Late last month, the international equestrian race in Spain that often offers the opportunity to run sunny for the best riders has taken a bad turn. An outbreak of disease infected many horses, leaving many weak and helpless and others displaying unusual aggressive behavior. At least 17 animals have died; others have had abortions or needed surgery to repair the damage to their organs.
The world of horses is preparing for the worst of times to come. Before researchers could identify the cause of the outbreak — a virus known as equine herpesvirus (EHV-1) —about 600 of the 750 horses that participated in the event were already on their way home, threatening to disperse what officials already called EHV-1 outbreak in Europe for decades. In an effort to stem the tide, the International Federation for Equestrian Sports (FEI), which oversees international equestrian competitions, has canceled all European events – including the World Cup – at least in mid-April. Horse owners, in turn, make strenuous efforts to vaccinate and separate their horses.
For scientists, the outbreak has raised a number of questions. They are investigating why EHV-1, a virus that tends to produce severe symptoms, appears to hit these animals, especially mares, with unusually high levels. Some wonder if the drug or vaccine against EHV-1 itself may have played a role. “The most important thing we must do is to address the impact of this terrible virus as soon as possible,” said Göran Åkerström, director of veterinary FEI. However, “It is also important to… expand our infectious disease information.” A special FEI working group to investigate the outbreak held its first meeting on March 18.
Investigators say the conditions at the monthlong competition, held in Valencia, Spain, were matured by the outbreak of EHV-1, which was mainly transmitted by inspired droplets. Horses are placed on tables that are full, and “All you need is for one horse with a hidden virus to have some form of stress, and then the virus will start to work and start to break down,” said equine pathologist Lutz Goehring of Ludwig Maximilian University of Munich.
Sick animals quickly filled the equine hospital at the nearby CEU Cardenal Herrera University, said Ana Velloso Álvarez, a veterinarian there. Tired trees treated up to 20 animals at a time, with many horses raised in the slopes, literally hanging between life and death. “I think I understand better what it was like for [COVID-19] doctors,” valvarez said.
Studies have shown that almost all horses have been exposed to at least one of the five EHV-1 viruses, and that animals can carry the virus for decades. Active infections often cause fever and shortness of breath, sometimes abortion. Particularly worrying variants, known as type 1, can cause severe nerve damage, giving the horses loose or unable to stand. Occasionally, it kills them.
Most outbreaks affect a number of horses before the farm is separated and disinfected. And less than 15% of infected animals usually show neurological symptoms. But in Valencia up to 40% of sick horses show signs of nerve damage, valvarez said. Also, with the unusual EHV-1 method, each horse had its own problem cocktail. Some had blood clots in their intestines and needed surgery. Some had swollen legs, walked as if they were drunk, or showed strange behavior. “This is completely different from what we’re used to [with EHV-1],” valvarez said.
Genetic sequence suggests that the rash was not a new form of EHV-1. That is what researchers look for in other things that can have serious consequences. Another move. Some horses have spent up to three days traveling to the event, and such a long trip can be “extremely stressful,” says Barbara Padalino, an equal scientist at the University of Bologna. A recent study by his team showed that after only a 12-hour journey, the horse’s defenses against EHV-1, which increase the risk of infection.
Some scientists are studying the role of sex. About 80% of Valencia’s most serious cases involve mares, valvarez said. Some researchers are wondering if the drugs used to prevent mares’ reproductive cycles – a treatment that some riders think make the horse easier to handle – may have contributed to the illness. Another popular drug, altrenogest, is based on progesterone, which has been shown to impair body function, notes Christine Aurich, an equine gynecologist at Graf Lehndorff Institute in Germany.
Researchers are also examining the EHV-1 vaccine, which has a sterile antibody record and requires booster shots every six months. Many sick horses were vaccinated, says strkerström – but previous research has shown that horses could be at greater risk for emotional symptoms in weeks after vaccination. The FEI working group collects vaccination records, as well as infection data and symptoms, in hopes of clarifying such issues — and building better treatment options and preventing future outbreaks.