a.k.a. scours or scouring. An increase in the fluid content of faeces often associated with increased bowel motility, faster transit times and decreased absorbtion of water and nutrients. Colic may be a feature. Diarrhoea can be acute or chronic, the latter term generally used for diarrhoea which persists for a month or more.

Causes are numerous and include some toxins including poisonous plants and fungal mycotoxins. Infectious agents both bacterial such as Salmonella, Clostridia and Aeromonas species and viral – Equine Coronavirus, Rotavirus in foals, and intestinal parasites. Drugs, particularly Non- Steroidal Anti – Inflammatory Drugs (NSAIDS) like Phenylbutazone, and antibiotics have been implicated as a cause as has ingested sand.

Nutritional diarrhoea is perhaps the most common and is associated with passage of too much NSC (starch & sugars mainly), or oil into the large intestine. High starch diets may cause some starch to escape enzyme digestion in the small intestine and pass down the tract into the large bowel (caecum & colon) where it stimulates the growth of certain bacterial populations and disturbs the normal balance of the microflora there. Lowered pH caused by acid forming bacteria may affect the gut lining and reduce the normal absorbtion of water and nutrients. Inflammation may cause secretion of water into the bowel, adding further to fluid content as well as promoting more rapid motility or ‘’Intestinal hurry’’ further limiting the opportunity for water absorbtion. Oil entering the large intestine tends to reduce the diversity of microbes in the gut microbiome with somewhat similar effects if not properly managed. Oil is a useful nutrient energy source but needs to be introduced gradually to avoid digestive upset. A good guide would be to feed no more than 0.1ml per kg bodyweight for the first couple of weeks, increasing gradually to a maximum of 1ml per kg bodyweight.

For severe acute adult diarrhoea, and diarrhoea in foals of more than 24 hours duration, dehydration may ensue and can rapidly become serious requiring veterinary attention. Further, the longer any diarrhoea persists, the more difficult it becomes to treat due to ongoing damage to the gut lining, so it should always be regarded as potentially serious.

Chronic diarrhoea can result from changes to the structure of the bowel wall caused by inflammatory processes, parasitic damage or neoplasia and may be diagnostically difficult. Overall, the actual cause of diarrhoea is only confirmed in less than 50% of cases.

Nutritional management for affected horses generally includes avoidance of oil in the feed, provision of good quality low NSC roughages such as Lucerne hay, soy hulls or non-molassed sugar beet pulp, and possibly the use of prebiotics and / or probiotics. Pasture access may best be avoided for a time as pasture may contain high levels of fructans which could exacerbate the problem. Lush pasture may itself precipitate a transient diarrhoea de to the high water content, especially if the horse has just been turned out onto it, but it is normally self-limiting.

Diarrhoea is a symptom of gut disturbance with many potential causes and can be simple and transient or complex and intractable requiring veterinary attention, preferably at an early stage.

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