by the horse ‘to get more oxygen in’, however, it’s clear from a number of studies that blowing/breathing after exercise is directly related to body temperature and not oxygen levels. Rapidly reducing body temperature by aggressive cooling results in a more rapid cessation of blowing.   

When should heat stress and overheating be suspected? A horse that is hot to touch, blowing very hard and also ataxic (wobbly) when pulled up should be suspected as suffering from overheating, and cooling should be started immediately. If possible avoid turning in tight circles but keep walking as this helps increase blood pressure. Even in cases where overheating is not the main problem, cooling is extremely unlikely to have any negative effects. As mentioned previously, overheating is frequently not considered as a possible cause for post-race ataxia/collapse and may therefore not be recorded as such. 

Some time ago Professor Tim Parkin and I examined data from the British Horseracing Authority over three seasons of diagnosed cases of post-race heat stress. Over a three-year period, 108 cases had been recorded by on-course veterinary surgeons post-race. A number of factors significantly increased the risk of a horse suffering overheating. Perhaps not surprisingly, races run in the spring or summer were eight times more likely to include at least one horse with heat stress as races run in the autumn or winter. National Hunt races were almost three times more likely to have a horse with heat stress due to the longer duration of the races compared with the flat. Also, for any type of race, there was an increasing risk for every five furlongs (1,000m). Races run in the afternoon were also three times more likely to have a heat stress case than a race run in the evening. Finally, faster races also increased the risk of horses suffering overheating. 

This should all be expected: long races and/or faster races in the afternoon on warm days in spring and summer carry an increased risk of overheating/heat stress and collapse. As not racing in such conditions is not likely to be an option, it’s essential that racecourses and trainers are aware of the signs and risks of overheating and the risk of post-race collapse and take appropriate and prompt action if necessary. 

Aggressive cooling is now used extensively in professional endurance racing and eventing, as well as in all equestrian disciplines at major events such as FEI World Championships and Olympic Games. The principle is simple. Applying cold water (0-5°C), either from a hose or from a large container of ice in water, rapidly cools the blood in the skin which in turn more slowly cools the muscles. In horses that are very hot and at risk of heat stroke/collapse, there is no requirement or benefit to scraping water. The key to minimising risk is continuous application of water over as much of the body surface as possible until the breathing starts to recover (i.e., until the blowing reduces). This is the best and most practical indicator of the effectiveness of the cooling. It’s also essential to continue aggressive cooling for 5-10 minutes to bring a horse’s temperature down 1-2°C. As mentioned previously, the cessation of blowing is the best indicator of effective cooling.   

Applying cold towels, fans, shade, ice packs on large blood vessels, ice in the rectum, spraying with alcohol are all minimally effective in comparison with continuous application of large volumes of cold water all over the body. In contrast to widely held misconceptions, this approach to cooling does not cause the horse to heat up due to constriction of blood vessels in the skin nor does it cause muscle or kidney damage. The key to preventing collapse and or permanent injury due to heat stroke is rapid instigation of cooling. Literally, seconds count. Delaying cooling by thirty seconds may result in a collapsed horse.

Even in cases where the cause of ataxia/collapse is not primarily due to overheating, starting cooling until veterinary help arrives will not make the situation worse. 

Compared with their jockeys, horses are actually able to tolerate much higher body temperatures. A jockey with a temperature of 41°C would be comatose and at risk of serious injury or even death, whilst a horse at 41°C would still be running. However, it is possible for both jockeys and horses to acclimatise to heat. Acclimatisation is the process whereby the body becomes more tolerant of heat as a result of regular daily exercise in the heat. Of course racehorses are most commonly trained early in the morning in the cooler part of the day, yet the majority of races are held in the warmer times of the day, so it’s conceivable that most racehorses are not heat acclimatised. It may also be of interest that heat acclimatisation also improves performance.

In summary, overheating of horses during races is more likely in longer, faster races at warmer times of the year. Horses that are hotter than normal are at an increased risk of heat-related collapse, often when returning to the paddock and standing. Horses that are very hot to touch, blowing hard, wobbly and possibly ‘excited’ are likely to be at risk for collapse. Starting cooling aggressively immediately can lead to rapid recovery and prevent collapse and the risk of more serious injury. 

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Why are leg boots not commonly used in racing?

