Lameness in Endurance Horses

Chapter 118Lameness in Endurance Horses



Martha M. Misheff



Evolution of Endurance


Modern endurance is the most rapidly evolving equine sport. It has its roots in the long distance races of the ancient Bedouin across the desert. Later British and American cavalry used endurance tests as part of their military training, and nineteenth century Austrians had a Vienna to Budapest ride. Despite this long history, organized endurance riding is a young sport. The first modern endurance ride, the 100-mile (160-km) Tevis Cup from Nevada to California, has been run every year since 1955. The Tom Quilty Gold Cup, a prestigious 100-mile endurance race named for its founder, was established in 1966 and is held every year in a different part of Australia. The oldest endurance organization, the American Endurance Ride Conference, has been in existence only since 1972. Since 1984, all international endurance competitions have been held under the auspices of the Fédération Équestre Internationale (FEI). The Endurance World Championships are held every 2 years in a different host country, and endurance is now the fastest growing FEI sport. More countries participate in international endurance championships than in the international championships of any other FEI discipline.



Description of the Sport


The FEI Rules for Endurance describe the sport as a test of “the competitor’s ability to safely manage the stamina and fitness of the horse over an endurance course in a competition against the track, the distance, the climate, the terrain and the clock.” It is the clock, however, that determines the winner because the winner is the competitor who completes the course in the shortest time and passes the final veterinary inspection and forensic testing. As the sport of endurance has grown, it has come under intense scrutiny with regard to equine welfare. In addition, there is a great deal of debate within the endurance community as to whether the more media- and spectator-friendly events favoring increasing speeds over shorter distances (120 km) are preferable to the more traditional sport of slower speeds over longer distances and more challenging terrain. In March 2008, an Endurance World Forum was held in Paris to discuss these and other concerns, and a task force was appointed to revamp current rules governing the sport. The new rules went into effect in 2009; the current championship distance remains unchanged at 160 km (100 miles). The 2008 Endurance World Championships in Malaysia marked the first time that this event was internationally televised.


Each country has its own National Federation, and national competitions may be run under national rules. All international and championship competitions are run under FEI rules, and there are rigorous qualification requirements that must be met to participate. Concours de Raid d’Endurance International (CEI) competitions are divided into 4-star (championship level) through 1-star (low-level) events, depending on the distance covered and age requirements of the horse. Horses must be at least 5 years of age to compete in qualifying events, with age requirements increasing through the star rating. Horses must be at least 8 years of age to compete in championships and CEI 4-star competitions.



Veterinary Controls


Veterinary controls are an integral part of endurance events, and horses must undergo and pass an initial veterinary inspection and subsequent inspections after each phase of the competition. The total distance of the ride is divided into segments of 20 to 40 km, usually laid out in a cloverleaf pattern with a central Vet Gate area. After riding each section, the competitor must pass the Vet Gate or veterinary checkpoint, where panels of veterinarians evaluate horses for lameness and metabolic criteria. The heart rate must be at or less than 64 beats per minute (bpm) within 20 minutes of arrival from the course to each Vet Gate and within 30 minutes of arrival to the final inspection after crossing the finish line. FEI rules allow the Veterinary Commission the latitude to lower the heart rate criterion if extreme weather conditions are deemed severe enough to warrant it.


The time is taken when the competitor comes in off the course into the crewing area. The competitor has 20 minutes in which to “clock in” to the Vet Gate, or the horse will be eliminated on a time penalty. Heart rate decreases to less than 64 bpm within 1 to 3 minutes of arrival in most elite endurance horses. The time is recorded again when the horse enters the Vet Gate. There is a mandatory hold time after each phase of the competition, which begins from the time of entry into the Vet Gate. Therefore speed of entry into the Vet Gate is a critical opportunity to gain time. If the horse’s heart rate is higher than 64 bpm on initial inspection he is “spun” or required to go out of the Vet Gate and return for a reinspection when the heart rate meets the 64-bpm criterion. A horse is permitted one reinspection within the 20-minute time limit at the Vet Gates, but no second chance to meet the heart rate requirement is permitted at the finish. Intensive crewing (rapid cooling with copious amounts of ice water) is common at high-level competitions, and top-level horses generally come in off the course, have saddles removed, are cooled, have the heart rates checked, and go directly into the Vet Gate. Most championship venues have instituted digital heart rate monitors on which the heart rate for each horse is publically displayed at the end of the trotting lane. A second heart rate is measured by direct auscultation 1 minute after the commencement of a 40-meter trot, and the difference between the first and second heart rates is calculated. This is known as the cardiac recovery index (CRI) or Ridgway Test, after Dr. Kerry Ridgway. Although not a criterion for elimination in and of itself, a positive CRI, in which the second heart rate is elevated more than several beats above the first one, is an indication that the horse merits further evaluation. Other metabolic parameters, such as mucous membrane color, capillary refill time, skin turgor, and auscultable intestinal borborygmi, are checked and recorded. The presence of any saddle sores, girth rubs, or bitting lesions is also noted. A heart rate above the maximum allowed, injury, or “consistently apparent” lameness are grounds for elimination. Synchronous diaphragmatic flutter is a common condition in endurance horses and may be grounds for elimination if other metabolic abnormalities are concurrently present. Time-keeping systems for arrival from and departure to the course are automated at high-level events, with riders presenting a digitized “slash card” at timing checkpoints. After passing the veterinary examination, the horse is returned to the crewing area, where it is fed, groomed, and watered before called to return to the course. The horse is not permitted to leave the exit gate until the hold time for each phase is completed. A computerized record of the competitor position, exit time, time taken for the horse to present, and average speed is available at each Vet Gate and at the finish. Endurance horses are not permitted to compete on performance-enhancing medication, and medication control programs are in place to test for substances that are not permitted, in accordance with FEI rules for all equestrian disciplines.


