MSD Manual

Please confirm that you are a health care professional

honeypot link
Pet Owner Version

Intestinal Disorders Other than Colic in Horses


Allison J. Stewart

, BVSc (Hons), PhD, DACVIM-LAIM, DACVECC, School of Veterinary Science, University of Queensland

Reviewed/Revised May 2019 | Modified Oct 2022
Topic Resources

Diarrhea, weight loss, abdominal pain, and protein loss are common signs of intestinal disorders in horses. These signs can be caused by many different disorders, including infectious diseases, parasites, inflammation, tumors, reactions to poisons or drugs, stress, changes in diet, and certain types of colic Specific Causes of Colic and their Treatment Over the years, colic has become a broad term for a variety of conditions that cause horses to experience abdominal pain. Because it is such a broad term, it is used to refer to conditions that... read more . Because the signs are similar for many intestinal disorders, determining the cause can sometimes be challenging and is often best left to a veterinarian. A correct diagnosis will lead to better treatment and, in many cases, a faster recovery.

Diarrhea in adult horses can be acute (sudden and often severe) or chronic (persisting a month or more). A variety of infectious diseases, ingestion of toxins, sand colic, use of certain medications, and a condition called colitis-X can cause acute diarrhea. It is often difficult to diagnose the cause of chronic diarrhea. It can occur due to inflammatory or cancerous conditions involving the intestine (such as sand colic or inflammatory bowel disease) or disruption of normal digestion (such as changes in the diet or in the normal bacteria found within the intestines). The body’s response to certain components of feed may play a role in chronic diarrhea of horses due to bowel inflammation, but has not frequently been established as a cause. Longterm heart and liver disease can also cause chronic diarrhea. Diarrhea in adult horses should be considered a serious event ( see Diarrhea in Horses Diarrhea in Horses Diarrhea in Horses , below).

The causes of weight loss in horses are numerous and can involve many body systems. This discussion will cover only diseases of the digestive tract. Protein loss (called hypoproteinemia) may or may not be associated with weight loss. The disorders commonly associated with either of these signs are tumors, inflammatory bowel disease, and adverse reactions to treatment with nonsteroidal anti-inflammatory medication.


Salmonellosis is one of the most common infectious causes of diarrhea in adult horses. It is caused by many species of Salmonella bacteria and can cause severe disease if the bacteria or toxins it produces enter the bloodstream (septicemia). It can also cause inflammation of the small or large intestine (enteritis or colitis). Signs range from none to sudden, severe diarrhea, and even death.

The usual route of entry by the bacteria is by mouth. After entry, the organism multiplies in the intestines and causes intestinal inflammation. Young horses are the most susceptible. Penetration of bacteria into the intestinal surface contributes to tissue damage and diarrhea. Entry into the bloodstream may follow. The bacteria can also live in the lymph nodes and can cause infection in other parts of the body, such as the brain, liver, lungs, joints, or bones, especially in foals.

Some horses that show no signs of disease may be carriers of Salmonella. Although most horses clear the infection in a few weeks, some can carry the bacteria for prolonged periods and shed it in the manure, especially when stressed. These horses may show signs of illness after stress. In adults, most cases of disease develop after the stress of surgery or anesthesia, alteration in diet, treatment with antibiotics, illnesses (especially digestive disorders), or transport. Mares may shed the bacteria while giving birth and infect the newborn foal. Salmonellosis in horses hospitalized for other causes is a major problem for equine clinics because horses that are hospitalized experience one or more of the stressors listed above. In these circumstances, a large number of vulnerable horses are present and carriers may shed the bacteria.

Diarrhea in Horses

Because of the large volume of the intestines of horses, massive fluid losses can occur in a short time. Thus, diarrhea in adult horses can be a serious event, and rates of illness and death may exceed those in other animals and humans. Any case of diarrhea in your horse that lasts more than a day or two, or that occurs repeatedly, is a cause for concern. Your veterinarian should be contacted for evaluation of the horse. Although a definitive cause can be determined in less than 50% of cases of diarrhea, treatment of most horses and foals with diarrhea is similar and can be started despite the lack of a precise diagnosis.

In newborn foals, the bacteria or its toxins are more likely to spread beyond the intestine into the bloodstream. Illness can develop suddenly and be severe; signs generally include lethargy, reduced suckling, and a high fever. Death occurs in 24 to 48 hours. In some foals these sudden signs of illness are not noticed. If the bacteria spread to joints or bones, the foal can become lame and have a fever.

