Equine Grass Sickness

Equine Grass Sickness (EGS) is a disease of horses, ponies and donkeys which causes damage to the nervous system, resulting in paralysis of the gut. EGS cases have been reported across the UK and Europe. Cases can occur throughout the year although the most common period is between April and July with a peak in May.

What are the clinical signs of Equine Grass Sickness?

There are three forms of equine grass sickness – acute, subacute and chronic – with the acute form being most severe. Survival rates are very low with the chronic cases most likely to survive.

• Acute grass sickness – horses display signs of colic as they are affected by complete gut paralysis. Other clinical signs of acute grass sickness include muscle tremors and inability to eat.
• Subacute grass sickness – horses display clinical signs similar to acute grass sickness but with less severity
• Chronic grass sickness – Chronic grass sickness in horses can appear as severe and rapid weight loss and a select portion of these cases may survive

The clinical signs of equine grass sickness can be obvious, but diagnosis and a comprehensive treatment plan for grass sickness requires veterinary attention.

What causes Equine Grass Sickness in horses?

The exact cause of equine grass sickness is still not known but various factors that appear to contribute to a horse’s susceptibility have been identified:

• Access to grazing – even a short period of time at grass can be sufficient especially if other risk factors are in play
• Horse has recently moved field – up to 2 months after moving fields appears to be a high risk time
• Soil disturbance – using mechanical poo-pickers, harrowing etc
• Stress – recent travel, worming and mixing with unfamiliar horses for example
• Overweight – those in good to fat condition seem to be more predisposed
• No conserved forage – feeding some form of conserved forage can have a protective effect
• Weather – greater differences in day and night-time temperatures

Some of these risk factors are inter-connected and it is important to note that correlation does not mean causation. For example, horses and ponies that are overweight are less likely to be fed conserved forage and so it may be the lack of forage rather than their bodyweight that predisposes them to the disease.

Why general digestive health might be crucial

Over time, the minimum amount of forage recommended for maintaining gut health has increased, largely in recognition of the increased prevalence of gastric ulcers. Fibre provided by forages has other important functions, not least that it is utilized by the microbiota or ‘bugs’ that live in the horse’s digestive system. The bugs are an important part of the horse’s immune system. Signs of dysbiosis – a disruption of the population of bugs in the gut – are often very visible. Loose droppings, poor condition and colic are some of the most common problems that we can see but what we tend to overlook is that changes to the microbiota might have been occurring for weeks, months or even years prior to those problems becoming apparent on the outside. At some point, a perfect storm of events occurs, or a tipping point is reached, where the horse changes from looking in good health to having a problem.

The beneficial microbes in the digestive tract help to keep potentially harmful ones at bay by out-competing them for resources – a phenomenon known as competitive exclusion. If we keep the good bugs well fed and in an environment they prefer i.e. not too acidic, they should thrive. If we don’t give them enough to eat however, they will start to look for food elsewhere and that includes the mucus that lines and protects the gut. If the mucus layer is eroded, the gut cells are exposed to things they shouldn’t be which compromises their tightly organized structure – a phenomenon known as leaky-gut syndrome. Whilst harmful bacteria are always a threat to an animal’s health, if the mucus layer is eroded, bacteria that are normally present in the gut performing important functions, may become problematic too. This is an important point as it might not be inherently harmful microbes that are the cause of disease.

Strategies for reducing the risk of Equine Grass Sickness

• Feed plenty of fibre
  For long term health and well-being, a minimum of 1.5% of current body weight on a dry matter basis is recommended to be fed. If reducing risk of EGS is the specific aim then conserved forage should make up a significant proportion of this amount. If your horse or pony gains weight at this level of intake then it is better to use a lower nutritional value forage rather than reducing the amount fed. Straw can be a great option for making a difference to calorie intake – replacing one third of the weight of an average hay with straw reduces a horse’s calorie intake by around 16%. Replacing a better quality forage such as haylage with straw can make an even bigger difference. Recent studies have shown that up to 50% of the forage ration can safely be fed as straw with no increased risk of gastric ulcers. Do check with your vet before introducing straw if your horse or pony has a history of impaction colic or doesn’t have the best teeth!
• Use conserved forage at high risk times
  If you can’t provide access to conserved forage all year round then ensure it is supplied at high risk times such as introducing new horses to the herd, moving fields, greater differences in temperatures etc.
• Ensure the the diet is balanced
  Good doers are often underfed vitamins and minerals as they are kept on restricted rations. These essential nutrients are an important part of the horse’s immune system. Low calorie ways exist to supply a balanced diet in the form of broad spectrum vitamin and mineral supplements or balancers.
• Try to keep stress to a minimum
  Adding a digestive aid to the ration at times of stress may help to minimise the disruption to the microbiome. Click here for more information about digestive aids.

Feeding the horse with Equine Grass Sickness

Should you find yourself in the situation of caring for a horse with EGS, the main aim is to use feeds that are easy to chew, highly digestible and palatable to the horse. It may be necessary to try a variety of different feeds to see which appeals to the horse. Turning the feed into a slurry or gruel that the horse can drink often helps and so feeds that lend themselves to this are ideal:

• Mashes are designed to be fed soaked so tend to contain small particle sizes. A greater proportion of water can be used to make their consistency thinner.
• Pelleted feeds have a small particle size as they have been ground before being turned into a pellet. Water can be added to make them into a gruel. Products containing pure dried grass are naturally sweet which can help to tempt horses to eat.
• A cooked cereal meal can also be a useful way of supplying lots of highly digestible energy in an easy-to-chew-and-swallow form.

If the horse’s appetite starts to increase then the aim is to increase the energy and protein levels in the feed to try and restore the weight and condition that has inevitably been lost as a result of EGS. A phenomenon called refeeding syndrome exists whereby if too much is fed too soon, a rapid rise in blood glucose and associated surge in insulin ensues. The insulin drives glucose and potassium into cells which also utilises other minerals and electrolytes. These are depleted because of the horse’s illness and so the sudden shift away from other functions can cause organs to fail – most commonly kidney, lungs and the heart. The most common time for this to occur is 3 to 7 days after refeeding starts.

Independent research from the US has shown that one way to reduce the risk of refeeding syndrome is to use alfalfa as it is low in starch and so doesn’t cause the increase in glucose and insulin associated with feeding cereals, but is also relatively abundant in minerals such as potassium, magnesium and calcium. It also provides good levels of quality protein to support the horse’s recovery. Although not included in the study, the same can be said of unmolassed sugar beet pulp which is also low in starch and sugar and contains a valuable level of calcium.

Using a digestive supplement is also recommended. A prolonged period of little or no forage intake will have caused a disruption to the population of microorganisms in the digestive tract. Using a digestive aid to try to help establish a healthy population is key.

Given the unique circumstances of EGS we recommend contacting a nutritionist for advice and guidance for each horse as the amounts of each feed will vary considerably.

References

• Witham And Stull (1998) Metabolic responses of chronically starved horses to refeeding with three isoenegetic diets. J Am Vet Med Assoc. 212: 691-696