Although national competitions may use the same veterinarians for control and treatment, FEI competitions require separate veterinarians dedicated to each. There are specific requirements regarding the number of commission and treatment veterinarians per number of competitors according to the star rating of the competition. Typically, a horse that is eliminated by a control (Commission) veterinarian for a lameness or metabolic problem is referred to the treatment veterinarian for further evaluation. If the treatment veterinarian deems that treatment is prudent, it is provided at a field clinic at the ride site, or in some cases the horse is transported to a local referral veterinary clinic or to its home stable for treatment. Team veterinarians are usually so busy with other competitors that have not been eliminated that they are happy to have the assistance of the treatment veterinarian. However, it is important that treatment and team veterinarians closely liaise with one another to provide the best possible care. Ride site clinics at large venues or championships offer sophisticated treatment (e.g., blood analysis, fluid therapy, digital radiography, ultrasonography, electrocardiography, endoscopy), and only those horses that require continued care, further diagnostics, or surgical treatment are referred.


Top-level international modern endurance is an extreme sport. Competitors and experienced endurance veterinarians recognize that proactive provision of postcompetition fluid therapy is beneficial in rapidly returning the horse to normal hydration and homeostasis. In my opinion, fluid therapy should not be withheld in an effort to reduce “treatment rates,” and competitors whose horses receive fluid therapy to optimize recovery should not be penalized.



Type of Horse


Most endurance rides are open to horses of any breed, but horses of Arabian extraction are most popularly used for endurance riding because they have light build and stamina for long distance rides. Larger, heavier framed horses are less able to sustain the speed required over the distances traveled. Endurance horses, compared with other athletic horses, tend to be small and light, although in the past 10 years there has been a trend toward taller more robust Anglo-Arab types. Like human marathon runners, good endurance horses tend to be ectomorphs, but speed, cardiac recovery capacity (the ability to rapidly decrease the heart rate after a fast gallop over long distances), and conformation that is likely to withstand the pressure of the demands to which endurance horses are subjected are currently the most sought-after qualities.



Training Methods and Competition Speeds


Training methods differ considerably among trainers, but most would agree that it takes 2 to 3 years of training to make a good endurance horse. Training distance and frequency depend largely on individual circumstances (terrain, weather, availability of pasture, or turn-out paddocks), but most regularly competing endurance horses are ridden 6 days a week. A typical program might include daily rides of 10 to 20 km, with weekly or 10-day interval 30- to 80-km rides. Horses are ridden at a walk, trot, and canter, with the proportion of the time spent at each gait varying considerably. Horses being prepared for a 100- to 160-km ride will usually perform a 60- to 80-km training ride on at least two occasions before competition. New qualification requirements mandate that horses complete successive rides at lesser distances before being permitted to compete at the next increased distance level. Overtraining of endurance horses tends to be more of a problem than undertraining, particularly by novice trainers.


