Grass sickness: how new research is shedding light on the disease

  • A new approach to equine grass sickness research aims to shed light on the causes and risk factors of this disease. Andrea Oakes reports

    CASES of equine grass sickness (EGS) typically peak during early summer months. While the causes of this often fatal disease are still unclear, more than 100 years after its discovery, a new biobank project aims to provide researchers with the information they need to make a breakthrough.

    Dr Kathy Geyer, EGS research fellow at Moredun Research Institute, explains that the disease mainly affects horses that have access to grass – although there have been isolated cases in stabled horses.

    “EGS has an extremely high mortality rate, with 80% of cases resulting in euthanasia on welfare grounds,” she says. “The clinical symptoms are non-specific [not due to any single known cause] and, without a reliable diagnostic test, diagnosis is challenging. As a result, many cases are mis- or undiagnosed, most likely leading to an underestimation of actual disease prevalence.

    “What is evident is that EGS is a highly complex and enigmatic disease and therefore very challenging to work with,” adds Kathy. “Decades of research have led to various different hypotheses on the cause, but with a single causative agent yet to be confirmed, EGS is now thought to be of multifactorial origin.”

    Locations of the 27 incidences of EGS that so far have been reported to the biobank project team in 2021


    SO what is the biobank – and how might it help?

    “Progress in EGS research has been hampered by the lack of coherent sample collections,” says Kathy, saying that the Moredun Foundation and the Equine Grass Sickness Fund, with support from the British Horse Society, have launched the initiative.

    “By collaborating with vets and owners, the biobank project aims to collect samples from EGS cases and co-grazer control horses that would otherwise be lost, so that horses have not died in vain. These samples will be available to any researcher working on EGS and will encourage an interdisciplinary approach in the search for the potential causes.”

    These samples may be biological, such as blood, saliva or urine, or environment related, adds Kathy, and will be collected at no extra cost to the owner. Tissue samples are especially valuable. Since the course of EGS can be swift and frequently devastating, owners are encouraged to familiarise themselves with the project ahead of time – should the worst happen.

    “There is just a small window of opportunity to collect the samples when grass sickness is suspected, so it would be helpful to think about and prepare for this in advance so these valuable samples are not lost,” she says.

    “In addition to collecting samples, we are working with owners to encourage the completion of our case report questionnaire, which will give us valuable information about potential risk factors and general understanding of the disease.”

    Many chronic EGS cases
    have a tucked-up appearance and this recognisable
    stance. Sadly, few survive


    HORSES can be affected by EGS at any time, but the spike in cases in spring and early summer is often followed by a second peak in autumn. Kathy explains that horses in eastern parts of the UK are at higher risk, with the north-east of Scotland having the highest incidence.

    “There may be a weather-related aspect,” she says. “Vets and owners experienced with EGS are on high alert after a dry, but cool [around 10°C] spell lasting around 10 days, or periods with sunny days and intermittent ground frosts. Furthermore, younger horses appear to be at a higher risk, with a peak between two and seven years of age.

    “The disease involves damage to the nerves in the digestive tract and other sites throughout the body, severely affecting the horse’s ability to swallow and impairing movement of gut contents,” she adds. “This can lead to complete paralysis of the digestive tract from the oesophagus downwards.

    “Based on the underlying nerve damage and disease progression, there are three main forms of the disease: acute, subacute and chronic. Although euthanasia is generally the only option for acute or subacute EGS, under certain circumstances, treatment may be considered for chronic cases.

    “With intensive and skilled care, around 50% of them survive,” Kathy says. “However, nursing a chronic case is physically, emotionally and financially demanding.”

    For details about the biobank project and relevant downloads, visit: grasssickness.org.uk/biobank/biobank-for-horse-owners

    This report can also be read in Horse & Hound magazine, on sale date 24 June 2021

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