In adult horses and in foals older than a week, intestinal inflammation is the most common form of the disease. Initially, there is fever, followed by severe, watery diarrhea. Straining during defecation can occur. The feces may have a foul odor and contain mucus, shreds of mucous membrane, and in some cases, blood. Abdominal pain and colic can also occur. Affected horses are severely dehydrated and many die within 24 hours of the onset of diarrhea if not treated promptly. A milder illness may develop in some adult horses. The signs include mild fever, soft feces, poor appetite, and dehydration. Pregnancy loss can also occur in mares.

A diagnosis of salmonellosis is based on signs and identification of the organism after laboratory examination of feces, blood, and tissues from affected animals.

Early treatment is essential for cases of salmonellosis in which the horse or foal has signs of dehydration, pain, or bacteria in the bloodstream (septicemia). Broad-spectrum antibiotics are given intravenously to treat septicemia. Oral medication may be provided in drinking water because affected horses are thirsty due to dehydration, and their appetite is generally poor. Horses with severe or ongoing fluid and electrolyte (salt) loss will require treatment with intravenous fluids. Additional medications may be given to protect the digestive tract and treat the damaging effects of bacterial toxins in the body (endotoxemia).

The intestinal form of salmonellosis is difficult to treat effectively. There is controversy regarding the use of antibiotics for intestinal salmonellosis (the type seen more commonly in adults). Oral antibiotics may prevent the spread of bacteria into the blood, but they will not decrease shedding into the environment or change the course of colitis. In addition, they may damage the normal population of intestinal bacteria, which aid in digestion and help protect the animal from other, disease-causing bacteria. There is also concern that antibiotic use may increase the number of bacteria that can resist antibiotics and make the infection more difficult to treat. Horses with acute intestinal salmonellosis will probably be given fluids containing electrolytes (salts) intravenously to correct deficiencies of sodium and potassium. Plasma with a high concentration of antibodies against Salmonella may also be given. Some horses may require treatment with medications given via a tube that is passed into the stomach. These medications are designed to help heal the intestine and absorb toxins produced by the bacteria.

Controlling Salmonellosis

Owners can take several steps to control salmonellosis:

  • Buy horses only from sources that are free of the disease.

  • Isolate new horses when they arrive.

  • Have a veterinarian check all new horses when they arrive to determine if they appear healthy.

  • Protect feed and water supplies from fecal contamination.

  • Be sure housing areas are thoroughly cleaned and disinfected between uses.

    Minimize stress .

If you have an ill or known infected horse:

  • Isolate the ill horse.

  • Have your horse examined and tested by your veterinarian to determine the cause of illness.

  • Start hygiene and cleaning routines for personnel working with infected horses.

  • Remove all feces promptly and dispose of them away from horse housing or grazing areas.

  • Thoroughly clean and sanitize all contaminated living areas.

    Minimize stress for all horses on the property.

  • Follow your veterinarian’s instructions for medication and care.

Although the signs of disease may disappear, eliminating the bacteria from the body is difficult, particularly in adult horses.

Control and prevention are focused on reducing the likelihood of exposure to the bacteria and promoting the overall health of the horse. A horse could be exposed to the bacteria by contacting the feces of an animal (for example, another horse or a bird or rodent) that is shedding the bacteria. It is advisable to make every effort to avoid introduction of a carrier animal that is shedding the bacteria into the environment. Horses should be purchased from farms known to be free of the disease and should be housed away from the resident horses for about 2 to 3 weeks while their health status is monitored. Feed and water supplies should be high quality and must be protected from fecal contamination by rodents, birds, or any other animal that may be shedding the bacteria in its feces. Should the bacteria be detected in the environment, any contaminated living areas should be thoroughly cleaned and disinfected. It is critical that a well-thought-out plan for cleaning be made and all label instructions on disinfecting products be followed. Because Salmonella can also infect humans, anyone working with infected horses or wastes from infected horses should be aware of the risks and the need for good personal hygiene.

