Faecal Water Syndrome in Horses: Causes, Symptoms and Solutions

You know the picture. A horse passes a perfectly formed ball of manure, and then a jet of brown liquid follows it down the back of both hind legs. The tail is stained. The skin above the hocks gets scalded and sore. You wash it off, and by the next morning it's back. Faecal water syndrome in horses rarely makes a horse acutely ill, yet it drives owners to distraction, because it looks awful and nothing seems to shift it.

Research on this condition has moved fast in the last five years. Some of what owners were told a decade ago has since been overturned.

What faecal water syndrome in horses is (and isn't)

The technical name is free faecal liquid, and the definition is mechanical rather than diagnostic: faeces are voided in two separate phases, one solid and one liquid (Lindroth et al., 2020). The liquid can come before, during or after the solid part.

This is not diarrhoea. The manure itself is usually well formed. The water simply travels separately, and that distinction changes where you look. In true diarrhoea the colon has failed to absorb water at all. Here the colon is absorbing water reasonably well, but fibre and fluid are separating somewhere along the way instead of leaving as one mass.

A survey of 339 affected horses in Sweden and Norway found no type that gets spared. Every breed, age, colour, discipline and housing system was represented (Lindroth et al., 2020). One number in that survey deserves attention: 23% of affected horses had a history of colic, which is high compared with published data from general horse populations. Whatever is going on, it isn't purely cosmetic.

The dysbiosis story got more complicated

For years the standard explanation was gut dysbiosis. The hindgut bacteria are out of balance, fermentation goes wrong, water separates out. It's a tidy story. It also failed its first three tests.

Lindroth and colleagues (2021) compared faecal bacteria in 50 case and control pairs kept on the same farms, and found the overall composition much the same. Laustsen et al. (2021) reported the same null result in Danish and Dutch horses. A Canadian group repeated the work in 2024 with 14 affected horses and 11 healthy stable mates, and again found no significant difference (Wester et al., 2024). Three studies, three times nothing.

Then the picture shifted. Porter and colleagues (2025), working with horses in the midwestern United States, did detect a significant difference in beta diversity between affected and control horses, along with several bacterial groups enriched in each. One of them was Alloprevotella, the same genus the Swedish study had flagged back in 2021 even while reporting no overall difference.

So where does that leave us? Not at "dysbiosis causes faecal water syndrome", and not at "the microbiome is irrelevant" either. The current position is narrower and more useful. There is a subtle, repeatable microbial signature in affected horses, and nobody yet knows whether it is a cause or a consequence. One finding tips the scales slightly. When Laustsen's team gave affected horses a faecal microbiota transplant from a healthy donor, symptom severity fell, even though those horses had shown no measurable dysbiosis to begin with. Responses varied and some improved only temporarily. Changing the microbial community still changed the signs.

It's probably not the haylage

Ask around any yard and someone will tell you to take the horse off haylage. It's the most common advice given for this condition, and the best-designed study on the question doesn't support it.

Lindroth et al. (2021) compared feeding and management on 50 private farms in Sweden and Norway, matching each affected horse to a control on the same yard. Wrapped forage intake was near identical between groups (p = 0.97). What differed was the rest of the ration. Affected horses were fed roughly twice as much concentrate per 100 kg of bodyweight per day. They took in more starch and more water-soluble carbohydrates, and less digestible crude protein and less neutral detergent fibre. They also received less straw and less lucerne.

That reframes the whole management conversation. The variable tracking with the condition was not the forage type. It was how much hard feed was displacing fibre.

To be fair to the haylage theory, owner-reported data does point at forage. In the 2020 survey, 58% of owners said signs eased after switching from wrapped forage to hay. Owner reports aren't blinded though, and a batch change alters far more than the wrapping. Hay quality itself has been documented as a factor in individual clinical cases (Cavallini et al., 2022).

What the manure itself reveals

The most concrete mechanical clue comes from analysing the faeces directly. Lindroth and colleagues (2022) measured chemistry and physical properties in case and control horses across two countries. Most variables came back the same: dry matter, pH, minerals, total short-chain fatty acids. Two did not. Affected horses had a lower proportion of lactic acid, and their faeces had a lower water-holding capacity.

That second one is worth holding onto. The fibre matrix leaving an affected horse is less able to bind water. Whatever upstream cause you eventually land on, this is the endpoint. Fibre that can't hold its own fluid, so the fluid leaves on its own.

It also explains why fibre quality and quantity keep resurfacing as the practical lever.

How to manage faecal water syndrome in horses

In practice, the results come from unglamorous changes applied long enough to judge them.

Start with the ration. Cutting concentrate and rebuilding the diet around long fibre is the highest-value move available, and adding straw or lucerne addresses exactly the deficit the case-control data identified. There's more on getting that balance right in our piece on what the latest research says about feeding. Change forage batches one at a time, and give each change three to four weeks. Week-to-week variation in this condition is large, and a fortnight tells you nothing.

Rule out the other causes of loose manure first. Dental disease, sand accumulation, parasites and any colic history all deserve a look. If you're unsure what normal should look like, our guide to reading horse manure walks through it.

Then support the hindgut environment. Live yeast carries the strongest recent equine evidence here. Carter et al. (2025) supplemented 30 geldings on a high-starch diet with Saccharomyces cerevisiae for 32 days and measured a higher faecal pH, a more diverse microbiome, and increases in fibre-digesting bacteria including Ruminococcus and Fibrobacter. An in vitro study using equine faecal inoculum pointed the same way: yeast buffered the pH drop during a sugar challenge and reduced D-lactate production (Sheridan et al., 2026). Steadier hindgut pH and better fibre fermentation are precisely what a horse with poor faecal water-holding needs.

The gut lining itself deserves attention too. Glutamine is the primary fuel for the cells lining the intestine, and equine work shows the horse's own gut ramps up glutamine uptake when it comes under nutritional stress (Aldridge-Dean et al., 2025). Feeding it supports the barrier those bacteria live against.

Nobody has solved faecal water syndrome. Some horses respond to a ration change within a month, others stay stained for years despite everything anyone tries. The current evidence does give you two things worth acting on: get the concentrate down and the long fibre up, and support hindgut fermentation while you wait. That beats swapping haylage and hoping.