Find out what causes watery feces in horses and whether you should be concerned.

Muck bucket and rake in hand, each day’s stall cleaning gives you a measure by which to monitor your horse’s well-being: The number and quality of his manure piles. You’re probably highly aware of the amount you collect.

Have you ever noticed “horse apples” that are not completely normal, having a watery component along with formed fecal balls? Then you investigate your horse’s hind end and find manure stains where there should be none. Is this a cause for concern?

One reason a horse might have manure soiling his rump and rear legs is free fecal water syndrome (FFWS). Alicia Long, DVM, of the University of Pennsylvania’s New Bolton Center School of Veterinary Medicine, in Kennett Square, explains how FFWS differs from diarrhea: “Diarrhea tends to be frequently passed loose or watery feces lacking in solid fecal matter. In contrast, a horse experiencing FFWS defecates a relatively solid pile of manure, which is then followed by wet feces or liquid. The liquid phase doesn’t always come just at the end—it may also pass at the beginning of or during ­defecation.”

What Are the Consequences of FFWS?

Jamie Higgins, DVM, Dipl. ACVIM, of Idaho Equine Hospital, in Nampa, notes that FFWS is mostly just bothersome to owners, making it difficult to keep horses—especially show horses—clean. “There are not many significant effects to the horse,” she says, “other than possible irritation or scalding of skin (dermatitis) from manure adhered to the hind limbs, tail, and perineal region around the anus. Some horses seem irritated while passing manure, likely due to fluid dripping on their legs.”

Another concern, Long notes, is liquid manure staining the legs and hind end might attract more flies, adding to the annoyance. “Reproductive health may be affected in mares with poor perineal conformation,” adds Higgins. “They are at greater risk of fecal contamination of the vaginal vault.” 

Horses with FFWS often show no other clinical signs, appetite changes, differences in weight or body condition, or discomfort aside from irritation/tail-swishing when voiding fecal water, says Long. “The amount of water lost does not appear to be substantial enough to cause clinical dehydration,” she adds.

“There also doesn’t seem to be a strong association with development of colic and FFWS in the literature, and this fits with my clinical experience,” she says. However, in one study (Kienzle et al., 2016) the authors note about 25% of horses with FFWS had a history of colic, compared to the general population colic incidence of 3.5-10.6%.

“In horses recovering from colitis (colon inflammation) from a variety of causes, there may be a prolonged period of free fecal water during the recovery period, but this is secondary to the primary colon disease and not the cause of colon disease,” says Higgins. 

What Might Cause FFWS?

Leaky Gut Syndrome

Leaky gut in horses results from altered intestinal permeability due to damage to tight junctions between intestinal cells that line the hindgut (the cecum and large colon ). Tight junctions act as gatekeepers between gastrointestinal contents and the systemic circulation. Such permeability allows ions, nutrients, inflammatory cytokines, and/or intestinal microbes to “leak” into the systemic circulation.

Higgins suggests a possible correlation between FFWS and leaky gut syndrome: “When other causes of excess fecal water have been ruled out, another consideration may be damage to the tight junctions. This can result in increased water loss between these cells into the lumen (cavity) of the colon and cecum, along with decreased water reabsorption through the hindgut.”

“Leaky gut syndrome occurs secondary to issues with intestinal immune function, disturbances in microbiota, acute or chronic inflammatory disease, and mechanical and functional intestinal obstructions,” says Long.

Any irritation to the bowel lining, such as from sand accumulation, long-stem or coarse hay, or non-steroidal anti-inflammatory or antimicrobial drugs, can also alter intestinal permeability. However, she notes that current studies in horses with FFWS have not shown evidence of leaky gut indicators, such as disturbances in hindgut fermentation, changes in microbiota composition or diversity, or evidence of hindgut acidosis.

Dental Health

Veterinarians have often considered dental health to be a potential cause of FFWS. Higgins says excessive forage fiber length—1 inch or longer—is more difficult for hindgut microbes to ferment for appropriate digestion. She says effective dentition is important for grinding fiber to more digestible lengths, and issues such as worn or missing teeth, sharp enamel points, or dental arcade imbalance potentially undermine this ability.

Although compromised forage grinding might affect microbial fermentation, Long cites results from studies on dental health and FFWS: “In a study (Kienzle et al.), recent dental correction did not change the occurrence of FFWS, according to owners of affected horses. Severe dental problems can alter fecal particle size, but another study (Zwirglmaier et al., 2013) shows that moderate dental problems don’t affect particle size. Therefore, it is unlikely that dental abnormalities are a consistent cause of FFWS.”

Intestinal Parasites

In regard to the possible effect of internal parasite infection, Long says: “Multiple studies have not shown any differences in fecal egg counts or deworming strategies between horses on the same farms with and without FFWS. However, diagnosis of certain equine intestinal parasites (particularly small strongyles and tapeworms) is challenging and, therefore, hard to rule out completely.”