Potomac Horse Fever

Potomac horse fever (also called equine monocytic ehrlichiosis) is a syndrome producing mild colic, fever, and diarrhea in horses of all ages, as well as loss of foals in pregnant mares. It is caused by Neorickettsia risticii bacteria. The bacteria are found in parasites called flukes, which have been isolated from freshwater snails and appear to be present in a number of insects. One route of exposure is believed to be the horse’s accidental ingestion of aquatic insects containing infected flukes. The disease is seen in spring, summer, and early fall and is associated with pastures bordering creeks or rivers. Having lights on in barns at night attracts insects, which may fall in feed or water buckets.

Infection of the small and large intestine results in inflammation of the large intestine (colitis), which is one of the principal signs of the disorder. Early signs of Potomac horse fever include mild depression and loss of appetite, followed by a mild to high fever. At this stage, a veterinarian may be able to detect decreased intestinal sounds.

Within 24 to 48 hours, a moderate to severe diarrhea and abdominal discomfort develops in about 60% of affected horses. The feces range from soft to watery. Some horses develop severe blood poisoning and dehydration. Founder (laminitis Laminitis (Founder) Among the many disorders that can affect the foot of a horse are laminitis, navicular disease, puncture wounds, infections, keratoma, pedal osteitis, pyramidal disease, quittor, sandcrack, scratches... read more ) can occur as a severe complication of the disorder in 20% to 30% of affected horses.

Several months following disease in pregnant mares, miscarriage of the foal due to fetal infection with Neorickettsia risticii may occur. The miscarriage is accompanied by an enlarged and retained placenta.

The diagnosis of Potomac horse fever can be suspected based on the history and presence of typical signs. A definitive diagnosis is made by identifying the Neorickettsia risticii bacteria in the blood and feces of infected horses using a DNA test.

Sick horses are not contagious and can be housed with other horses. Potomac horse fever can be treated successfully with an appropriate antibiotic, if it is given soon after the disease begins. A response to treatment is usually seen within 12 hours. This is associated with relief from fever, followed by an improvement in attitude, appetite, and bowel sounds. If treatment is started early, signs frequently resolve by the third day of treatment. Generally, antibiotics are given for no more than 5 days. In horses with signs of enterocolitis, fluids and a nonsteroidal anti-inflammatory drug will likely be prescribed. Founder, if it develops, is usually severe and often resistant to treatment. Overall, the disease is fatal in approximately 5% to 30% of cases.

Several vaccines are commercially available; however, they do not appear to be very effective. Minimizing insect ingestion by turning off barn lights at night may help prevent some cases. No risk to humans from this disease is known.

Clostridia-associated Intestinal Inflammation

Clostridium bacteria have been diagnosed as a cause of intestinal inflammation (enterocolitis) in horses and foals. Although some Clostridium bacteria are normally found in the intestines of many healthy horses and foals, certain variants of the bacteria that produce toxins appear to cause the disease. The exact role of the bacteria is still unclear, and it may be that multiple factors—such as stress, antibiotic use, and altered diet—are involved in the development of this disorder. Disease due to this infection is more common in foals but appears to occur in adult horses as well.

Signs of clostridia-associated intestinal inflammation include:

  • diarrhea with or without blood

  • colic

  • poor appetite

  • fever

  • lethargy

  • sudden death

Signs can range in severity from non-existent to fatal. Damage to the lining of the intestines can lead to bacterial infections in the bloodstream. These signs are similar to those seen in other causes of intestinal inflammation. Affected foals (usually less than 3 days old) often have bloody diarrhea and colic. In many cases, severe lethargy and rapid worsening of the horse's condition is followed by death in 1–2 days. Several foals on the same farm may be affected, but usually only one case occurs on a farm at a time. Diagnosis is based on signs and identification of the bacteria or the toxins produced by the bacteria in feces or tissue samples from the infected horse or foal. If you are asked to bring in a fecal sample for testing, follow your veterinarian’s instructions in regard to the handling of these samples to preserve the bacteria.

In humans, Clostridium bacteria have been identified as an infection common in hospitals. This may also be seen in horses receiving hospital care. Veterinary hospital personnel are aware of this possibility and take special precautions to prevent the spread of the disease.

Treatment with an antibiotic called metronidazole appears to be beneficial for this infection. Supportive care is similar to that for other causes of intestinal inflammation and includes intravenous fluids and anti-inflammatory drugs. Newborn foals may be given medication to help prevent stomach ulcers. Broad-spectrum antibiotics may also be prescribed. Foals with colic associated with ingestion of milk often require intravenous fluids and nutritional support. Drugs that are given via a tube into the stomach may help absorb the toxins produced by the bacteria and help the intestine heal. Intensive treatment may be required for several days until the condition improves. The death rate in horses with clostridial enterocolitis can be high, even with prompt medical treatment.