Endurance competition has rapidly evolved from amateur operations where the same person owned, trained, and rode the same horse for many years, competing for pleasure, with small prizes in kind, to large stables with numerous horses run by professional trainers competing for international recognition, luxury automobiles, and hundreds of thousands of dollars in prize money. The value of a horse winning or running successfully in a prestigious international competition increases exponentially. Speeds at high-profile rides over mountainous terrain have not substantially changed, but as the sport has continued to develop, speeds of competition over flat terrain have increased dramatically. Essentially, at the highest levels the sport has changed from endurance riding to endurance racing. The winner of the 2008 World Championship in Malaysia averaged 18 km/h (5 m/sec) for 160 km, at night, in a torrential downpour, and with high humidity. A world record time for 160 km of 6 hours, 28 minutes, 28 seconds riding time was set in January 2008. Although the total average speed for all phases was 24.71 km/h (6.86 m/sec), the final 19-km loop was covered at an average speed of 30.29 km/h (8.41 m/sec). It must be borne in mind that as an average over 19 km, actual speeds for short bursts can be substantially higher. As winning speeds at the classic 160-km distance have increased, speeds at the shorter 120-km and 100-km distance have increased even more. In these races, average speeds for some phases reach more than 36 km/h (10 m/sec), which is the equivalent of an “open gallop.” It is not unusual to have flying finishes with fractions of seconds separating finishing places. Ten years ago this would have been unheard of. Opinions differ as to whether this is desirable progress, but from a veterinary standpoint, at rides where speeds are increasing, it means that we are beginning to see more flat-racing types of musculoskeletal injuries, as well as serious metabolic abnormalities. Although water is available on the course every 5 to 8 km, riders competing at distances less than 160 km rarely stop on the course to water their horses. The horses are watered when they come into the crewing area.



Course Terrain


Endurance horses compete and train over some of the most highly variable terrain of any sports horses, including rocky mountains, through creeks, across sandy deserts, along tarmac, gravel, or dirt roads, and across grassy fields, depending on where the ride is held. Course terrain clearly has a bearing on the type of injury, generally in a straightforward way. Stone-bruised feet and painful joints are more common when horses are worked on hard, rocky ground, whereas ligament and tendon injuries are more often observed in horses worked on soft or sandy ground. Downhill grades, if slippery or taken at speed, tend to produce a plethora of dorsal carpal injuries as a result of falls. A relatively recent innovation is the grading and harrowing of long stretches of the course, much like a racetrack without the cushion. Many large endurance stables now train horses on racetracks. This has resulted in an increase in racing-type fractures and superficial digital flexor tendon (SDFT) injuries.



Lameness and Conformation


Lameness in endurance horses can be divided into three categories involving (1) ligaments or tendons, (2) muscle, or (3) joints, bones, or feet. Lameness affecting endurance horses can also be separated into transient problems that may be cause for elimination on the day of competition but then resolve and more persistent problems that are likely to be recurrent. In the relatively recent past, many endurance horses competed over an 8- to 10-year period, and horses with the most glaring conformation defects tended to weed themselves out by attrition. Developments in the sport, most notably increases in speeds of competition, have paralleled the shortening of the competitive life of top-level endurance horses by several years. Astute veterinary advisors make a concerted effort to avoid young horses with flat, asymmetrical, or contracted feet, major angular limb deformities, offset cannon bones, and long, slack, or short, upright pasterns because these may contribute to later unsoundness problems.1 Although poorly conformed horses may be successful in the short term or at less-demanding levels of competition, it is an exceptional poorly conformed horse that stays sound and is successful over the time that it takes to reach championship level.2 Flaws that are not extreme may be present without compromising the overall function of the horse but are preferentially avoided when possible. Horses with toe-in conformation appear to be more likely to develop splint exostoses that may impinge on the suspensory ligament (SL), causing desmitis, and horses with recurrent suspensory desmitis are ultimately unsuitable for endurance.



The Lameness Examination and Proceeding without a Diagnosis


A lameness examination in any horse is most easily and efficiently performed when the lameness is visible, and the endurance horse is no exception. It is usually not possible to make a specific diagnosis if a horse is not lame at the time of examination. Endurance horses are problematic because often a mild transient lameness that resulted in elimination from competition may not be visible the next day or even later the same day, although many horses have painful superficial digital flexor tendons (SDFTs) and SLs after a race. Low-grade, inconsistent, nonspecific lameness that cannot be localized is a frustratingly common scenario faced by veterinarians charged with keeping endurance horses sound. When possible, watching the horse trot while lame, application of hoof testers, noting the presence of any palpable abnormalities, and response to flexion or other manipulative tests help to narrow the possible causes. Diagnostic analgesia, adhering to the principle of starting at the bottom and working upward, is the standard most useful means of localizing the source of pain causing lameness when no palpable abnormalities are present. Sometimes, despite a careful and complete lameness examination, diagnostic analgesia, and ancillary diagnostic aids such as radiography, ultrasonography, scintigraphy, and even magnetic resonance imaging (MRI), a specific diagnosis cannot be reached. Some horses never “block out.” A careful neurological examination should then be performed because subtle neurological deficits may mimic or produce lameness. In other horses, pain causing lameness can be isolated to a specific region, but no lesion can be found. Like other athletic horses, endurance horses may have subtle gait abnormalities emanating from numerous areas, all contributing to suboptimal performance but difficult to identify individually. The advent of MRI has elucidated some previously elusive conditions of the distal aspect of the limb, especially in the proximal palmar metacarpal region and the foot.


