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An absolute favourite in the equine advisory service is and remains the topic of Free Faecal Water Syndrome (FFWS). It feels like 9 out of 10 horses we are asked about are more or less severely affected by FFWS. Many of the owners were between resigned and desperate because they cannot wash their horse as often as it becomes dirty again.

But why are so many horses nowadays suffering from this digestive disorder? Is it the feed? The way the horse is kept? The stress?

If you try to search the scientific literature on the subject of FFWS, you will hardly find anything. Although widely recognized in German-speaking countries, there have been few studies conducted to date. As soon as you look beyond national borders, FFWS seems nonexistent—most countries don’t even have a term for it. The research situation is accordingly thin, which is surprising considering the apparent prevalence of the condition in many horses.

Possible causes of FFWS include:

  • Stress
  • Feeding
  • Intestinal mucosal inflammation
  • Disorders of the intestinal flora
  • Stomach ulcers / gastritis
  • Dental anomalies, parasite infestation, sand deposits in the large intestine or “metabolic problems” are also repeatedly mentioned.

FFWS is certainly not a mere ‘blemish’ but rather the visible symptom of a substantial imbalance in the large intestine— even if it ‘only’ occurs in winter.

These intestinal disorders often lead to secondary diseases, ranging from the detoxification disorder kryptopyrroluria (KPU) to immune system disorders and psychological problems in affected horses.

Therefore, FFWS should not be taken lightly; instead, the causes should be identified as early as possible, remedied, and the intestines supported therapeutically to restore their natural balance.

FFWS and stress

Regarding stress, there is, in fact, a study that has shown that low-ranking horses in group housing are particularly prone to FFWS. Stress results in less optimal blood supply to the intestinal mucosa. Constant stress is particularly detrimental here, as the intestinal mucosa never gets the opportunity to calm down and regenerate. Small, everyday damage to the intestinal mucosa cannot be repaired in time, resulting in inflammation and, consequently, apparent FFWS. Stress can manifest in various ways in horses. For example, a low-ranking horse can experience significant stress, particularly when kept in cramped conditions in a small winter turnout. However, high-ranking horses overwhelmed by their position can also experience stress, just as horses in groups lacking a sovereign herd leader can.

In group housing, it is often observed that piebald and grey horses not only experience particularly frequent FFWS but are also often excluded. This is likely linked to an evolutionary survival program: Light-colored horses attract the attention of predators to the group. If the herd excludes such a horse, this behavior ensures the survival of the group but unfortunately leads to stress for the grey or piebald horse. This behavior is not observed in groups with only white or piebald horses, and such groups are usually much calmer and more social with each other.

Eye of a stressed horse
Constant stress in particular is fatal. © Adobe Stock / mani

Being kept alone (in a box) without social contact can also cause stress, as can having an unliked box or paddock neighbor. This is compounded by a lack of exercise. Free exercise in the pasture or a large turnout is a great way for horses to relieve stress by running, bucking, and playing. However, in many stables, this is not possible or hardly feasible, especially in winter. Controlled exercise on the lunge line or under the rider is no substitute for free ‘bucking out’ to reduce stress, nor is long, slow movement in the pasture.

Stress in horses caused by a lack of eating or sleeping opportunities is often underestimated. Studies show that reducing roughage to ‘meals’ with longer breaks in between is just as stressful as offering too few feeding spots Offering more roughage, possibly with a close-meshed hay net, can therefore prevent stress in such cases. In addition, horses require bedded lying areas to ensure they can sleep sufficiently. The trend, especially in open yards, but also increasingly in box stables, to dispense with bedding and instead only offer rubber mats or ‘horse mattresses’ as insulation against the cold floor must be viewed critically. As a result, the horses do not lie down sufficiently to sleep, missing the important REM deep sleep phase. Narcolepsy, i.e., falling asleep while dozing, is the unpleasant consequence and, of course, causes a lot of stress. Proper bedding and sufficient space to keep a distance from neighbors are essential to avoid sleep deprivation stress.

Finding out why a horse is stressed is, therefore, not that easy. It is usually even more difficult to switch off this stress trigger. If it’s just the annoying neighbor in the box, you might be able to swap boxes with someone else. However, if it is the lack of roughage, then stress is usually already pre-programmed for the owner to negotiate with the stable manager and the other horse owners. In some cases, you will have no choice but to change stables—and lo and behold, for many horses, the FFWS disappears as soon as the stress trigger is no longer present.

Feeding, intestinal mucosal inflammation and intestinal flora

These three points cannot be separated from each other, as they usually occur together. In most cases, what they all share is that the affected horses suffer from low pH values in the large intestine. The hyperacidity of the large intestine (‘hindgut acidosis’) is increasingly being scrutinized and questioned by scientists. It is commonly linked to improper feeding practices, a disrupted intestinal flora, and/or inflammation of the intestinal mucosa.