The most important strategy for prevention is good farm hygiene. Clostridial spores survive in the environment and are resistant to many disinfectants. Keeping the foaling area and mare as clean as possible during the period before and after foaling and ensuring the ingestion of colostrum (the first milk) within the first hour after birth have reduced incidence of disease on some farms. The mare’s hind legs, tail, and udder can also be washed in soapy water immediately after foaling to decrease ingestion of fecal material by newborn foals. Affected animals should be isolated to limit the spread of infection and contamination of pastures and stalls. Vaccines are available, but the safety and efficacy of these vaccines are not well established.


Colitis-X is a term used to describe undiagnosed causes of an extremely rapid, fatal intestinal inflammation of horses that causes a sudden onset of profuse, watery diarrhea and development of shock. Many affected horses have a history of stress. The cause of colitis-X is unknown, but an undiagnosed case of severe salmonellosis or infection with clostridia (which can be difficult to identify) may be to blame. Disease onset is often closely associated with stress such as surgery, anesthesia, strenuous athletic events, or transport.

The disease may begin with a short period of fever, but the horse’s temperature soon returns to normal or below normal. Rapid breathing, rapid heart rate, and lethargy are present. Severe diarrhea develops, followed by extreme dehydration and shock. Death may occur within 3 hours of onset of signs. In some horses the progression of the disease is so rapid that the only sign is sudden death. In less sudden cases, death occurs within 24 to 48 hours.

Treatment for colitis-X usually is not effective but is similar to that for salmonellosis Salmonellosis Diarrhea, weight loss, abdominal pain, and protein loss are common signs of intestinal disorders in horses. These signs can be caused by many different disorders, including infectious diseases... read more . Large volumes of intravenous fluids are needed to treat the severe dehydration, and electrolyte (salt) replacement is often necessary. Drugs to reduce inflammation, bind bacterial toxins, and treat bacterial infections are often used.

Coronavirus in Horses

Coronavirus has been identified in the feces of healthy foals and those with intestinal disease. However, recent outbreaks of diarrhea and colic in adult horses are thought to be caused by coronavirus.

Signs of coronavirus include:

  • unwillingness to eat

  • lethargy

  • fever

  • possibly abdominal pain (colic) and stool changes

Occasionally, the disease worsens rapidly and leads to death (or euthanasia), but most cases resolve with supportive care (such as fluid administration and nutritional support). The diagnosis of coronavirus is made by identifying the virus in a horse's feces.


Intestinal Disease Caused by Eating Sand or Dirt

Consumption of large amounts of sand, which then accumulates in the large intestine, can produce diarrhea, weight loss, or colic Obstruction of the Small or Large Intestine Over the years, colic has become a broad term for a variety of conditions that cause horses to experience abdominal pain. Because it is such a broad term, it is used to refer to conditions that... read more . Sand may be eaten accidentally along with food when the horse or foal is kept on sandy pasture or is fed hay or grain in a sandy area (paddock, stall, or pasture). Some horses or foals develop a habit of eating dirt and sand if it is in their environment.

A veterinarian can diagnose this condition based on history of exposure to a sandy environment, the presence of sand in the feces, “sand sounds” in the abdomen, and abdominal x-rays or ultrasonography. Treatment involves administering a fiber product (usually psyllium seed hull) by a tube inserted up through the nostril and into the stomach or added to the concentrate feed daily. Diarrhea generally resolves within 2 to 3 days after the start of treatment; however, 3 to 4 weeks of treatment is often necessary to remove all of the sand. The treatment may need to be repeated if the horse or foal is not removed from the source of sand. Preventive treatment with a fiber product may be recommended in areas where this condition is common.

If enough sand accumulates in the small colon, an obstruction can occur. These sand impactions can be treated medically by pumping psyllium and warm water into the stomach, with pain medications, and with intravenous fluids. Surgery is necessary if the sand completely obstructs the colon. The outlook after surgery is usually good.

