• Dr Beth Wells

Equine Grass Sickness (Equine dysautonomia)

Dr Beth Wells, Knowledge Exchange Specialist and Principal Research Scientist and, Dr Kathy Geyer EGS Research Fellow both from the Moredun Research Institute and, Kate Thomson, Administrator of The Moredun Foundation Equine Grass Sickness Fund, describe the condition and what we can do to help with on-going research


Equine grass sickness (EGS) affects grazing horses, ponies and donkeys and presents as a frequently fatal neurodegenerative disease involving widespread damage to neurons (nerve cells) in the nervous system of the gut (as well as other anatomical sites), severely compromising the propulsive capacity of the gut. This affects the function of the gut involved in the transit of food. Despite much research, the precise cause remains elusive and is the subject of active research along with seeking new prevention and treatment strategies.


The disease was first described in 1909 at Barry Buddon Army camp, near Carnoustie on the east coast of Scotland, a remounting facility to supply horses to the British Army. The disease has a strong association with grazing and peak incidences (in the northern hemisphere) tend to occur in the spring and early summer, with around 60% of cases being recorded at this time. Data from recorded EGS cases sent to the EGSF over the last 10 years confirm this trend for higher disease incidence in April, May and June (see Figure 1).

Figure 1


As a result of the high losses of horses due to EGS and the importance of horses as working animals on farms, there was a lot of research effort in the early years, 1920-1950, to try to find out the cause of the disease. The UK has the highest incidence of EGS compared with other countries across the world. It occurs mainly in Northern Europe, but similar diseases, such as mal seco, in South America and tamboro in Colombia, have been recorded.

Within the UK, EGS has a higher occurrence nearer the east coast compared with the west, particularly in Scotland, as shown in Figure 2.

Figure 2

These figures should be interpreted with care as the higher number of cases in some counties may be influenced by a relatively greater horse population in that particular county. Figure 2 illustrates differences in disease occurrence relating to geography in the UK, with the highest number of cases recorded in Aberdeenshire. Therefore, it is possible that climate, soils, pasture, indigenous plants and fungi related to specific geographical regions may help provide some clues about the causative agent and associated risk factors. Climate related risk factors include cooler, drier weather and irregular ground frosts, which prevail more frequently in the eastern side of the country. It should be noted that these data relate to owner reported cases over the past 7 years, illustrating the importance of reporting diagnosed cases to the EGSF. In reality we suspect the actual number of EGS cases to be much higher than is reflected here.


Equine grass sickness has been reported in many different horse and pony breeds, with a recent study in Scotland suggesting an increased susceptibility amongst native Scottish breeds, but there are many confounding factors which may influence this apparent association, including a higher number of native and native breed crosses within the UK. Many studies confirm that a key predisposing factor is age, with disease being more frequently seen in young adult animals (Figure 3).

Figure 3

Key points

  • Equine grass sickness (EGS) affects grazing equids in many countries, but is most prevalent in the UK during the spring and summer months

  • It is a complex, frequently fatal disease involving damage to the nerves of the gut (as well as other anatomical sites), compromising the animal’s ability to swallow and the transit of food through the gut

  • The causes are still unknown but likely to be multi-factorial and there is evidence supporting the potential involvement of neurotoxins from Clostridium botulinum and fungi

  • Three overlapping manifestations of the disease are recognised – acute, sub-acute and chronic; these are based on clinical severity

  • Euthanasia is warranted for acute, sub-acute and severe chronic cases, based on welfare grounds; around 80% of diagnosed cases reported to the Equine Grass Sickness Fund (EGSF) result in death

  • Around half of chronic cases, which are assessed to be suitable for treatment, survive with intensive and skilled nursing over a long time period and around 80% of those survivors return to work

  • The clinical signs vary in severity depending on the disease category and are largely reflected by the severity of nerve cell loss in the gut; greatest in acute and sub-acute cases and less in chronic cases

  • Diagnosis can be challenging and is often reliant on the presence of a pattern of clinical signs. The gold standard diagnostic approach involves microscopic study of tissues taken from the ileum by biopsy (pre- or post-mortem) and specific nerve ganglia (post-mortem) to look for changes associated with the disease

  • There are certain management practices that can be implemented that can help reduce the risk of EGS

  • There are currently no preventive vaccines or treatments that can cure the disease

  • New technologies, used in other areas of disease research, along with an inter-disciplinary approach, will be used to take a fresh look at this disease and progress research

Unfortunately, despite extensive research efforts in the field of EGS over the last century, the cause of this disease is still unknown and euthanasia on welfare grounds is still the most common outcome of EGS cases. In order to finally get to the bottom of this complex multifactorial disease, about a year ago, the Moredun Foundation (TMF) and the Equine Grass Sickness Fund (EGSF) joined forces and launched a three-year EGS Fellowship to encourage novel and inter-disciplinary research approaches in the field of EGS.


Since the lack of a coherent sample collection of EGS cases has most likely hampered progress in EGS research so far, one of the main aims of the fellowship project was the establishment of a national EGS Biobank and Database. The EGS Biobank, which is heavily supported by the BHS, was launched in April last year and has been a great success so far. With the enormous support from horse owners and vet ambassadors, these critical samples from affected equines which would otherwise be lost, are now being donated to the biobank and therefore available to any scientist working on EGS.

In the second year of the EGS Biobank, we intend to further increase awareness of the disease itself as well as the Biobank project in order to extend the current sample collection which will allow directed and robust research into the disease.


Due to the acute nature of the disease, the time window for sample collection is quite small and we therefore appreciate if owners of horses and ponies are aware of our biobank project and engage with their vet or veterinary practice if they consider a potential sample donation in the event of their horse or pony developing EGS. This will also help to raise awareness amongst other vets and horse owners who may be clients at the practice.


Sample kits are free and can be requested by veterinary practices via email: kathy.geyer@moredun.ac.uk.


In addition to sample collection, we also encourage the reporting of all EGS cases, including chronic survivors. Collecting this data will help us to estimate the prevalence and location of the disease, and cross reference against weather and soil data. It also helps identify potential risk factors and close in on the causal agent.

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