In humans, the connections between diet, intestinal flora, intestinal mucosal irritation, and irritable bowel diseases (IBD, IBS, FGID) are now relatively well-researched. Studies on humans demonstrate a complex interaction between the intestinal flora and the immune system. The microorganisms naturally residing in the intestine work in harmony with the mucus produced by the intestinal mucosa, as well as the IgA, antimicrobial peptides, and lysozymes it contains. However, they also engage in symbiosis with the epithelial cells of the intestine, i.e., the mucosal surface, and the components of the immune system (dendritic cells, lymphocytes) integrated into the intestinal mucosa. In the human colon, if there is ‘dysbiosis,’ a disturbance of the natural intestinal flora, then the interaction between the intestinal symbionts and the ‘CD1d natural killer cells’ (NKT) of the immune system is disrupted. In humans, disorders of the intestinal flora consequently result in NKT-mediated inflammation of the surface of the intestinal mucosa. It is assumed that similar processes also take place in the horse’s intestine.

Specifically, ensiled feeds such as haylage, corn silage, etc., along with structured chopped feeds, substantial amounts of concentrates, or abundant juice feeds (such as carrots, apples, or bananas), are often the cause of a disrupted intestinal flora. This disruption leads to acidic pH values in the large intestine and corresponding secondary disorders such as FFWS

However, the effect on the intestinal flora or intestinal homeostasis varies depending on the feed.

Ensiled feed

Ensiled forage such as haylage is based on lactic acid fermentation. Each portion of haylage introduces lactic acid bacteria and lactic acid into the digestive tract on a large scale. However, lactic acid bacteria are not a natural part of the horse’s large intestine flora and should only be present in very small quantities, if at all. In wild horses, they are generally not detectable. In small quantities, the horse can compensate for the lactic acid produced in the large intestine because bacteria colonize in this case, immediately continuing to utilize the lactic acid. However, if large quantities of lactic acid bacteria are introduced into the intestine through haylage or other ensiled feed, they can colonize to a correspondingly high degree and significantly lower the pH value, especially in the large intestine. This gradually displaces the natural intestinal flora, which requires a neutral pH value, leading to further acidification and the colonization of other pathogenic and acid-loving germs. Generous quantities of concentrated feed, high-fructan pasture grass, and similar feeds provide the ideal basis for the lactic acid bacteria now present to continue multiplying and further disrupt intestinal homeostasis. Moreover, alongside the disrupted interaction between the gut flora and the immune system, the resultant acids also corrode the intestinal mucosa, triggering inflammation. The owner then notices FFWS as a symptom.


Pectins, the ‘building materials’ found in fruits, vegetables, and young pasture grass, can contribute to increased acidity in the large intestine and disrupt the natural balance of intestinal flora. However, in this case, the pathway involves protozoa more than lactic acid bacteria. These are always present in the horse’s intestines but in small quantities. This way, they do not interfere. However, when provided with an ample supply of pectin-rich feed, their multiplication occurs, leading to acidification of the large intestine (hindgut acidosis). This, in turn, paves the way for the colonization of other pathogenic and acid-loving microorganisms, resulting in inflammation of the intestinal mucosa and ultimately contributing to FFWS. When feeding fruits and vegetables like carrots or apples, it’s crucial to remember: The poison is in the dose. While a small amount can be a tasty treat, in large quantities, they pose problems for the digestive system. In stark contrast to humans. Pectins act as probiotics in humans, stabilising the flora in the large intestine. In horses, they have the opposite effect, severely disrupting the microbial balance in the large intestine. From this perspective, the feeding of apple pomace or (demelassed) sugar beet pulp to horses should be regarded with caution, as these feeds consist almost exclusively of pectin.

Structured chaff

Structural chaff typically comprises hay or lucerne, ‘chopped’ into pieces ranging from 1 to 5 cm in length. This chaff is then provided as a standalone feed or mixed into various horse mueslis. They are identifiable by their short, green stems. Horses cannot chew these short, dry chaffs adequately. These are swallowed as oversized roughage particles, subsequently disrupting peristalsis in the large intestine. Research has demonstrated that providing structured chaff results in the feed remaining in the large intestine for up to a week. The large intestine retains these long fibers, anticipating further nutrient extraction. However, the lack of mechanical comminution hinders the intestinal symbionts from adequately breaking them down. Fibre lengths ranging from 2 to 5 mm, also naturally produced during the chewing process, would be optimal for the large intestine. The retention of food chyme caused by structural chaff leads to improper fermentation, resulting in disruptions to the microbiome, and subsequently, as previously described, presumably immune-mediated inflammatory processes.

In this context, it is worth noting that chronic stress also induces inflammation of the intestinal mucosa and increases the immune system’s readiness to ‘overreact,’ potentially resulting in feed allergies, autoimmune disorders, and disruptions of intestinal homeostasis.

When stress and feeding errors coincide, colon disorders are often pre-programmed.