Gastrointestinal Tumors

Several types of tumors can affect the digestive tract, the most common of which are squamous cell carcinoma and the intestinal (alimentary) form of lymphosarcoma. The primary sign is usually chronic weight loss. However, gastrointestinal tumors are rare, so veterinarians often investigate other more common causes of weight loss first. Longterm diarrhea and low protein levels in the blood (hypoproteinemia) may also be seen. Diagnosis is usually made by excluding other possible causes of weight loss and by examining tissue collected during exploratory surgery. Some cancerous conditions can be diagnosed using endoscopy, ultrasound, rectal examination, or microscopic examination of abdominal fluid. Treatment of cancerous gastrointestinal tumors in horses is generally not attempted because the outlook for longterm recovery is poor. However, a few cases of gastrointestinal cancer have been treated, which improved the well being of the horses at least temporarily.

Inflammatory Bowel Disease

This disease occurs when inflammatory cells accumulate in the small and large intestine and regional lymph nodes. The inflammation may be limited to only a short segment of the bowel or it may be more widespread. This condition interferes with absorption of nutrients and causes a loss of protein from the blood. Affected horses may have repeated episodes of colic or widespread skin disease. Diarrhea may or may not be present. Diagnosis is based on signs, physical examination, tests for low blood protein or malabsorption, and intestinal or rectal biopsy. Your veterinarian may detect thickened intestines or enlarged abdominal lymph nodes based on rectal examination or ultrasonography. The cause of this disease is not well understood. An altered immune response to a common intestinal exposure (such as feed, parasites, or bacteria) has been suggested. Standardbreds appear to have a higher risk for developing some forms of inflammatory bowel disease.

Medical treatments that may reduce the inflammatory response in the intestine have been tried with limited success. Supportive nutritional care is often prescribed. The usual program involves the frequent feeding of good-quality, high-energy feeds. The longterm outlook is frequently poor. If only a limited and accessible section of the bowel is affected, your veterinarian may recommend surgery to remove the affected portion of the intestine.

Nonsteroidal Anti-inflammatory Drug Toxicity

If the drugs phenylbutazone, flunixin meglumine, or other nonsteroidal anti-inflammatory drugs (often called NSAIDs) are administered at high doses or for prolonged periods, they can cause damage to the digestive tract or kidneys. Some horses are more sensitive to these drugs and can experience side effects even at low dosages. Although drug-induced damage can occur anywhere in the digestive tract, the stomach and large colon are most likely to be affected. Ulcers Stomach (Gastric) Ulcers in Horses A gastric ulcer is a sore in the stomach lining that occurs when the lining has been damaged by stomach acid and digestive enzymes. Ulcers can also be seen in the lower esophagus and in the... read more (sores within the lining of the digestive tract) within the large colon can cause protein to be lost from the blood into the intestines. This protein loss can also occur in horses that do not have ulcers.

Signs of NSAID toxicity include:

  • mouth sores

  • loss of appetite

  • lethargy

  • weight loss

  • diarrhea

  • accumulation of fluid under the skin of the limbs or underbelly

  • drooling

  • colic

Your veterinarian can make a tentative diagnosis based on the history of drug administration, signs consistent with this condition, and the presence of reduced blood protein. Stomach ulcers can be confirmed using an endoscope inserted through the mouth or nasal passages and then into the stomach. Treatment includes discontinuing the use of phenylbutazone or any other nonsteroidal anti-inflammatory drug. Reducing the production of stomach acid with medications may be beneficial. Intravenous fluids may be necessary, especially in dehydrated horses or those with kidney damage. In cases of sudden toxicity, mineral oil may be administered with a stomach tube to decrease the absorption of the offending drug. Changes in diet can help some horses. Surgery may be required if scarring of the intestines has resulted in partial obstruction.

To prevent NSAID drug toxicity, it is important to always follow your veterinarian's treatment and monitoring recommendations closely. Give all medications as directed, and do not give them more often than recommended. Blood tests to monitor the level of protein in the blood may be necessary. Finally, let your veterinarian know right away if there are any changes in the consistency of your horse's feces.

Small-intestinal Fibrosis in Horses

Some adult horses on pasture in northern Colorado have developed fibrosis (the excessive formation of fibrous connective tissue) within the wall of the small intestine. Signs include weight loss and repeated episodes of colic. All affected horses died or were euthanized because of their deteriorating condition. The cause is unknown.

Malabsorption and Maldigestion

Sometimes horses have a defect in the ability of the digestive tract to incorporate nutrients into the body. There are two main causes: malabsorption and maldigestion.

