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Drugs Used to Treat Diarrhea in Monogastric Animals


Patricia M. Dowling

, DVM, MSc, DACVIM, DACVCP, Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan

Reviewed/Revised May 2023
Topic Resources

Treatment for diarrhea in dogs, cats, horses, and other nonruminants includes fluid therapy, electrolyte replacement, maintenance of acid-base balance, and control of discomfort.

Antiparasitic drugs or dietary modification can also play an important role in the treatment of some types of diarrhea.

Additional treatment may include the administration of intestinal protectants, motility modifiers, antimicrobials, anti-inflammatory drugs, and antitoxins (see the table Antidiarrheal Drugs Antidiarrheal Drugs Antidiarrheal Drugs ).


Mucosal Protectants and Adsorbents Used to Treat Diarrhea in Monogastric Animals

Kaolin-pectin veterinary formulations are popular for supportive care of diarrhea. Kaolin is a form of aluminum silicate, and pectin is a carbohydrate extracted from the rind of citrus fruits.

Kaolin-pectin is purported to act as a demulcent and adsorbent in the treatment of diarrhea. This action is claimed to be related to the binding of bacterial toxins (endotoxins and enterotoxins) in the GI tract. However, clinical studies have not demonstrated benefits from the administration of kaolin-pectin. It may change the consistency of the feces, but it neither decreases the fluid or electrolyte loss nor shortens the duration of illness. Nevertheless, it is often administered to small animals, foals, calves, lambs, and kids.

Kaolin-pectin products may adsorb or bind other drugs administered orally and decrease their bioavailability.

Activated charcoal is derived from wood, peat, coconut, or pecan shells. The material is heated and treated in such a way that many large pores form, dramatically increasing the internal surface area. Activated charcoal is available in a variety of pore sizes. The formulations sold for drug and toxicant adsorption typically have pore sizes of 10–20 Å. Activated charcoal is effective for adsorbing bacterial enterotoxins and endotoxins that produce some types of diarrhea. It also adsorbs many drugs and toxins and prevents GI absorption, so it is a common nonspecific treatment for intoxications. Because activated charcoal is not absorbed, overdose is not a problem.

Although other mucosal protectants have questionable efficacy, bismuth subsalicylate is considered by many human gastroenterologists to be the supportive treatment of choice for acute diarrhea. Its efficacy has been proved in controlled clinical trials in humans with acute diarrhea (due to enterotoxigenic Escherichia coli, also called traveler’s diarrhea).

Bismuth may have some ability to adsorb bacterial enterotoxins, and it may produce some gastric or intestinal protective effect. The salicylate component is believed to have anti-inflammatory action. Five sources of salicylate in this product account for a salicylate concentration of ~9 mg/mL. Practically, all of the salicylate from bismuth subsalicylate is absorbed systemically when administered to dogs and cats. There is no precise dose for animals. Some animals may not like the taste, and owners should be warned that bismuth subsalicylate turns the feces black. This color change may interfere with evaluating the feces for hemorrhage.

Salicylate toxicosis is possible, especially in cats.

Motility-Modifying Drugs Used to Treat Diarrhea in Monogastric Animals

Anticholinergic drugs are common ingredients in antidiarrheal preparations because they appreciably decrease intestinal motility and secretions. Their parasympatholytic effects decrease segmental and propulsive intestinal smooth muscle contractions and relax spasms of smooth muscle. Although they do not alter the course of disease, anticholinergic drugs decrease the urgency associated with some forms of diarrhea in small animals as well as the amount of fluid secreted into the intestine and abdominal cramping associated with hypermotility.

Because of the types of diarrhea observed in animals (few cases can be classified as hypermotile), anticholinergic drugs are limited for use in veterinary medicine. Intestinal motility is already impaired in many patients with diarrhea, and these drugs may actually worsen the diarrhea by creating a stovepipe effect. Anticholinergic drugs also have profound systemic pharmacological effects. If they are administered in sufficient doses to affect intestinal motility, possible adverse effects include severe ileus, xerostomia, urine retention, cycloplegia, tachycardia, and CNS excitement. Longterm administration of anticholinergic drugs may lead to serious intestinal atony.

Atropine is the best-known anticholinergic drug. Because it has many other systemic effects, including CNS excitement, it is not ordinarily used for its antidiarrheal effect.

Hyoscine butylbromide is an antispasmodic and anticholinergic drug that relaxes the smooth muscle of the GI tract. It is approved for the treatment of uncomplicated, spasmodic colic in horses. Initial relief of colic pain occurs within 5–10 minutes, and pain relief lasts 3–4 hours. Because of its parasympatholytic effects, hyoscine butylbromide produces transient tachycardia; therefore, heart rate is not a useful indicator of response to treatment for up to 30 minutes after administration. Hyoscine butylbromide also decreases gut sounds for 30 minutes after administration. Rectal relaxation, another effect of the drug, makes rectal palpation easier. Hyoscine butylbromide may also be beneficial in cases of choke Esophageal Obstruction in Large Animals Esophageal obstruction, commonly known as choke, occurs secondary to obstruction of the esophagus with food or foreign objects. Symptoms include nasal discharge of feed, coughing, bloat, and... read more Esophageal Obstruction in Large Animals , and it relieves acute bronchoconstriction in horses with equine asthma. It can be administered concurrently with NSAIDs and sedatives.

