11/29/2025
What do we know about F***l Water Syndrome !
๐๐ก๐๐ญ ๐๐จ ๐ฐ๐ ๐ค๐ง๐จ๐ฐ ๐๐๐จ๐ฎ๐ญ ๐๐ซ๐๐ ๐๐๐๐๐ฅ ๐ฐ๐๐ญ๐๐ซ ๐ฌ๐ฒ๐ง๐๐ซ๐จ๐ฆ๐ ๐ข๐ง ๐ก๐จ๐ซ๐ฌ๐๐ฌ?
There is one condition I get the most calls about every winter and that is free f***l water syndrome (FWS/FFWS). This condition is observed in horses who defecate a relatively normal pile of manure followed by liquid manure or f***l water (not diarrhea).
Overall, this condition has not been found to have further negative health implications for the horse as it has not been tied to dehydration, weight loss, changes in appetite, or further gastrointestinal discomfort. However, it can be a difficult and inconvenient condition to manage for horse owners as it often results in manure staining the legs and tail which can irritate skin as well as attract flies during warm weather or lead to frozen tails during cold weather.
But this leads us to the biggest challenge we face with FWS โ we currently have no idea what is causing it and therefore we do not have a standardized or consistently effective treatment. From conversations I have had with veterinarians and fellow Ph.D. nutritionists, there seems to be a long list of possibilities that we try with hopes that one will โstickโ. These can range from changes in feed or forage as well as a variety of supplements. However, while a specific option may work for one horse, there is not a single solution that works for every case. Individual horses sometimes improve, but there is no single solution that works across the board, and even helpful methods tend to reduce symptoms rather than eliminate them. This uncertainty is what pushed me to dig deeper into the research on this topic, and I wanted to take you along on that journey.
๐๐จ๐ซ๐ฌ๐๐ฌ ๐๐ญ ๐๐ข๐ฌ๐ค
One of the first questions I wanted to answer was: which horses are most commonly impacted by this condition? In a study of German horse owners (Kienzle et al., 2016), horses with FWS were paired with a healthy stablemate for comparison. The study found that Paint Horses, geldings, and horses lower in the herd hierarchy appeared more often in the FWS group while f***l egg counts did not play a role.
However, this study included only 42 horses with FWS and 37 controls, so the authors ultimately concluded that while social stress may be a contributing factor, larger studies are needed.
๐๐ข๐๐ซ๐จ๐๐ข๐จ๐ฆ๐
Because of the nature of FWS and the central role the hindgut microbiome plays in digestive health, researchers have explored whether microbial differences might help explain the condition.
A European study evaluating around 15 case-control pairs found no differences in the f***l bacterial microbiota community of FWS versus control horses (Schoster et al., 2020). These findings are supported by a Canadian study evaluating 14 FWS horses compared to controls (Wester et al., 2024) as well as a Danish study evaluating 10 horses undergoing f***l microbiota transplantation (Lausten et al., 2021).
However, a NorwegianโSwedish study evaluating 50 pairs found enriched populations of Alloprevotella in FWS horses and enriched Bacillus spp. in controls (Lindroth et al., 2021). A U.S. study also reported increased Alloprevotella in FWS horses (Porter et al., 2025). This study suggested these alterations in bacteria populations may serve as a biomarker, rather than a cause, of the condition.
Across these studies, findings remain inconsistent, and larger sample sizes are needed to fully understand whether the microbiome plays a meaningful role.
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๐๐๐๐ฅ ๐๐ข๐๐ซ๐จ๐๐ข๐จ๐ญ๐ ๐๐ซ๐๐ง๐ฌ๐ฉ๐ฅ๐๐ง๐ญ๐๐ญ๐ข๐จ๐ง (๐
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F***l microbiota transplantation was explored as a way to restore hindgut microbial balance. In the Danish study (Lausten et al., 2021), The FMT protocol used in this study included treating 10 horses with omeprazole for 10 days to reduce the acidic environment of the stomach and increase the likelihood that the microbes would successfully reach the hindgut. Meanwhile, f***l samples were collected from healthy donor horses, mixed with saline, and filtered through a sieve to remove large particles and create an inoculum to be administered to the FWS horses. During the last five days of the omeprazole treatment, the inoculum was administered to FWS horses via a nasogastric tube daily. Horses were then visited multiple days during a 24-week period to monitor their status and take f***l samples and questionnaires were completed by owners on days 168 and 335.
