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Equine grass sickness (EGS) is a rare and often fatal disease of the grazing equids, with the majority of clinical signs associated with failure of normal gastrointestinal function. EGS has the highest incidence in Great Britain but is also frequently reported in other northern European countries. A very similar condition called mal seco has been recognised in South America. The precise aetiology of the disease is unknown, but is thought to be associated with potent neurotoxins, produced in the gastrointestinal system by Clostridium botulinum type C, a bacteria commonly found in the soil, under certain environmental conditions. EGS occurs almost exclusively in horses that have an access to grass and it is acknowledged that certain premises are associated with a higher occurence of EGS. Most often only one animal is affected, however, outbreaks in a larger number of horses in a limited period of time are not uncommon. The disease can affect any age of the horse but is most commonly diagnosed in younger animals between two and seven years of age. Older horses may develop resistance to the causative agent and therefore not get affected. It was found that periods of dry, sunny and frosty weather result in an increased number of cases and occurence of the disease is most prevalent in the spring months between April and June, with a peak in May. The risk of developing EGS is higher if the affected horses have recently had a change of feeding regimen, been moved to a new pasture or treated with anthelmintics. EGS occurs in three different forms; acute, subacute and chronic. Clinical signs are a result of extensive neuronal degeneration of autonomic and enteric nervous system, while also affecting brain and spinal cord. Symptoms in the acute and subacute cases are severe and occur rapidly. Gastrointestinal paralysis results in signs of acute colic, accumulation of large amounts of gastric reflux, gasseous distention of the large intestine and lack of fecal output. Muscle tremors, ptosis and patchy sweating are often observed. The prognosis is usually bleak and most of the time the horses diagnosed with EGS are subjected to euthanasia. Chronic form is less severe and develops more slowly. The symptoms are those of a mild intermittent colic, reduced appetite, difficulty in swallowing and rapid weight loss. Rhinitis sicca can also be observed. Some of these cases may survive under certain conditions and with an intensive supportive care. It is often difficult to distinguish EGS from other causes of colic. A definite diagnosis is confirmed with histopathologic examination of myenteric or submucosal plexus of the ileum. Loss of neurons can be observed in all parts of the gastrointestinal system. Commonly used ancillary diagnostic procedure is a ptosis test, which involves topical application of phenylephrine eye drops to the cornea, causing reversal of the drooping eyelids. This report describes three acute cases and one subacute case of EGS which were presented to Højgård Hestehospital, Denmark in May 2020. Time between onset of clinical signs between the first and the last case was 15 days, following a 14-day period of dry, cold and sunny weather conditions. Age of admitted horses ranged from two to seven years and all of them were treated with ivermectin two days before the onset of clinical signs. The first three cases had been living together on the same grass field and were presented to the hospital with nearly identical acute colic signs. They were tachycardic, had decreased peristaltic, developed a lack of fecal output and accumulated large amounts of gastric reflux. An exploratory laparotomy was performed on the first admitted case where moderate distention of small intestine and signs of enteritis were found, with proximal enteritis being considered the most likely diagnosis at the time. Postoperative treatment consisted of antimicrobial and antiinflammatory therapy, administration of intravenous fluids, prokinetics and frequent gastric decompression. There was no significant improvement after six days of supportive care and the horse was subsequently euthanised. Post mortem samples of small intestine were collected and pathohistological findings revealed autonomic neuronal depletion and neuronal degeneration of ileum, which confirmed diagnosis of an acute case of EGS. Second and third cases were kept on medical treatment and were euthanised after four and three days, respectively, after their condition had progressively worsened. The fourth case was turned out daily on the paddock next to the grass field where the first three horses were being kept and was referred to the hospital after being unsuccesfully treated for an impaction by the referring vet. Dullness, droopy eyelids and muscle tremors were noted upon presentation, along with other clinical signs, consistent with an impaction of large intestine. No improvement was observed after medical treatment and the decision for surgical intervention was taken later in the day, being aware of the fact that the horse had a high probability of being affected by EGS as well. Cecum and large colon impaction was found and pelvic flexure enterotomy was performed to empty the contents of the intestine. The horse continued on supportive therapy after. Patchy sweating over shoulder and gluteal regions was noted on the second day after the surgery. On the third day the horse was euthanised due to lack of clinical improvement and poor prognosis. Based on the clinical appearance and findings, the first three presented cases were examples of an acute form of EGS, while the last case was a subacute form. The last official report of EGS in Denmark goes back to 1948, however, many anecdotal reports of the disease appearing in certain regions of the country exist. The main purpose of this case report is to draw more attention to EGS as it appears to occur more often than being noted, in order to set a correct diagnosis, avoid unnecessary treatment and therefore not prolong animal suffering. Suspicion of EGS should be considered whenever a horse is presented with clinical signs of acute colic, nasogastric reflux, intestinal hypomotility and decreased fecal output, especially in the spring months, with high suspicion when muscle fasciculations, patchy sweating and bilateral ptosis are observed four to five days after the onset of the disease. [ABSTRACT FROM AUTHOR]