Opiates have both antisecretory and antimotility effects by action on the mu- and delta-opioidergic receptors of the GI tract. Opiates decrease propulsive intestinal contractions and increase segmentation, for an overall constipating effect. They also increase GI sphincter tone. Some evidence suggests that opiates inhibit colonic motor activity in horses. In addition to affecting motility, opiates stimulate the absorption of fluid, electrolytes, and glucose. Their effects on secretory diarrhea are probably related to the inhibition of calcium influx and decreased calmodulin activity. They are frequently used to treat diarrhea in dogs; their use in cats, however, is controversial because they may cause excitement. The constipating effects of morphine and codeine have been known for many years, but they are not used clinically as antidiarrheal drugs. Butorphanol, an opioid agonist-antagonist, has minimal effects on GI motility. Opiates can have potent effects on the GI tract and should be used cautiously.

Paregoric is a tincture-of-opium product and a controlled substance (5 mL of paregoric corresponds to ~2 mg of morphine).

Diphenoxylate and loperamide are two synthetic opiates that have specific action on the GI tract without causing other systemic effects. They have been used in small animals and large animal neonates. Diphenoxylate is a controlled substance in a formulation that contains atropine to discourage abuse; at therapeutic doses, the atropine has no effect. Loperamide is available over the counter.

Opiates are contraindicated in infectious diarrhea because slowing the GI transit time may increase the absorption of bacterial toxins. In dogs, constipation and bloat are the most common adverse effects. Potentially, paralytic ileus, toxic megacolon, pancreatitis, and CNS effects could occur (especially in cats).

Loperamide should not be used in dog breeds known to be sensitive to ivermectin (eg, Collies, Australian Shepherds, Old English Sheepdogs) without genetic testing. These dogs may have a gene mutation (ABCB1 gene deletion) that produces a functional defect in P-glycoprotein, which controls drug movement in many tissues. In humans and genetically normal dogs, large doses of loperamide do not result in the typical CNS effects due to opioids, because P-glycoprotein–mediated efflux of loperamide prevents it from achieving high concentrations within the CNS. Dogs with the ACBC1 gene deletion develop clinical signs of ptyalism, panting, ataxia, and recumbency at doses of loperamide that do not affect healthy dogs. IV lipid emulsion is an antidote for loperamide toxicosis.

Antimicrobials Used to Treat Diarrhea in Monogastric Animals

The efficacy of antimicrobials in the treatment of diarrhea is unknown or unproved in most clinical situations. In most cases of diarrhea in small animals, a bacterial etiology is not identified.

In large animals, antimicrobial treatment has not been shown to alter the course of bacterial enteritis, and in some cases, it is thought to perpetuate the disease by producing “carrier” animals (eg, salmonellosis). Nonabsorbed antimicrobials are frequently combined with motility modifiers, adsorbents, and intestinal protectants in some antidiarrheal preparations.

Campylobacter enteritis, due to infection by Campylobacter jejuni, occurs in cats and dogs and can be zoonotic. Treatment alleviates clinical signs; however, animals usually remain carriers. Antimicrobials suggested for the treatment of diarrhea include erythromycin, enrofloxacin, clindamycin, tylosin, tetracycline, and chloramphenicol.

Intestinal bacterial overgrowth is usually due to Escherichia coli or Clostridium spp. Provision of fluids and electrolyte replacement are the key treatments for enterotoxigenic E coli infections. For clostridial infections, if indicated, an oral drug that is effective in the GI lumen and has anaerobic activity should be selected; examples include metronidazole, amoxicillin, ampicillin, tylosin, and clindamycin. An invasive E coli is associated with granulomatous colitis in Boxers and French Bulldogs. It is typically responsive to fluoroquinolone treatment unless resistance has emerged.

Enteritis due to a variety of pathogens is common in young animals. When integrity of the intestinal mucosa is lost, septicemia or endotoxemia is likely. Clinical signs of sepsis include severe bloody diarrhea, fever, scleral injection, dehydration, and alteration in the leukogram (early leukopenia in endotoxic shock, followed by leukocytosis). If septicemia or endotoxemia is suspected, systemic antimicrobials are warranted, along with NSAIDs. Neonates with diarrhea deteriorate rapidly, often before bacteriologic culture and antimicrobial susceptibility test results are available. Therefore, broad-spectrum antimicrobial treatment should be initiated. Suggested antimicrobials (depending on the species) include fluoroquinolones, a penicillin or cephalosporin plus an aminoglycoside (gentamicin, amikacin), ampicillin or amoxicillin, tetracyclines, potentiated sulfonamides, chloramphenicol, or florfenicol. In septic animals, GI absorption is likely to be altered, so parenteral administration is preferred.

NSAIDs Used to Treat Diarrhea in Monogastric Animals

The antiprostaglandin activity of NSAIDs may be beneficial in the treatment of some types of diarrhea and may be important in the treatment of septicemia or endotoxemia. Prostaglandins are important intracellular messengers for stimulating hypersecretion by the intestinal mucosa, possibly by stimulating an increase in cyclic adenosine monophosphate (cAMP). Antiprostaglandin drugs may directly inhibit fluid and electrolyte hypersecretion by the intestinal cells.

NSAIDs should be administered cautiously because they have adverse GI, hepatic, and renal effects.

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