The study found substantial individual variation, and overall, horses with FWS did not show altered f***l microbiota compared to controls. However, FMT did temporarily reduce symptom severity. The authors suggested that FMT may influence other factors, such as metabolites like butyric acid, that could offer short-term relief. They recommended that future studies include a control group and evaluate metabolites more thoroughly.
๐๐ข๐๐ญ
Diets are also important to consider, as they can strongly influence the hindgut microbiota and broader gut health. To evaluate this variable, a study in Sweden and Norway comparing FWS horses with controls found that FWS horses consumed twice as much concentrate. This resulted in higher starch and water-soluble carbohydrate intake and lower crude protein and neutral detergent fiber (Lindroth et al., 2021).
In addition to concentrates, some supplements have been recommended to help FWS horses, but there is little to no evidence that they provide consistent, long-term benefits. These supplements include psyllium, beet pulp, probiotics, prebiotics, and bentonite clay. One study found that 26% (13/50) of FWS horses showed a reduction or elimination of clinical signs when provided one of these supplements (Lindroth et al., 2021). This demonstrates that while improvements can occur, they are not predictable or reliable across horses.
Forage is also a component that needs to be considered, as it is primarily fermented in the hindgut and plays an important role in gastrointestinal health. One study found that switching forage could impact FWS symptoms as 58% of horses who switched from haylage to hay showed improvements and 46% improved when switched from haylage to pasture (Lindroth et al., 2020).
Additionally, one of the most consistent management strategies for FWS is transitioning horses off a long-stem forage source (hay, haylage, or pasture) and onto a pelleted forage replacer for some or all of their diet. While research has not yet identified why this approach is so effective, it has been proposed that changes in stem length, and how that fiber is processed in the hindgut, may play a key role. It is also important to note this dietary change requires direction and supervision from a veterinarian or Ph.D. nutritionist to ensure it is done correctly.
๐๐จ๐ง๐๐ฅ๐ฎ๐ฌ๐ข๐จ๐ง๐ฌ
Free f***l water syndrome remains a frustrating condition because it is common yet difficult to manage and poorly understood. The current research suggests no single factor can fully explain why some horses develop FWS while others do not. While certain interventions may reduce symptoms for individual horses, long-term, reliable solutions are still lacking.
I can accept at this point I sound like a broken record when I say we simply need more research to understand this condition better. I am currently working on quite a few local cases and I am collecting and compiling some data and plan to share what I find out later this year!
Do you have a horse with f***l water syndrome? What has worked for you in terms of managing this condition?
Cheers!
Dr. DeBoer
Kienzle E, Zehnder C, Pfister K, Gerhards H, Sauter-Louis C, Harris P. Field study on risk factors for free f***l water in pleasure horses. Journal of equine veterinary science. 2016 Sep 1;44:32-6.
Schoster A, Weese JS, Ge**er V, Nicole Graubner C. Dysbiosis is not present in horses with f***l water syndrome when compared to controls in spring and autumn. Journal of veterinary internal medicine. 2020 Jul;34(4):1614-21.
Lindroth KM, Dicksved J, Pelve E, Bรฅverud V, Mรผller CE. Faecal bacterial composition in horses with and without free faecal liquid: a case control study. Scientific Reports. 2021 Feb 26;11(1):4745.
Porter MM, Davis DJ, McAdams ZL, Townsend KS, Martin LM, Wilhite C, Johnson PJ, Ericsson AC. Alterations in the Microbiome of Horses Affected with F***l Water Syndrome. Veterinary Sciences. 2025 Jul 31;12(8):724.
Laustsen L, Edwards JE, Hermes GD, Lรบthersson N, van Doorn DA, Okrathok S, Kujawa TJ, Smidt H. Free faecal water: analysis of horse faecal microbiota and the impact of faecal microbial transplantation on symptom severity. Animals. 2021 Sep 23;11(10):2776.
Lindroth KM, Lindberg JE, Johansen A, Mรผller CE. Feeding and management of horses with and without free faecal liquid: a caseโcontrol study. Animals. 2021 Aug 30;11(9):2552.
Lindroth KM, Johansen A, Bรฅverud V, Dicksved J, Lindberg JE, Mรผller CE. Differential defecation of solid and liquid phases in horsesโA descriptive survey. Animals. 2020 Jan 1;10(1):76.
