Chronic diarrhea in horses demands a broader diagnostic lens

A new The Horse article is putting fresh attention on a familiar but often messy equine medicine problem: chronic diarrhea that isn’t always what it seems. Reporting from the 2026 Veterinary Meeting & Expo, held January 17-21 in Orlando, Lucile Vigouroux summarized a presentation by Amy Stewart, DVM, PhD, Dipl. ACVIM-LA, who argued that veterinarians should be cautious about labeling horses with fecal water syndrome before ruling out more consequential disease. Her central point was simple but important: intermittent fecal water, soft manure, weight loss, colic, low protein, or reduced appetite can reflect very different underlying processes, and the distinction shapes both treatment and prognosis. (thehorse.com)

That framing builds on long-standing frustration in equine internal medicine. Chronic diarrhea in adult horses has many potential causes, and even thorough workups do not always deliver a definitive diagnosis. Merck notes that a precise cause is identified in fewer than half of diarrhea cases overall, even though treatment often has to begin before that answer is clear. The Horse’s earlier background coverage similarly described chronic diarrhea as a broad syndrome tied to causes ranging from sand accumulation and parasitism to inflammatory bowel disease and lymphoma. (merckvetmanual.com)

The newer article adds a more specific clinical distinction between true chronic diarrhea and fecal water syndrome. Stewart described fecal water syndrome as intermittent passage of liquid before, during, or after otherwise normal feces in horses that may appear healthy. By contrast, true chronic diarrhea raises concern for colonic disease and broader systemic consequences. According to the report, Stewart recommends a progressive diagnostic approach: start with history, physical examination, CBC and chemistry testing, fecal egg counts, and Salmonella testing; if those are unrevealing, move to abdominal ultrasound, radiographs for sand burden, abdominocentesis, and rectal mucosal biopsy to help separate inflammatory from neoplastic disease. (thehorse.com)

The differential list is wide. Infectious causes include cyathostomes, Salmonella, and Clostridium-associated disease, while noninfectious causes include sand enteropathy, infiltrative bowel disease, lymphoma, and right dorsal colitis after prolonged NSAID exposure. Stewart noted that fecal egg counts may underestimate parasite burden when cyathostomes are encysted in the colon wall, and that chronic Salmonella shedding can be intermittent enough to require repeated or pooled samples. AAEP guidance on clostridial diarrhea in adult horses underscores the infection-control side of the equation, advising isolation and hygiene precautions because diarrheic horses can spread enteric pathogens through fecal-oral transmission. (thehorse.com)

Research into fecal water syndrome adds nuance, but not clarity yet. A 2021 study indexed in PubMed found that horses with free fecal water did not show a clearly altered hindgut microbiota compared with controls, although some horses had temporary symptom improvement after fecal microbiota transplantation. More recent microbiome work indicates there may be detectable microbial shifts in affected horses, but the field still lacks a settled mechanism, validated biomarkers, or standardized treatment protocols. In other words, microbiome disruption is a plausible piece of the story, but not yet a dependable clinical shortcut. (pubmed.ncbi.nlm.nih.gov)

Expert reaction in this case comes mainly through Stewart’s presentation rather than outside commentary, but the message aligns with broader equine guidance: don’t assume a dirty tail equals a benign condition. That matters because some horses compensate surprisingly well despite ongoing intestinal disease, and because delayed workup can miss treatable problems such as parasitism, sand accumulation, inflammatory disease, or NSAID-associated colitis. It also matters for client communication with pet parents, who may normalize chronic manure changes if the horse otherwise seems bright and eating. (thehorse.com)

Why it matters: For veterinary professionals, the practical implication is a more disciplined triage mindset. Horses with chronic loose manure need evaluation not just for symptom control, but for underlying pathology, hydration status, protein loss, infectious risk, and management factors such as diet, forage type, supplements, and medication exposure. The article also reinforces that fecal water syndrome remains a diagnosis of exclusion, not a catch-all label. In ambulatory and referral settings alike, that can help clinicians justify serial testing, imaging, biopsy, and biosecurity steps that may otherwise seem excessive for a horse that is still eating and active. (thehorse.com)

What to watch: The next developments are likely to come from three areas: better tests for hidden parasitism and intermittent infectious shedding, more rigorous microbiome research to define what fecal water syndrome actually is, and stronger evidence on which dietary or microbiota-based interventions produce durable improvement. For now, the most defensible approach remains the one Stewart outlined: rule out treatable causes first, then manage the residual cases as presumptive fecal water syndrome with careful follow-up. (thehorse.com)