Addressing acute diarrhea in horses

There are multiple causes of acute diarrhea in adult horses. In other species, diarrhea may not be considered an emergency. However, in horses, acute diarrhea can lead to fast dehydration, endotoxemia, systemic inflammatory response syndrome (SIRS) and death. Therefore, all horses with acute diarrhea should be considered risk patients and should be closely monitored.

The causes of diarrhea differ by geographic localization and age of the patient. Acute diarrhea can be caused by infectious agents such as Salmonella spp, Clostridium spp, Equine coronavirus, Neorickettsia risticii (Potomac Horse fever), Lawsonia intracellularis and encysted small strongyles (Cyathostomes). Non-infectious causes include administration of NSAIDs, exposure to toxins such as cantharidin, hoary alyssum, and arsenic, sand accumulation as well infiltrative diseases such as alimentary lymphoma or inflammatory bowel disease (which can also present as chronic diarrheas or weight loss). Some of these infectious organisms are contagious. Thus, horses should be isolated until the contagious causes of diarrhea are ruled out or a cause can be stablished. In our hospital, isolation is mandatory for horses that show up with diarrhea, neutropenia, or fever (2 out of these 3 signs).

There are multiple mechanisms for diarrhea: malabsorption, increased osmolarity of the GI content, increased secretion of water and electrolytes into the lumen, inflammation of the GI or altered GI motility. In adult horses, diarrhea results almost exclusively from disorders of the large intestine (although small intestine may be involved at the same time). This is less true in foals. Regardless of the mechanism, most of the treatment focuses on patient maintenance.

Clinical signs:

Aside from diarrhea, the clinical signs are typically those of dehydration and endotoxemia. Horses may show lethargy, anorexia, tachycardia, tacky mucous membranes that might be congested. Fever and colic are not uncommon. Some cases develop ileus and acute laminitis.

Diagnosis:

A definitive diagnosis is difficult to obtain in horses. A definitive diagnosis is not achieved in > 50% of the cases. Clinical signs are too similar between etiologies and are rarely pathognomonic. A proper history is critical to differentiate infectious (other horses infected, history of fever, travel, etc.) from non-infectious (NSAID administration, feeding on ground, potential exposure to toxins, etc.).

• Fecal samples (and blood for some pathogens) should be submitted whenever possible for culture, PCR, and/or toxin identification.
• Bloodwork is useful to assess acid base status, electrolyte abnormalities, dehydration, and overall organ function. Marked neutropenia and toxic changes in neutrophils are common.
• Abdominal ultrasonography may show fluid in the large colon/cecum, thickening of the wall and free fluid.
• Abdominal radiographs are useful to assess the presence of sand/gravel.

Treatment:

Treatment is aimed at correcting dehydration and electrolyte imbalances as well as the causative agent when identified.

Restoring fluid balance:

• Fluid therapy is typically administered IV in severe cases, but enteral fluid administration can also be performed using a small nasogastric tube. Both routes allow for electrolyte supplementation.
• Oncotic pressure: Low albumin is not uncommon in these cases. Hydroxyethyl starch solutions or plasma can be used to correct these. Hydroxyethyl starch solutions can cause coagulation abnormalities if administered >20 ml/kg or for multiple days.

Anti-endotoxin treatment:

• DTO smectite (Biosponge®) or activated charcoal can be administered via NGT to decrease endotoxin absorption from the GI.
• Polymixin B (6,000 IU IV q 6-8h) can be used to bind circulating endotoxin. This drug is expensive and can be nephrotoxic.

Analgesia:

• NSAIDs can be used to provide analgesia. However, they should be use cautiously as they are nephrotoxic and can cause mucosal damage of the colon.
• Butorphanol or other opioids can be added to the pain management plan.

Treating the underlying cause:

• In many cases, empirical treatment is selected based on regional incidence of disease.
• Systemic antibiotics are typically recommended in the case of Clostridium spp., Neorickettsia risticii and Lawsonia intracellularis. Broad spectrum administration in horses with other causes of diarrhea can worsen GI dysbiosis. Administration of systemic antibiotics to all neutropenic horses remains controversial.
• Clostridium spp: Metronidazole, 15 mg/kg PO q8h. IV formulations are available, but their price restricts its use in adult horses. Per rectum administration can be used in cases of ileus, doubling the dose to 30 mg/kg is recommended. Anorexia and increased liver enzymes are common after administration. Ataxia may be seen if the protein is low; more common with IV formulaitons.
• Neorickettsia risticii and Lawsonia intracellularis: Oxytetracycline IV (6.6 mg/kg IV q 12-24h for 3-5 days). Lawsonia may require longer treatment. Improvement is seen within 24h. Oxytetracycline is nephrotoxic and should be given after the patient is hydrated. or twice daily for 5 days (dilute in fluids or give slowly). If given fast, it can cause collapse and death. Thus, in the case of sick animals, it is best to use diluted in sterile water.

[...]