Malabsorption is the failure of nutrients to pass from the inside of the intestines to the bloodstream. Many diseases can cause a malabsorption syndrome by altering the normal function of the small intestine. Protein loss may coexist with this condition and prove more harmful than the malabsorption itself. Maldigestion is the inability to digest certain foods within the intestines. It can be caused by a missing or insufficient digestive enzyme, impaired stomach function, or abnormal bacterial growth in the small intestine. In horses, diseases of malabsorption are much more common than are diseases of maldigestion. Some diseases involve both maldigestion and malabsorption, such as is seen in foals with lactase deficiency, an inability to digest the sugar in milk (see below).


Signs of malabsorption and maldigestion vary, depending on the underlying disease condition. Insufficient energy, weight loss, and possibly low blood protein concentrations are characteristics of these syndromes. Longterm weight loss, muscle wasting, or a reduced growth rate are common. Excessive eating may be seen, because the nutrients that are taken in do not effectively stimulate the brain areas that normally indicate fullness. More commonly with small-intestinal malabsorption, loss of appetite is present. Diarrhea may occur; however, small-intestinal disease may be extensive before diarrhea develops because the large intestine can compensate and absorb the increased fluid. In adult horses, diarrhea usually indicates large-intestinal disease.

Abnormal pain may result from bowel inflammation, abscesses, adhesions, or partial obstruction. Abnormal fluid buildup under the skin (edema) or in the abdomen (ascites) and weakness may develop later in the disease, especially if protein loss is present. Skin and eye abnormalities, arthritis, and liver or kidney disease may indicate immune system reactions, particularly with inflammatory bowel disease. Skin abnormalities seen with malabsorption-related skin disease include a thin hair coat, patchy hair loss, and areas of scaling and crusting that are often symmetrical.

Small-intestinal malabsorption cannot be determined by physical examination or by routine laboratory data. The veterinarian must exclude more common causes of weight loss before a diagnosis can be made. Determination of the primary underlying disease process is also necessary to establish an appropriate treatment regimen and outlook. The veterinarian will typically ask about the horse's history (for example, deworming and feeding protocols), perform physical and rectal examinations, and recommend appropriate diagnostic tests (for example, blood, urine, and fecal tests; analysis of abdominal fluid; x-rays; ultrasonography; endoscopy; absorption tests; and tissue biopsies).


The causes of the disease leading to malabsorption must be determined before treatment can be started. Specific treatment for most causes is not available; however, if the cause is parasite damage, it can sometimes be corrected with antiparasitic medications. Anti-inflammatory agents (such as nonsteroidal anti-inflammatory drugs or corticosteroids) may help decrease inflammation within the intestine, although care must be taken in their use. Follow your veterinarian’s instructions about dosages and administration times carefully.

Supportive care may also be necessary. This may include a special diet to improve digestion in the large intestine. Horses that will not eat may have to be force-fed through a stomach tube. Intravenous feeding may be necessary for horses that refuse to eat or for those that cannot tolerate force-feeding. However, intravenous nutrition is difficult to continue on a longterm basis. For most adult horses with conditions causing malabsorption or maldigestion, the treatment is often unsuccessful and the outlook for recovery is poor.

Malabsorption and weight loss sometimes occur after viral enteritis, because the infection destroys the villi in the intestine. The villi are hair-like projections of the inner surface of the intestine that increase the area for uptake of nutrients. The loss of the villi means a reduction in the nutrients absorbed from food. Replacement of the villi may take weeks to months in severe cases.

Most of the conditions that cause malabsorption or maldigestion in adult horses have a poor outlook, and treatment is often unsuccessful. However, horses with parasite infections can be successfully treated. Short-term improvement may be seen with treatment in some cases.

Foals with Lactase Deficiency (Lactose Intolerance)

Lactose is a sugar found in milk and is broken down by the enzyme lactase. Horses with a lactase deficiency do not digest lactose properly. Lactase deficiency may be present at birth (congenital). However, typically it develops after infectious organisms (viruses, bacteria, or parasites) damage the cells that produce lactase. This results in temporary maldigestion. Foals with lactase deficiency commonly have bloating after intake of milk, dehydration, and weight loss.

