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.