Higgins notes that encysted cyathostomins (small strongyles) cause inflammation of the lining of the large colon and cecum, which can potentially alter manure consistency. In most horses with FFWS, it does not appear parasites are the underlying cause, but it’s important to work with your veterinarian to ensure internal parasites are not exacerbating the condition, she says. Long agrees: “Based on my experience and what the literature shows, internal parasites are unlikely to be the biggest causative factor of FFWS.”

Stress

Stress elicits a variety of health issues in horses. “As we learn more about the microbiome-gut-brain axis, we find that stress, both physical and mental, affects the microbiome diversity of horses,” explains Higgins. “Stress also may increase hindgut motility, with less time for processing of feed material and resorption of water. Stress arises from intense training, transport, excessive stall confinement, environmental stresses, heat stress, herd dynamics, and (herd) hierarchy struggles in competition for feed and shelter.”

Long concurs, noting, “Owners of horses with FFWS describe that only 37% occupied a high position in the herd social hierarchy, compared to 58% without FFWS (Kienzle et al.). The study authors postulate that a lower position in the hierarchy could contribute to increased stress and, in turn, affect gut motility.”

Researchers have found that geldings were more likely to be affected by FFWS than mares or stallions, which also fits with Long’s clinical experience: “It is possible that herd dynamics and social status explain this stress-related FFWS phenomenon.”

Intestinal transit time—not just that of the hindgut—increases with stress, too, with food and water moving faster through the intestines than they would normally, she adds. Consider the horse that produces small, frequent amounts of somewhat watery feces from a stressful event like a visit to an unfamiliar environment.

“The hindgut of the horse is not only responsible for metabolism and digestion of food, but it also secretes and reabsorbs a lot of water—about 30 liters (~ 8 gallons) per day in a standard pony,” Long says. “Hence, only about 10% of the liquid that enters the equine hindgut makes it into the feces. If feed and liquid move more quickly through the colon, fecal water content may increase. This still doesn’t totally explain a mechanism for FFWS, however, since a study (Lindroth et al., 2022) found that total fecal water content of horses with FFWS does not differ from horses without the condition; the difference is instead due to distribution of fluid within the feces.”

Seasonal Component

“In the available literature,” says Long, “there is not a proven seasonal component, and many horses have FFWS year-round. However, in my experience I see it most often in winter, and a study (Kienzle et al.) reports that 31% of horses had FFWS only during the winter.”

Higgins says she also sees it primarily in winter: “Winter weather corresponds with increased hay feeding, weather extremes, and reduced turnout time. The stress of extreme weather can be a trigger in some horses, while freezing temperatures complicate the ability to keep the horse clean.”

Endocrine Effects

Higgins points out that horses with pituitary pars intermedia dysfunction, or PPID (formerly called equine Cushing’s disease) frequently have FFWS that improves with PPID treatment using medications such as pergolide. 

Dietary Influences

“Long-stem roughage is often associated with the condition,” says Higgins. “Some horses have sensitivities to different hay types, with nutrient-rich alfalfa most associated with the condition. However, individual horses do better on one type of hay versus another. It helps to try a different type of hay, preferably less stemmy than what the horse consumed when the condition developed. At least several weeks is necessary to determine if there is improvement or not, once gut flora adjust to new feed.”

“A case-controlled survey (Lindroth et al.) questioned owners regarding feeding strategies,” explains Long. “Owners reported an improvement in FFWS when changing from bagged silage forage to hay or pasture and when changing between different cuttings of bagged silage. The water content of bagged silage is greater than that of traditional hay, but less than pasture. Since total water content in feces is not different in horses with FFWS, feeding forages high in water content is unlikely to be the sole reason for the occurrence of FFWS.”

Long says in her experience, horses with FFWS often improve or their signs resolve when pasture returns in the spring. “Pasture may benefit some horses due to stress mitigation from increased turnout time, grazing behavior, and social interaction,” says Higgins. “That said, a horse put on pasture too rapidly, without allowing adjustment of the gut flora, may experience a transient increase in FFWS.”

Feeding Strategies to Manage FFWS

One FFWS management strategy involves the maturity and digestibility of the fiber in the horse’s diet. Higgins says many horses improve with feed supplement products recommended for the condition. “Especially for aged horses with dental disease, switching to an all-pelleted complete feed, hay pellet or cube, or chopped hay provides stem length that is efficiently processed in the hindgut,” she explains. “Some horses benefit from a partial diet change that combines pelleted feed with reduced amounts of hay; this lessens the amount of long-stem roughage the hindgut has to process. Dietary changes are done gradually to allow adjustment by the gut microbiome; rapid changes may exacerbate FFWS.”

Long adds that while no specific data support the use of pelleted, chopped, or cubed feed to address FFWS, feces from horses with FFWS have a lower water-holding capacity than normal horses, and water-­holding capacity is related to the type, amount, and particle size of fiber present in the feces. “Fecal fiber content properties vary between individual horses fed the same diet, so it is possible that horses with FFWS may benefit from different particle-size feed that holds more water,” she says.

