Grass sickness is a devastating equine disease. Prevalent throughout the British Isles, it's fatal in both its acute and subacute forms and extremely distressing.
Along with the Equine Grass Sickness Fund and the Animal Health Trust, The British Horse Society is supporting an EGS vaccine trial was launched in March 2014. Our campaign is helping to provide valuable additional funding for the full three-year EGS vaccine trial project.
Most people with more than a passing interest in horses will have heard of grass sickness. Luckily, relatively few of them will actually come across a case, but those who do know just how devastating the disease can be.
Many of us assume that grass sickness is a disease that only affects the east of the UK, particularly Scotland. While it's true that historically there has been a preponderance of cases in these areas, the map of cases reported to the Equine Grass Sickness Surveillance Scheme shows that grass sickness can and does occur almost anywhere - see the map of known cases on the left.
Not only does the disease occur all over Britain, it can affect any horse of any sex or breed. Donkeys are also susceptible. Cases have been seen in horses of all ages although very young foals are rarely affected.
According to the Equine Grass Sickness Fund, most cases are seen in animals between the ages of two and seven, with a peak between three and four years old. It does seem that older horses develop some sort of natural immunity to the disease, probably through exposure to the cause, but even horses in their 20s have been lost to grass sickness.
In short, at present none of our horses are truly safe from the disease.
What is grass sickness?
The disease causes damage to the part of the nervous system that controls involuntary functions – things that don’t require conscious thought, like breathing and digestion. The organ primarily affected by the disease is the gut, as it essentially becomes paralysed. There are three forms of grass sickness, with the acute form being the most severe.
1 Acute form
With this form of the disease, the paralysis of the gut will lead to the horse showing signs of severe colic. Additionally, swallowing will be difficult and there will be excessive salivation (dribbling). In many cases the stomach fills with a deeply unpleasant liquid which may end up coming out via the nostrils. This can be hugely distressing for the horse and its owner. Unsurprisingly, the lack of movement in the gut leads to constipation. If the horse does pass any droppings they will be small and hard, usually covered by mucus. On top of all of this the horse may sweat in patches and exhibit muscle tremors. There is no hope for a horse with acute grass sickness. The horse will die and should be swiftly euthanased (within two days of the clinical signs appearing) to prevent suffering.
2 Subacute form
The subacute form of the disease has similar symptoms to the acute form but less severe. There is unlikely to be a nasal discharge but the horse will lose a large amount of weight very rapidly. In these cases the horse should be humanely destroyed within a week as, sadly, there is no hope of survival.
3 Chronic form
The final type of grass sickness is the chronic form. The Animal Health Trust states that about a third of all grass sickness cases are chronic. In chronic cases the symptoms develop far more slowly than with other forms and may be as subtle as intermittent mild colic. The paralysis of the gut manifests itself as a difficulty in swallowing, which in turn leads to a loss of appetite. The horse will lose a lot of weight and other signs may include a dry and crusty nose (rhinitis sicca, left), drooping eyelids (ptosis), patchy sweating and a tucked-up stance.
The positive news is that some chronic grass sickness cases can be nursed back to health: approximately 50 percent, according to the Animal Health Trust. However, the cases where nursing is attempted must be selected carefully (based on the severity of the clinical signs) and there are no guarantees that treatment will be successful. Given that all acute and subacute grass sickness cases will require euthanasia, it's obvious that there's no straightforward cure. Treatment of chronic cases revolves primarily around nursing and nutritional support (remember the horse will have lost a lot of weight and is likely to have a poor appetite). There are some drugs that can be given but these are to treat symptoms and relieve pain. They are not a cure.
It's clear that grass sickness is a truly appalling disease that kills the vast majority of horses that encounter it. Because of that, a great deal of research has been put into the disease, much of it financed by the Equine Grass Sickness Fund. Despite all of this research, the cause of the disease is not definitively known. Over the years there have been many suspected causes ranging from plants to chemicals, insects to vitamin deficiencies.
In recent times, attention has been very sharply focused on the bacterium Clostridium botulinum and more specifically, a neurotoxin that it produces. Clostridium botulinum can be found in the soil, which would explain why grass sickness affects horses at pasture.
What it doesn’t explain is why only some horses get grass sickness, or why horses that consume spoiled silage or contaminated water are susceptible to botulism and not grass sickness. The truth is that Clostridium botulinum produces a range of toxins and it is believed that grass sickness is actually a toxico-infectious form of botulism caused by the Clostridium botulinum Type C neurotoxin being produced locally in the horse’s gastro-intestinal tract.
It seems that the presence of Clostridium botulinum in itself is not enough to cause grass sickness and that a variety of other factors need to be in place for the disease to occur. This would explain why one horse grazing a particular field may die from grass sickness while his companion can remain completely unaffected.
If the consensus of opinion is correct and Clostridium botulinum Type C is the cause of grass sickness
then the best hope of defeating this dreadful disease is a vaccine.
Vaccinations against species of Clostridia have proven extremely effective – tetanus is the most notable example (although tetanus is caused by Clostridium tetani not Clostridium botulinum).
Case study: Ulla Balletta and William
Ulla Balletta’s horse William (pictured right in happier times) was hit by grass sickness in April 2011. He had been healthy and happy, but in the space of just 24 hours Ulla lost her best friend. Here, Ulla describes the 17-year-old Irish Draught x gelding’s sudden deterioration, and how some strange symptoms that she thought were colic turned out to be the devastating disease.
"I’d ridden William on the Thursday and he was fine, but when I went out to his field on the Friday he was acting strangely. He didn’t come over to me, which was unusual, and when I brought him in he didn’t try and dive to eat any of the grass like he normally did.
"I tied him up outside his stable and he tried to pass droppings, but they were just mucus-covered pellets. He wasn’t eating or drinking and he didn’t touch his hay – and then he started showing signs of what I thought was colic. He was looking at his flanks and scraping the floor, so I called the vet out straight away."
When the vet arrived, he too initially diagnosed mild colic – but after a full examination 12 litres of green reflux was drained from William’s stomach and his heart rate found to be dangerously high. Fearing life-threatening colic, William was rushed to hospital immediately. Devastatingly for Ulla, the hospital examination revealed something much more sinister – the vets suspected grass sickness. William was now gravely ill and needed to be operated on the next morning.
"That night, I went home and researched the condition. Nothing I read was positive. When the vets operated in the morning, they confirmed our fears – it was acute grass sickness, and nothing could be done for William. That was it. He was gone within 24 hours.
"I’d never heard of grass sickness until it happened to William. The sheer shock is what is so scary about it because before you know it, your horse is gone. If you don’t know what grass sickness is, you think you’re dealing with colic because the symptoms are so similar. That’s why it’s so important to research grass sickness and know how to minimise the risks. You don’t have the time to do it after it strikes."