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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

Salivation in a horse suffering from Grass Sickness

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

Rhinitis Sicca

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’s horse William was hit by grass sickness in April 2011. Read more about here story here.

Further Reading

Equine Grass Sickness Fund website


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