With this in mind, Long notes researchers have revealed horses with and without FFWS consume very different diets. “Based on Lindroth’s studies, horses without FFWS are fed a lower proportion and amount of concentrate,” she says. “Spillover of concentrate into the hindgut produces lactate, which can have an osmotic effect that pulls water into the colon. Similarly, horses without FFWS are fed lower starch and a higher-fiber diet than those with FFWS. Different feeding practices, as for example the number of times fed per day, did not differ between horses with and without FFWS.”

In summary, Long suggests owners might see benefits from feeding horses with FFWS higher-forage diets with less concentrates and lower starch levels. However, “based on studies thus far, high-concentrate and high-starch diets are not documented to actually cause FFWS,” she notes.

Prebiotics and Probiotics

Researchers debate the use of pre- and probiotics for managing FFWS. “Probiotics are ­microorganisms—bacteria and beneficial yeasts such as Saccharomyces species—that populate the gut and aid in digestion of fiber as well as support of the immune response,” Higgins explains. “Some gut flora produce butyrate, which is protective to and an energy source for colonocytes (colon cells). Prebiotics are substances that support the nutrition and health of the gut flora. Both pre- and probiotics may be beneficial if underlying gut dysbiosis, caused by stress, excess dietary starch, illness, or antibiotics have adversely affected normal, healthy gut flora.”  

“In a study (Lindroth et al.) evaluating the effects of different management and feeding strategies on FFWS, 26% of owners reported improvement or resolution of FFWS when adding a pre- or probiotic, psyllium, linseed, or thiamine supplementation,” adds Long. However, she says the jury is out as to whether commercially available probiotics are helpful in horses because they contain only a handful of organisms out of the thousands found in a healthy horse’s gut. Often these products contain many filler ingredients, she says, and don’t contain the amounts or organisms listed on the label.

“Research has shown that some commercial probiotics contain bacteria with antibiotic-resistant genes, which could also be detrimental to a horse’s future health in the face of an infection,” she adds. “That said, I am a proponent of psyllium ­supplementation—a prebiotic that provides nutrients for gut bacteria—for proper hindgut (colonic) health. Psyllium improves repair and regeneration of equine colon cells and is a precursor for butyrate production. In the Lindroth management survey study, Brewer’s yeast (Saccharomyces cervisiae)—another prebiotic—was fed to a significantly higher proportion of horses without FFWS than those with FFWS; this could be considered a potentially beneficial supplement.”

Transfaunation

Researchers have compared fecal bacterial microbiota in horses with FFWS to those without the condition and found minimal differences in hindgut bacterial populations in FFWS horses. “This suggests that FFWS may not be related to disturbances in the hindgut microbiome of affected horses,” says Long. “This possibly explains why fecal microbiota transplantation (transfaunation) has variable success in treating FFWS, with little lasting benefit for horses with FFWS. There are still many unknowns related to the best practices for performing fecal transfaunation; its efficacy is also variable when used for other gastrointestinal diseases in horses.”

In addition, Higgins says recent studies suggest what makes up healthy gut flora differs between farms, so it is more beneficial to obtain manure from healthy donors in the same environment as the recipient horse. “This ensures the presence of microbes that effectively process the same feed source,” she says, but stresses that if owners don’t address management issues such as high-concentrate diets and herd stress, gut flora improvements from transfaunation are not likely to last long.

Other Supplements

“Several supplements show promise in management of horses with FFWS,” says Higgins, noting that “any supplement likely takes at least four to six weeks to determine improvement.” These include:

Butyric acid/butyrate strengthens tight junctions between mucosal epithelial cells in the colon and protects colonocytes lining the gut by reducing inflammation and supporting immune function. Butyrate is also an energy source for colonocyte growth.

Zinc is another source for strengthening tight junctions between cells lining the gut.

Chia seeds are supplement sources with anecdotal evidence of improving FFWS.

Psyllium is helpful as a bulk laxative to move out sand accumulation and mitigate irritation that can lead to changes in fecal character.

Beet pulp is a highly digestible fiber source that provides increased branched-chain amino acids (BCAA) to the colonocytes. BCAAs help maintain intestinal barrier function and promote development of the intestinal microbiome and absorption of protein and glucose.

Marine-based omega-3 fatty acid supplements with high levels of docosahexaenoic acid (DHA) and eicosatetraenoic acid (EPA) might have anti-inflammatory effects on the intestine. Fresh forage is also a source of omega-3 fatty acids.

Spray-dried oral plasma (a functional ingredient from healthy animal blood used as a feed additive in livestock and pet nutrition) with bioactive proteins has purported uses for improving intestinal health.

Take-Home Message

Although veterinarians do not know the underlying triggers for FFWS in most horses, our sources say it is worth investigating a variety of strategies to help improve affected animals. Making even small adjustments in type and proportion of readily digestible forage might elicit welcome changes. Managing social dynamics, limiting stress and anxiety, and offering feed supplements such as butyric acid, probiotics, psyllium, omega-3 fatty acids, and beet pulp might provide additional benefits.