Horses with pain causing lameness localized using diagnostic analgesia or clinical examination to a specific region in which there are no detectable radiological or ultrasonographic lesions should be reexamined by the most appropriate conventional imaging modality after 2 to 4 weeks or should undergo scintigraphic or MRI examination if warranted by the degree of lameness present. Stress fractures are particularly notorious for not being detectable on the first radiological examination and being readily apparent later. However, stress fractures are usually detectable using scintigraphy or MRI. If the lameness has resolved and no lesion is detected on a recheck examination, it is reasonable to return the horse to light training. If lameness recurs, the lameness examination should be repeated.


Although “full-body” scintigraphy is ideally avoided, it is sometimes beneficial in horses that have an acute lameness that cannot be specifically localized. We have recognized the propensity for stress fractures to turn into catastrophic fractures; therefore obtaining a specific diagnosis is increasingly important. It should be borne in mind that the percentage of positive scintigraphic findings is higher for horses with acute, severe lameness than in those with chronic, low-grade lameness. Sometimes, despite every attempt at imaging and reimaging, the clinician is still left with a lame horse and no diagnosis. When this point is reached, a prolonged rest period (8 to 12 months) may be helpful.



Common Causes of Lameness in Endurance Horses


The principal cause of lameness is proximal palmar metacarpal pain,1-4 but the other top 10 causes of lameness in endurance horses tend to vary somewhat in ranking from season to season. Although the problems listed here are the most frequently encountered, not only is the ranking variable, but also pathology in the proximal palmar metacarpal region and in the fetlock can be divided into subcategories that may overlap and are difficult to separate. The advent of the availability of sophisticated imaging techniques, especially MRI, has made it more difficult to write a list of the 10 most common causes of lameness in endurance horses because of the ability to recognize to a greater degree the interrelatedness of the anatomical structures involved. In addition, some injuries appear to be part of a spectrum or continuum that progresses through phases with time. Bearing this in mind, the following are the lameness problems seen most commonly:


1 Proximal palmar metacarpal pain
Proximal suspensory desmitis

Proximal palmar metacarpal stress pathology

Pathology associated with the second and fourth metacarpal bones

2 Foot problems

3 Metacarpophalangeal and metatarsophalangeal osteoarthritis, synovitis, and periarticular pathology

4 Stress-related injury of the distal aspect of the third metacarpal bone

5 Superficial digital flexor tendonitis

6 Myalgia of the paravertebral and gluteal muscles

7 Osteoarthritis of the centrodistal and tarsometatarsal joints

8 Desmitis of the suspensory ligament body and branches

9 Other bone injury

10 Exertional rhabdomyolysis


Proximal Palmar Metacarpal Pain


The leading cause of chronic or recurrent lameness in endurance horses, regardless of terrain, is pain in the proximal palmar metacarpal region. Proximal suspensory desmitis (PSD) is often the primary problem. Pain associated with the lesions of the palmar aspect of the third metacarpal bone (McIII) or less commonly pain associated with the second (McII) or the fourth (McIV) metacarpal bones, or the interosseous ligaments between the McII and McIII and the McIV and McIII may contribute to lameness. Injury to the proximal metatarsal region of the hindlimb also occurs but is less common.



Proximal Suspensory Desmitis


Desmitis of the SL typically develops in the later stages of a race when the horse becomes fatigued but may occur in the earlier stages of a race in an unfit horse or a horse that has had previous SL injury. It is also a common training injury. Lameness caused by PSD may be sudden in onset and severe or be more insidious in onset. Often, there is little or no detectable swelling, but careful palpation may elicit a painful response. However, many horses resent palpation of the proximal aspect of the SL and will give a false-positive response to even the most gentle and careful palpation. Horses with forelimb PSD are usually positive to distal limb flexion. Diagnostic analgesia is critical to definitively localize pain to the proximal palmar metacarpal region. The proximal palmar metacarpal region may be desensitized by (1) blocking the deep branch of the lateral palmar nerve laterally at the edge of the accessoriometacarpal ligament distal to the accessory carpal bone and the medial palmar nerve (high two-point or Wheat block, after Dr. John D. Wheat), (2) direct infiltration, (3) performing a high palmar and palmar metacarpal nerve block (“high four-point”), or (4) blocking the lateral palmar nerve in the longitudinal groove on the medial aspect of the accessory carpal bone.5

Related posts:

  1. Lameness in Horses: Basic Facts Before Starting
  2. Thermography: Use in Equine Lameness
  3. Counterirritation
  4. The Carpal Canal and Carpal Synovial Sheath

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Jun 4, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Lameness in Endurance Horses

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