Grass sickness – a veterinary emergency needing immediate action

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    This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.
  • Grass sickness: Risk factors | Signs | Diagnosis | Treatment | Causes | Prevention

    Grass sickness is an often-fatal condition that typically occurs in grazing horses. It was first recognised in Scotland in the early 1900s, yet relatively little is known about the condition.

    Another name for the condition is equine disautonomia, so-called because it is a disease in which degenerative changes occur in the central nervous system and to the nerves of the autonomic (involuntary) nervous system. This results in malfunction of the whole digestive system.

    The United Kingdom has the highest incidence of grass sickness in the world with cases being more common in spring. Young adult horses, aged between two and seven years, appear to be at the highest risk of the condition, but it can affect horses of any age.

    With more horses being turned out at grass currently due to the Covid-19 restrictions, all owners should be aware of the signs and prepared to act promptly if they think their horse could be affected.

    If you suspect your horse may be showing signs of grass sickness then a vet should be called immediately.

    Grass sickness can be acute, subacute or chronic in nature. While around half of the chronic cases may recover with careful nursing, the more acute cases will not survive and are normally put down on welfare grounds. If acute cases are not euthanised, death often occurs within 48hrs of the onset of clinical signs.

    Grass sickness: which horses are at highest risk?

    • Horses between the ages of two and seven in good to fat body condition are at the highest risk, although equine of all types and ages can be affected
    • It is particularly prevalent during April, May and June, and later in the autumn, after a spurt of grass growth, but can occur at any time
    • Anecdotally, ground frosts and sudden weather changes are associated with disease outbreaks
    • There is evidence that the majority of cases occurred during periods of cold, dry weather
    • Equine grass sickness almost exclusively affects grazing animals. Risks include dietary changes, recent movement to new premises or pasture and frequent administration of wormers
    • If your horse is kept in a high-risk area, always consult your vet before choosing a worming treatment
    • Moderate worm egg counts have been associated with decreased risk of the condition
    • Frequent grass cutting and manual faeces removal appear to lessen the likelihood, but pasture disturbance, such as construction work, for example replacing fencing, digging ditched or even large numbers of mole hills, may increase the risk
    • Cases are more common on previously affected premises and pastures, and at large establishments such as livery yards, riding schools and stud farms
    • Grazing with high soil nitrogen and sand or loam soil types have been found at affected locations
    • There is some evidence that premises with domesticated birds or fowl may carry a heightened risk
    • Survival rates from chronic grass sickness are greater in Scotland (60%) than in England (45%), but the reasons for this are not yet known. Equally strangely, grass sickness has not been found in Ireland, which has a somewhat similar climate

    What are the signs of grass sickness?

    Grass sickness has acute, subacute and chronic forms.

    In acute grass sickness cases, the signs are suddenly and severe. They include colic, reflux of green stomach fluid down the nose, difficulty in swallowing, muscle tremors, abnormal sweating patterns, impaired gut activity and impaction of the colon with depression and distress.Horses suffering the subacute form will have milder signs than are seen in acute cases. Most subacute cases will require euthanasia if they are unable to swallow, but some will progress to the chronic form.In chronic grass sickness cases (typically horses that are still alive eight or more days after first showing signs), the signs develop more slowly and include marked weight loss, variable difficulty in swallowing and abdominal discomfort, muscle tremors, sweating, slow gut activity and a “snuffling” sound during breathing, due to blockage of the nasal passages with mucus. Some success can be achieved in treating this form of the disease with very intensive nursing over a long period of time.

    How is it diagnosed?

    While vets will diagnose the majority of horses with grass sickness based on clinical signs, the best way to definitively diagnose the disease in a live horse is to take a biopsy from the intestine while the horse is under a general anaesthetic. This is obviously very invasive, so recent research has looked at alternative biopsy sites, such as evaluating rectal biopsies1.

    There are other helpful tests that can provide a guide, such as applying phenylephrine eye drops to the conjunctiva of one eye – if the eyelid of the treated eye becomes less droopy 30minutes later, this increases the suspicion that the horse may be suffering from grass sickness.