By Dr. David Marlin

Lower leg injuries are extremely common in all types of race and sport horses. Risk factors for traumatic injury from shoes on other legs due to over-reaching or brushing or even from other horses in racing or polo include high speed, jumping and rapid turning. Injuries can also occur from sharp stones being thrown up from the ground. Whilst in sports such as eventing and showjumping, it’s unusual to see horses competing without leg protection at least over the forelimb tendon area; in racing the use of protective boots is much less common. This is also true for polo—a sport which would present a high risk of injury to the lower leg due to balls, sticks, high speed, turning and close proximity to other horses. So is the risk much lower than we might expect, or is their some other reason why boots are not commonly used in racing?

Epidemiological studies of injuries to racehorses in training and racing certainly don’t point to a large risk for interference-type injuries from shoes within or from other horses. In the younger flat racehorse population, fractures remain the greatest concern both in training and racing. Injuries due to forelimb Superficial Digital Flexor Tendons (SDFT) tendonitis and Suspensory Ligament (SL) desmitis are not uncommon, with a prevalence of around 10-20%. However, information on the rate of injuries due to interference in racehorses is generally lacking.

Whilst an injury to the lower forelimb where the skin has been cut and there is clear penetration is easily identified, this is not the only way that tendons may be injured. Blunt force which does not result in obvious superficial injury may still lead to internal bruising and inflammation. Repeated traumatic insults due to interference may therefore still contribute to tendon inflammation. As we now recognise that most tendon injuries are due to chronic inflammation and damage as opposed to isolated accidents, anything that contributes to tendon inflammation is a cause for concern.

The risk to tendons from the heat generated during exercise may be one of the reasons why racing has tended to shy away from the widespread use of boots, except perhaps in the case of individuals suffering repeated or severe injuries. Wilson and Goodship at Bristol Vet School showed in the 1990’s that equine tendons reached temperature of around 45°C during galloping. Tendons are essentially large elastic bands which store and release energy on each stride—one of the adaptations that makes the horse such a supreme athlete. In the same way that if we stretch an ordinary rubber band, it will heat up. Tendons have a poor blood supply, and so the heat accumulates and the tendon increases in temperature during the gallop; the longer and faster the gallop, the higher the temperature. Why is tendon temperature a concern?

Heat: Tendon cells appear to be sensitive to increases in temperature. When isolated tendon cells in culture were heated for 10 minutes at 45°C (113.0°F), around 10% died; but when they were heated to 48°C (118.4°F) for 10 minutes, then around 80% died. Similar results were found in a later study by a group from Japan. Even though the number of live tendon cells in a tendon is low, compared with the elastic matrix that makes up the majority of the tendon, injured or dead cells release inflammatory mediators which in turn can lead to tendon damage. And of course, anything that insulates the leg reduces heat loss and can lead to even higher tendon temperatures. This is likely one of the primary considerations for not using a protective boot, although there are others.

Abrasion/rubbing: Boots (or bandages) that do not fit correctly or that are applied incorrectly may lead to skin abrasion and an increased risk of skin infections. In addition, boots that allow the ingress of surface material between the boot and the skin will likely lead to rubbing.

Restriction of blood flow to and from the foot: Morlock, et al. (1994) observed pressure under bandages applied to the lower limb during galloping which they concluded were high enough to restrict blood flow. In bandages or boots applied over the fetlock and cannon, high pressures due to the method of application, the tightness of the application and the type of material used the lateral and medial digital arteries and veins could be compromised.

Restriction of range of motion: Restricting the range of motion of a joint will change the loading dynamics of the joint. This may be beneficial in the case of a joint that is injured, but this will reduce the extent to which that joint dissipates forces during the loading phase. This may in turn lead to overloading of other limb structures. If the restriction is only on one limb, then this may lead to asymmetry and an increased risk of injury in the un-restricted limb. The effect of various equine boots on range of motion both in vitro and in vivo has been reported (Balch, et al. 1998; Kicker, et al. 2004).

Contact dermatitis: Boots and bandages have the potential to cause contact dermatitis. Neoprene is commonly used in boots, but it has been estimated that around 6% of horses are allergic to neoprene.  Rosin (also known as colophony), which is commonly found in the resin of pine and conifer trees, can cause skin contact sensitisation. It is used in neoprene adhesives and may also pose a risk of contact dermatitis in horses. The risk of skin infection is also increased by boots and bandages that do not allow sweat to evaporate and therefore result in hyper-hydration of the skin under the boot or bandage. This results in both an increased susceptibility to mechanical trauma to the skin from friction and an increased risk of infection, particularly by fungi.

Increase in weight….

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