Chronic diarrhea in horses: a diagnostic approach

Defining chronic diarrhea (and distinguishing it from free fecal water syndrome)

Chronic diarrhea in the adult horse is best described as the continuous or intermittent passage of soft to watery feces for a prolonged period. There's no universally agreed cutoff in the equine literature, but most clinicians use roughly two to four weeks as the threshold that separates a lingering problem from a self-limiting upset. The exact day count matters less than the recognition that the diarrhea has outlasted what supportive care alone should resolve.

Before launching a workup, confirm you're actually dealing with diarrhea. Free fecal water syndrome is a separate entity: the horse passes normal, formed fecal balls accompanied by a distinct stream of free liquid that often stains the hindquarters and tail. The manure itself is normal. That distinction is clinically important, because free fecal water syndrome rarely reflects serious intestinal disease, while genuine chronic diarrhea can signal infiltrative, infectious, or parasitic conditions that demand investigation.

Why a structured workup matters

It's tempting to treat chronic diarrhea empirically, but that approach tends to chase symptoms while missing the cause. A definitive diagnosis is reached in fewer than half of equine diarrhea cases, largely because the large colon responds to many different insults in similar ways and because pathogens shed intermittently. A structured ladder still earns its keep: even when it doesn't name the culprit, it rules out the dangerous and the treatable, and it tells you when to stop testing and start managing. As The Herd has covered, chronic diarrhea in horses demands a broader diagnostic lens than a single fecal test or a course of empirical antibiotics.

Differential diagnoses

A useful differential list spans several categories. Inflammatory bowel disease covers a group of infiltrative conditions, including lymphocytic-plasmacytic, granulomatous, and eosinophilic forms, in which inflammatory cells invade the intestinal wall and cause malabsorption and protein loss; diarrhea may or may not be present. Parasitism, particularly larval cyathostominosis from encysted small strongyles, is a classic cause and can occur even in horses on a deworming program. Sand enteropathy develops in horses grazing sandy paddocks or fed on the ground, where accumulated sand irritates the colon. Infectious causes include Salmonella, Clostridioides difficile and C. perfringens, equine coronavirus, and Neorickettsia risticii (Potomac horse fever). Neoplasia, most often intestinal lymphoma, belongs on the list for older horses with weight loss. And dietary factors, from abrupt feed changes to poor-quality forage to disrupted hindgut fermentation after antimicrobial use, round out the picture. Non-intestinal causes such as chronic liver disease and abdominal abscesses can also drive chronic diarrhea.

The diagnostic ladder

Work from least to most invasive. History anchors everything: diet, deworming, travel, antimicrobial exposure, environment, and the timeline of signs. The physical exam assesses hydration, body condition, mucous membranes, and abdominal findings. Bloodwork, a complete blood count and biochemistry, screens for inflammation, organ dysfunction, and electrolyte derangement. Pay particular attention to total protein, albumin, and electrolytes: low protein concentrations point toward protein-losing enteropathy and infiltrative disease, while sodium, chloride, and potassium losses guide fluid therapy. Fecal testing comes next, including parasite egg counts, sand sedimentation, and pathogen testing by culture and PCR. Ultrasonography, both transabdominal and transrectal, is the best tool for detecting bowel-wall thickening, sand accumulation, and mesenteric lymphadenopathy. When infiltrative disease remains likely, biopsy provides the definitive answer: rectal mucosal biopsy is low-risk but diagnostic in only a subset of cases, so full-thickness samples via laparoscopy or laparotomy may be needed.

Biosecurity and zoonotic considerations

Several causes of chronic diarrhea are transmissible to people, so handle every case as potentially infectious until proven otherwise. Salmonella is the headline zoonosis; healthy horses shed it at under 2 percent, but hospitalized and clinically affected horses shed far more, and because the organism can't be cultured consistently, collecting three to five daily fecal samples improves detection. Owners should be explicitly counseled on the zoonotic risk. Clostridioides difficile and equine coronavirus also warrant caution. Practical measures include isolating the affected horse, using dedicated boots, gloves, and clothing, washing hands rigorously, and disinfecting contaminated surfaces. People who are pregnant, elderly, very young, or immunocompromised should limit contact entirely.

When to refer

Refer when the first-line workup is inconclusive and the horse continues to decline, when weight loss or hypoproteinemia signals significant infiltrative or protein-losing disease, or when definitive diagnosis requires intestinal biopsy or advanced imaging beyond the practice's capability. Referral is also the right call when a confirmed or suspected contagious agent demands isolation facilities the practice can't safely provide. Knowing the limits of an in-field workup, and acting on them early, protects the patient, the herd, and the people in contact with the horse.