    The Animal Health Trust launched a major nationwide blind trial of a vaccine for grass sickness in 2014 involving more than 1,000 equines across a two-year period. The results of the trial have not been released.

    Treatment for grass sickness

    Horses suffering from acute signs of grass sickness cannot be saved and should be euthanised on welfare grounds as soon as the diagnosis is clear. Most subacute cases also require euthanasia if they are unable to swallow. It can be extremely difficult for all concerned to make the decision, but the important thing is to ensure the horse does not suffer and discuss with your vet.

    Treatment for chronic grass sickness cases is extremely onerous, time-consuming and emotionally taxing, according to the Dr E Milne of the Equine Grass Sickness Fund2, who warns that even with the most dedicated nursing, many horses cannot be saved. Careful selection of appropriate horses for treatment is important. This should include willingness and ability to attempt to drink and swallow feed and the absence of continuous colic pain. The greatest obstacle to a good outcome is the profound lack of appetite that is commonly seen in many cases of grass sickness.

    Consequently a highly palatable feed that is high in energy and protein and is easily swallowed should be fed. Small appetising feeds should be offered at least four to five time a daily. Fresh water should always be available.

    Initially, horses should be kept stabled in a deep straw-bedded box, but short walks in hand two or three times a day are important to help stimulate the movement of the horse’s gut and to help keep the horse’s interest.

    As a chronic case improves, the horse can turned out on good grazing, starting with 20-30min and gradually increasing.

    Vets usually give horses with grass sickness pain relief, or use appetite stimulants to encourage them to eat.

    Many horses with chronic grass sickness have a low body temperature, so are best kept warm in a thick wicking rug that allows sweat to evaporate while maintaining body temperature.

    Potential complications include diarrhoea, choke and inhalation pneumonia. If the horse avoids these and other challenges, gradually weight gain can be expected three to five weeks from the onset on the disease. Return to full weight can take months and will never be achieved in some cases. Unusual sweating and problems with swallowing should also gradually improve.

    If the horse is doing well six to eight weeks after first succumbing, the chances of survival are good. The recovery rate in chronic cases is around 50%.

    What causes grass sickness?

    It has been believed that a toxin produced by the bacterium Clostridium botulinum causes damage to parts of the nervous system, which control involuntary functions, producing the main symptom of gut paralysis.

    However, this was questioned by recent research published in the Equine Veterinary Journal (October 2016)3 that identified key differences between grass sickness and botulism. The study suggests that grass sickness is unlikely to be caused by neurotoxins from this bacterium and concludes that further investigation of an alternative cause is needed.

    The high levels of the condition in Scotland may be partly associated with the particular composition of macro and trace elements in the soil in this region, according to another study published in the Equine Veterinary Journal (October 2016)4.


    It may help to keep horses indoors for at least part of every day, and/or feed supplementary roughage during periods of peak incidence. It is thought it may be more of a problem in areas where the grazing is sparse. It therefore makes sense to provide additional nutrition or bring horses in some of the day when the grazing is poor, especially with young or newly arrived horses, or those on grazing where a case of grass sickness has already occurred. If there is an option to do so, it is best to move horses off grazing paddocks where grass sickness is a known concern.

    There are some suggestions that mixed grazing with sheep or cattle may reduce the risk, which also may help paracite control. In high-risk areas, it may make sense to removed droppings from pasture by hand, rather than using mechanical devices. Overall anything that minimises stress of horses in high-risk areas is likely to beneficial.

    In November 2019 the disappointing news was released that a four-year field trial of a vaccine, which it was hoped would be effective in preventing grass sickness, had proved inconclusive5, so currently there is no vaccine available.  The nationwide research had involved more than 1,000 horses and ponies at 120 premises.


    1: Histological assessment of β-amyloid precursor protein immunolabelled rectal biopsies aids diagnosis of equine grass sickness

    2: Management of Chronic Grass Sickness Patients

    3: Equine grass sickness, but not botulism, causes autonomic and enteric neurodegeneration and increases soluble N-ethylmaleimide-sensitive factor attachment receptor protein expression within neuronal perikarya

    4: Equine grass sickness in Scotland: A case-control study of environmental geochemical risk factors


    Also: Equine grass sickness

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