In the fall of 2015, Queensland veterinarian Andrew Van Eps encountered an unusual case.

Wendy Sullivan brought her horse to the University of Queensland at Gatton because its hooves had started flaking and its ankles had swollen to the point of bleeding when it was being ridden. After examining the horse, Van Eps determined the cause of these symptoms to be coronary band dystrophy. Sullivan, however, was concerned that a Hendra vaccination was to blame.

“Anything that happens within a few months after the vaccine gets blamed on the vaccine,” Van Eps told The Atlantic.

Hendra is a virus that has affected the equine population in Australia since the early ’90s, although outbreaks of it have been sporadic. Flying foxes, a type of bat native to Australia, have been identified as the origin of the virus. Although it is rare, Hendra is a serious biosecurity threat that has wreaked havoc in the equine industry, affecting horses and humans alike.

Hendra was first documented in Australia in early September 1994, when horse trainer Vic Rail brought an ill, pregnant mare back to his stables. At the time, Rail was concerned that the mare would abort her foal and had veterinarian Peter Reid examine the mare. He found that the horse was running a high fever, in addition to having swelling around her lips and jaw and a partially paralyzed tongue.

Reid concluded the examination without pinpointing a specific cause of the mare’s illness. The next day, Rail decided to check on the mare. After he had opened the door, the mare stumbled out, collapsed and died.

Nearly 2 weeks later, 12 of Rail’s horses came down with similar symptoms. Some had to be euthanized because the fluid pooling in their lungs was slowly drowning them. Unfortunately, this did not stop the spread of the disease. In late September 1994, the disease’s progression made an unexpected leap. Rail, who had started experiencing flu-like symptoms while the disease was being investigated, passed away from cardiac arrest in the hospital.

The Australian Animal Health Laboratory in Geelong soon determined that a then-unidentified paramyxovirus was the cause. The virus was named “Hendra” after a Brisbane suburb. By the time the first outbreak was over, more than 20 horses had been infected and 13 had died.

“ … Hendra’s fatality rate equals or exceeds that of Ebola. Unlike Ebola, however, Hendra is not highly contagious: horses only cough in the final stages of infection, and human-to-human transmission has never been documented,” according to The Atlantic. However, transmission from horses to humans is a significant concern with this disease, especially if the virus mutates.

When dealing with a potential case of Hendra, veterinarians are required to wear protective gear and limit care until Hendra can be ruled out. Although instances of transmission between horses and humans are rarer than those between horses, it still happens. Since 1994, seven cases of humans contracting Hendra from horses have occurred. As a result, four people died.

Zoetis, well-known in the farrier industry as the maker of Dormosedan gel, has been working on a Hendra vaccine for years. In 2012, the company was able to release a vaccine after the Australian government fast-tracked its release. It was then marketed under a “minor-use” permit. Since the release of the vaccine, 20 horses that contracted Hendra have died. However, none of those horses were vaccinated.

Since its release, the vaccine has caused a great deal of contention between veterinarians and horse owners. There are several reasons horse owners don’t vaccinate their horses, including the cost and other factors. But the reason many horse owners have for not vaccinating, and the one that has caused the most contention, is that they believe “the company exaggerated the vaccine’s ability to prevent Hendra outbreaks and that it failed to properly test the vaccine’s effectiveness or risk of side effects.” In March 2018, a class-action lawsuit was filed against Zoetis, alleging that the vaccine’s ability to prevent outbreaks was exaggerated. Zoetis denies these allegations.

Natalie Boehm, a former veterinary nurse, tells a different story. In 2008, Boehm contracted the Hendra virus, likely from a horse that she had treated for a fungal infection. But instead of succumbing to the virus, she somehow recovered. The Atlantic suggests that she is probably one of the few humans living who is immune to the virus.

Since recovering, Boehm has partnered with Zoetis to educate people about the vaccine and the benefits it can have. She also works with veterinarian Janine Dwyer at All Horses Veterinary Services. Together, they work to bring the vaccine to the horses who need it.

“It’s quite amazing what it can do — that it can protect people and horses from dying,” she says. “We just try to teach people about it. But at the end of the day, it’s their decision.”