Late last month, an international horse jumping competition in Spain that usually offers a sunny getaway for elite riders took a grim turn. A disease outbreak sickened scores of horses, leaving many so weak they couldn’t stand and others exhibiting unusually aggressive behavior. At least 17 animals have since died; others have had abortions or needed surgery to repair organ damage.
The equestrian world is bracing for worse to come. Before researchers were able to identify the outbreak’s cause—a known pathogen named equine herpesvirus type 1 (EHV-1)—some 600 of the 750 horses participating in the event were already heading home, threatening to spread what officials already call the most serious EHV-1 outbreak in Europe in decades. In a bid to contain the damage, the International Federation for Equestrian Sports (FEI), which oversees international equestrian competitions, has canceled all European events—including its World Cup—through at least mid-April. Horse owners, meanwhile, are frantically trying to vaccinate and isolate their horses.
For scientists, the outbreak has raised a host of questions. They are examining why EHV-1, a familiar virus that typically produces milder symptoms, appears to have hit these animals, particularly mares, unusually hard. Some are wondering whether drugs or the vaccine against EHV-1 itself may have played a role. “Our top priority must be to deal with the immediate impact of this terrible virus,” says Göran Åkerström, veterinary director of FEI. But, “It is also crucial that we … expand our epidemiological data.” A special FEI working group to study the outbreak had its first meeting on 18 March.
Researchers say conditions at the monthlong competition, held in Valencia, Spain, were ripe for an outbreak of EHV-1, which is primarily spread by exhaled droplets. The horses were housed in tightly packed stalls, and “All it takes is for one horse carrying a latent virus to have some sort of stress, and his virus gets activated and starts shedding,” says equine disease specialist Lutz Goehring of Ludwig Maximilian University of Munich.
Sick animals soon overwhelmed an equine hospital at the nearby CEU Cardenal Herrera University, says Ana Velloso Álvarez, a veterinarian there. Exhausted medics were treating up to 20 animals simultaneously, with many horses hoisted in slings, literally hanging between life and death. “I think I understand more what it’s been like for [COVID-19] doctors,” Álvarez says.
Studies have found that nearly all horses have been exposed to at least one of EHV-1’s five major strains, and animals can carry inactive virus for years. Active infections usually cause fever and mild respiratory disease, sometimes abortion. One especially worrying variant, known as type 1, can cause serious neurological damage, rendering horses wobbly or unable to stand. Occasionally, it kills them.
Most outbreaks affect just a handful of horses before a farm is quarantined and disinfected. And less than 15% of infected animals typically exhibit neurological symptoms. But in Valencia up to 40% of sick horses have shown signs of neurological damage, Álvarez says. And, in an unusual twist for EHV-1, each horse had its own cocktail of problems. Some had intestinal blood clots and needed surgery. Others had swollen legs, walked like they were drunk, or exhibited unusual behavior. “This is completely different from what we’re used to [with EHV-1],” Álvarez says.
Genetic sequencing suggests the outbreak wasn’t caused by a new strain of EHV-1. That has researchers looking at other factors that might have worsened outcomes. One is travel. Some horses spent up to 3 days journeying to the event, and such long trips can be “a huge stressor,” says Barbara Padalino, an equine scientist at the University of Bologna. Recent studies by her team have shown that after just a 12-hour trip, a horse’s immune defenses against EHV-1 drop, increasing the chance of infection.
Other scientists are examining the role of sex. About 80% of the most severe Valencia cases involved mares, Álvarez says. Some researchers wonder whether medications used to stop the mares’ reproductive cycles—a treatment some riders think makes a horse easier to handle—might have contributed to illness. One popular drug, altrenogest, is based on progesterone, which has been shown to weaken immune function, notes Christine Aurich, an equine gynecologist at the Graf Lehndorff Institute in Germany.
Researchers are also scrutinizing the EHV-1 vaccine, which has a spotty record of preventing disease and requires booster shots every 6 months. Many of the sick horses had been vaccinated, Åkerström says—but past studies have hinted that horses may be at higher risk of neurological symptoms in the weeks after vaccination. FEI’s working group is gathering vaccination records, as well as infection and symptom data, in hopes of clarifying such issues—and developing better ways to treat and prevent future outbreaks.