Exploring the cause of grass sickness

Why would one horse develop equine grass sickness (EGS) and not another? Owners who have nursed an animal through this debilitating disease — or lost one to it — are left puzzling over what they could have done to prevent it.

Sadly, there are still no answers. A vaccine trial has proved inconclusive and the cause of EGS remains a mystery. So what do we know about this troubling condition — and where, exactly, do we stand?

Risk factors

“EGS is a degenerative disease of the nervous system that affects horses, ponies and donkeys, resulting in impaired motility of the gut,” explains Dr Tim Mair MRCVS. “Clinical signs vary, depending on how much nerve damage has occurred.

“Less extensive damage results in a milder form of the disease called chronic EGS,” he adds. “Greater damage results in permanent paralysis of the gut and severe clinical signs, and is termed acute or sub-acute EGS. While a proportion of animals affected by chronic EGS can survive, the disease is always fatal in its acute and sub-acute forms.

“All ages are at risk, but especially young horses with access to pasture in springtime,” says Tim, adding that the highest incidence occurs in animals between two and seven years old. “It is most common between April and July, with a peak in May and a second, smaller peak in autumn.

“Many potential causes have been considered, including poisonous plants, chemicals, bacteria, viruses and moulds,” says Tim. “A favoured theory is that EGS is associated with intestinal overgrowth of a toxic soil-associated bacterium (C. botulinum), which damages nerve cells in the digestive tract.”

While EGS occurs in most areas of the UK, eastern counties are considered particularly at risk. Certain paddocks are thought to be hotspots for the disease, with cases occurring in consecutive years — for no known reason.

“Research has identified a number of factors that increase the risk of the disease,” explains Tim. “These include recent pasture changes or disturbance, such as construction or harrowing, and sites where there has been an outbreak previously.

“Studies indicate that EGS is non-contagious,” he adds. “It is more likely to reoccur where there are a high number of horses, especially young animals.”

Intensive care

With so little scientific evidence to work with, experts are hard-pressed to offer advice about preventative measures — although precautions can be taken.

“Stabling animals 24/7 during spring and early summer will reduce risk, especially when dry weather with a temperature of 7-11˚C has persisted for 10 consecutive days,” says Tim. “It is also advisable to stable horses new to premises where the disease is known to occur for two weeks, before introducing grazing slowly.

“If a case occurs among a group of horses, it is best to move the others out of that field.”

While there is no specific treatment for EGS, chronic cases benefit from intensive nursing.

“Those affected by acute and sub-acute EGS cannot survive and should be euthanised,” says Tim. “Once clinical signs are apparent, the nerves will be permanently affected. In chronic cases, however, the damage is relatively mild and the remaining nerves may be able to compensate.

“Treatment should only be attempted if the horse is not in pain, is still able to eat and is interested in life,” he adds. “Constant nursing care and human contact are vital, along with the provision of palatable, high-energy and easily swallowed food. With such intensive management, around 70% of treated cases can survive.”

Focus on fungi

The recent nationwide field trial for a vaccine for the prevention of EGS involved more than 1,000 animals residing at 120 premises across the UK which had been previously affected by a high incidence of the disease.

The study was co-ordinated by the Animal Health Trust, in collaboration with key university veterinary schools and support from the Moredun Foundation Equine Grass Sickness Fund. Sadly, the trial did not show a significant protective effect of the C. botulinum type C vaccine.

So what’s the next step?

“Our current focus is to test the theory that EGS is caused by a toxin produced by one or more of the numerous fungi growing on pastures,” says Professor Bruce McGorum FRCVS of the University of Edinburgh. “Many risk factors for EGS are consistent with involvement of such a fungal toxin, including the seasonal occurrence, the clustering of cases in certain locations and the climatic conditions that often precede the development of cases.

“This research follows work from the 1990s, which identified increased numbers of particular fungi on grasses from EGS fields and within the intestine of EGS horses. The potential for this previous work to identify a fungal cause was likely hampered by the limited laboratory tools available.

“Within our current three-year project, funded by the Horse Trust, we are adopting a more sensitive genomic approach which allows the detection and identification of thousands of different types of fungi within the intestinal tracts of EGS and healthy horses, thus permitting comparisons to be made,” adds Bruce. “It is hoped this comparative genomic approach might increase the chance of finding the elusive ‘needle in the haystack’ that causes this devastating disease.”

‘I never thought it would happen to me’

“I didn’t put two and two together,” says Felice Chilcott of the early signs in 2015 that her six-year-old warmblood gelding Jaguar (Jack) had developed chronic EGS. “He was always quite a ‘hot’ chestnut, but that didn’t explain him coming in on a cold day in a sweat. When he started picking at his feed, I thought he must be eating more grass.

“The day he started trembling, the vet identified Jack’s drooping eyelids and almost instantly suspected EGS. I felt as if I’d been pushed off a cliff.”

With support from the Royal (Dick) School of Veterinary Studies, Felice nursed him through the nightmare months ahead.

“They said he would be better at home, as the trauma of a journey would potentially set him back,” she says. “I had to feed him every four hours, encouraging him to eat anything I could. We worked our way through all sorts — haynets of strimmed grass, windfall apples — everything well-soaked as he wouldn’t eat anything dry.

“He became painfully thin and looked so miserable, standing at the back of his stable with his head down,” adds Felice. “I thought he would die. When I realised he might survive, I wondered if he’d ever be strong again.”

After months of round-the-clock feeding, Jack gradually gained weight and could return to light work. He is now winning at British Dressage elementary level, with Area Festival and Scottish national championship titles to his name.

“He is now healthy looking but will never be a good-doer,” says Felice. “What he went through is always at the back of my mind. We’re based south of Aberdeen — known as an EGS hotspot — but we have good, relatively old pasture that has been grazed by young horses with no problems,” she adds. “It’s still a puzzle.”

Ref Horse & Hound; 16 January 2020