FFWS and stomach ulcers/gastritis

The topic of gastric ulcers or gastritis in horses is still relatively new. The fact that there is a connection here with FFWS is more due to coincidence. The ‘deacidification cure’ of the body, aided by bicarbonate (commonly known as ‘alkaline powder’), is widely embraced among alternative practitioners. The theory behind this is that bicarbonates are able to bind acids. If you feed them to the horse, they do so in the stomach. They bind stomach acid and are irreversibly broken down in the process, essentially decomposing into CO2 and water. As a result, they do not reach the large intestine or the connective tissue, where their supposed deacidifying effect is intended. Bicarbonates simply neutralise the stomach. Interestingly, it has been observed time and again that horses experienced significantly fewer or no further instances of FFWS after the administration of bicarbonate.

In such horses, the cause can be traced to disorders of gastric homeostasis, meaning they suffer from gastritis or ulcers in the gastric mucosa. They are sensitive to the harsh effects of stomach acid. The resulting pain induces stress in the body. This pain-induced stress, in turn, may lead to FFWS (refer to ‘FFWS and Stress’). Neutralizing the stomach acid with bicarbonate eliminates the triggering pain stimulus. As the stress diminishes, so does the occurrence of FFWS. When the administration of bicarbonate is stopped, the inflamed areas of the stomach mucosa come into contact with acids again, reliably triggering the reappearance of FFWS

The use of bicarbonate can thus be seen as a form of ‘therapeutic diagnosis’. If FFWS disappears following the administration of bicarbonate, it suggests that the horse may have inflammation or ulcers in the gastric mucosa. Bicarbonate is not suitable as a therapy for the stomach, as the prolonged neutralization of the gastric environment leads to other digestive issues, such as inadequate protein hydrolysis and/or the introduction of undesirable microorganisms, resulting in colon problems.

Tail stained by FFWS
Inflammation of the stomach lining (gastritis) can be a cause of FFWS. © Adobe Stock / Grubärin

A horse with gastritis or ulcers should be treated specifically for gastric ulcers. This involves not only regenerating the protective mucus layer in the stomach and mitigating inflammation but also, primarily, identifying and addressing the root causes of the stomach problem. The causes are typically traced back to errors in housing or feeding management and range from insufficient roughage breaks (>4h) to pure stress.

Other possible causes of FFWS:

Dental anomalies

Many dental practitioners consistently note that FFWS issues improve or vanish after addressing dental concerns. This is likely because horses with malocclusions often struggle to grind their roughage adequately. When inadequately ground roughage enters the large intestine, it results in peristalsis disorders. This places ‘dental problems’ as a potential cause of FFWS in the same category as the feeding of structured chaff. The only distinction is that, in this scenario, the horse generates the coarse chaff itself during chewing due to inadequate chewing performance. Regular dental examinations and corrections by a suitably trained equine dental practitioner should, therefore, be routine.

Parasite infestation

Elevated levels of endoparasite infestation, commonly known as ‘worms,’ are recognized to induce symptoms such as diarrhoea, flatulence, and colic. Particular attention should be paid to strongylids in horses with FFWS. However, this raises the question of chicken and egg: horses with FFWS often exhibit worm infections Whether the worms are the primary cause of the intestinal problems or, conversely, if the disturbed immune system resulting from intestinal problems is the cause of the worm infestation, has not yet been conclusively clarified. However, horses with FFWS should undergo regular testing of faecal samples for worm infestation, and appropriate measures should be taken if the results are positive.

Sand deposits

Sand deposits in the large intestine are more common than one might assume. On the other hand, as a steppe animal, the horse has evolved mechanisms to cope with a certain amount of sand in the gut. The primary issue is an excessive intake of sand, a phenomenon observed more frequently in our housing conditions. Nevertheless, studies have yet to establish a direct link between sand in the gut and FFWS. However, practical experience shows that many horses that ingest too much sand do not get enough roughage and/or have stomach problems. Both, in turn, act as stressors, a known trigger for FFWS. So, it is likely not the sand in the colon. Instead, it is housing and feeding errors that contribute to excessive sand intake, among other factors.

Metabolic problems

In certain instances, there is mention of ‘metabolic problems’ associated with FFWS. Yet, these can be quite diverse, with varied causes and consequences. Conversely, in almost every case, FFWS leads the affected horse to develop metabolic problems.

When intestinal homeostasis is disrupted—definitely the case with FFWS—the horse experiences a long-term deficiency of essential nutrients typically provided by the intestinal symbionts.

One of the most prevalent outcomes is the detoxification disorder known as kryptopyrroluria (KPU), which subsequently triggers additional health problems and symptoms of illness.


The multitude of causes and variations in progression make FFWS challenging to comprehend as a disease. Each case must therefore be considered individually. What works well as therapy for one horse may be precisely the wrong approach for the other. A comprehensive anamnesis of the prior history, feeding, and housing conditions is essential, alongside tailored therapeutic measures aimed at assisting the horse in regaining its natural digestive balance over the long term.