Riders arrive at the Santa Anita track in Southern California on 8 March.

IRFAN KHAN/LOS ANGELES TIMES/GETTY IMAGES

Wave of horse deaths on famed racetrack stumps scientists

The Southern California racetrack famous for historic wins by thoroughbred greats such as Seabiscuit, John Henry, and Zenyatta is struggling to explain a series of fatal accidents in horses. In less than 3 months, 22 horses have died on the Santa Anita tracks in Arcadia, most of them from catastrophic limb fractures, leading managers to shut it down on 14 March.

The string of fatalities has spurred outcries from animal welfare activists and caused major economic loss—but it also mystifies scientists who study horse racing and racetracks. Some believe heavy rainfall may have caused irregular compaction of the dirt track layers, increasing the risk of fractures when horses' hooves penetrate the ground at high speeds. "Dirt tracks are particularly dangerous because they can seem fine on the surface but hide the compaction deep below," says Nathalie Crevier-Denoix of the French National Institute of Agricultural Research and the National Veterinary School in Alfort, near Paris. But a battery of tests by U.S. experts has failed to show anything unusual.

Injuries so serious they cause death or require immediate euthanasia because they can't be repaired occur on every racetrack. The most common type is a fracture of the front fetlock, a hinge joint between the foot and the lower leg bones that is "an important shock absorber, like airplane landing gear," says Susan Stover, a veterinary researcher at the University of California, Davis. At Santa Anita, the catastrophic injury rate has doubled compared with last year.

A horse's age, sex, and racing experience can all affect its risk of injury, as can preexisting stress fractures. Some experts also suspect veterinary drugs. Joe Pagan, president of Kentucky Equine Research in Versailles, recently found that furosemide, used to stop airway hemorrhaging as a result of exertion, and omeprazole, a treatment for stomach ulcers, both affect calcium excretion and absorption, which could theoretically weaken bones; the Stronach Group, which owns Santa Anita, recently said it would ban the use of drugs on racing days. But Pagan says a connection to the injuries is "a big stretch." Others have noted that more than 90% of racehorses nationwide have stomach ulcers, and most are treated with furosemide, so the drugs' effects wouldn't be limited to Santa Anita.

Instead, many scientists think something about Santa Anita's dirt track must be to blame. After the 21st death this season, the Stronach Group invited Mick Peterson of the Racing Surfaces Testing Laboratory, a nonprofit organization in Lexington, Kentucky, to study the problem. Peterson ran chemical and x-ray diffraction studies on track samples, testing, among other things, the soil's density, moisture content, and mineralogical qualities.

He also examined the consistency of the track's layers. The top layer, called the cushion, is soft and granular, to dampen the impact on the horse's foot; the one below, called the pad, is harder and more compact, allowing for more "push-off " of the hoof. (Below that are two more layers called the hardpan and the base.) Horses probably adjust to different surfaces as long as they're consistent, Peterson says, but injuries could result if the track characteristics change from stride to stride. His team brought in a machine, towed behind a vehicle, that mimics a galloping horse's forelimb plunging into the track and collects data on deceleration, sliding, surface elasticity, and energy absorption. They also used ground penetrating radar to measure the depths of the layers every 10 centimeters along the track.

Researchers used a machine that mimics a galloping horse foot to do biomechanical tests on Santa Anita's dirt track.

SANTA ANITA PARK

None of the tests revealed anything unusual. "There's nothing that we know, based on what we know, that's wrong with the track," Peterson says. Santa Anita reopened its tracks on 11 March, after his results had come in; within days a 3-year-old filly sustained fractures in both forelimbs and was euthanized, and the shutdown resumed.

Peterson says current testing methods could miss problems with moisture management on the dirt track. The cushion layer works best when it contains about 14% water, he says. Track managers have an array of techniques for managing moisture, such as sealing water out during wet weather by rolling the surface overnight, or "harrowing" and watering the track during dry weather to offset evaporation. But moisture levels can still change dramatically throughout a race day, especially when heavy rainfall alternates with bright sun and desert winds. Complicating matters further, Santa Anita's grandstand casts a large, evolving shadow across part of the dirt track. The departure of Dennis Moore, a seasoned surface manager who retired from Santa Anita in December 2018, may also have played a role, although it's not clear how much practices have changed since then. "If the maintenance wasn't perfect, that may have been a factor," Peterson says.

Peterson is now studying how well different management techniques work after rainstorms. Necropsies of the fallen horses may also offer clues by revealing whether certain types of injury were more common.

Santa Anita officials didn't respond to Science's request for comment. But they have evidently decided they can't wait for science to come up with definitive answers. Moore has been brought back as a consultant, and as Science went to press, Santa Anita was slated to reopen on 29 March.