Foals with temporary lactase deficiency caused by diarrheal diseases often respond well to feeding of lactase-treated milk until the small-intestinal lining has regenerated. Foals that can tolerate it should be fed small amounts of high-quality roughage or grain to help meet their energy needs. Young foals that do not respond to lactase-treated milk may benefit from short-term intestinal rest (withdrawal of milk feeding for less than 24 hours) while the intestinal lining heals. Your veterinarian will recommend an alternative source of energy and nutrients during this time. Dietary change to a soy-based, non-lactose-containing milk replacer and early weaning may be necessary for foals with lactase deficiency that does not respond to other treatments.

Other Causes of Diarrhea in Horses

Some horses develop soft feces when first introduced to lush pastures, alfalfa hay, or a temporarily stressful situation, such as a trailer ride, racing, showing, or visit to a veterinary hospital. This change in fecal consistency is not of medical significance as long as the horse is healthy in all other regards. It is important that horses with diarrhea have a physical examination and appropriate laboratory tests to exclude infectious causes and to determine whether treatment is required. Usually, the fecal consistency returns to normal when the horse adapts to its new diet or the stressful situation resolves.

Other causes of diarrhea or semiformed to watery feces in horses include grain overload, heart failure that results in thickening of the intestinal wall with retained fluid, blood clots within the intestinal blood supply, peritonitis (inflammation of the lining of the abdomen), antibiotic treatment, kidney or liver failure, and numerous poisons (such as certain chemicals, plants, and insects). Diarrhea can also be seen while an impaction of the large intestine is resolving or being treated.

Diarrhea in Foals

Diarrhea in foals may be caused by bacterial or viral infection, parasites, and nutritional or environmental changes.

Foal Heat Diarrhea

At about 4 to 14 days after birth, foals often develop a mild diarrhea that resolves on its own. Although the cause is unknown, it may be associated with changes in the normal bacteria in the foal’s intestines or alteration in diet as the foal begins to eat small amounts of hay and grain. Eating its mother's stool (a normal behavior for foals) may also play a role. The condition is often referred to as foal heat diarrhea because it appears at about the same time as the dam is undergoing her first estrous cycle after the foal’s birth.

The foal remains active and alert and has a normal appetite. Vital signs remain normal. Feces are semiformed to watery and not foul-smelling or passed more frequently than is normal. Monitoring is important to ensure the foal’s condition does not worsen. Specific treatment is usually not necessary, but application of a protectant to the skin around the buttocks helps prevent irritation from the diarrhea.

Bacterial Diarrhea

Several bacterial infections can cause intestinal inflammation and blood infection in newborn and young foals. Organisms that can be involved in diarrhea include Salmonella and Clostridium species, Escherichia coli, and others. Intensive antibiotic treatment, correction of fluid loss and electrolyte (salt) abnormalities, and nursing care are usually needed. If a veterinarian determines that transfer of antibodies from the mother during suckling was not sufficient, a blood plasma transfer into the foal’s blood may be performed. Your veterinarian will prescribe medication based on the diagnosis, the condition of the foal, and other factors.

Intestinal infection with the bacteria Lawsonia intracellularis has been associated with outbreaks of diarrhea, rapid weight loss, colic, accumulation of fluid under the skin (edema), and lowered levels of protein (due to loss from the intestines) in weanling foals. The bacteria infects foals from 3 to 12 months of age, but 4- to 6-month old foals are most commonly affected. Severely ill foals may also develop infections in other parts of the body, including the lungs and skin. Stress may cause foals to be more prone to infection. Most foals recover with proper treatment, but sudden death can also occur. The diagnosis involves fecal and blood tests. Because it is difficult to diagnose this condition, some veterinarians will initiate treatment for this bacterium in foals when other causes of diarrhea have been excluded and there is evidence of exposure. Treatment with appropriate antibiotics has been successful, and a response to this treatment is considered confirmation of the diagnosis. With treatment, ~90% of foals survive.

Viral Diarrhea in Foals

Viruses appear to cause diarrhea in foals but rarely affect adult horses. Rotavirus is the main cause of viral diarrhea in foals, but other viruses (such as coronavirus) may also be responsible. Signs of rotaviral infection can include lethargy, colic, loss of appetite, and profuse, watery, foul-smelling diarrhea. It is usually seen in foals less than 2 months old. Younger foals typically have more severe signs. The diarrhea usually lasts 4 to 7 days, although it can persist for weeks. Rotavirus destroys cells in the lining of the small intestine, causing poor absorption of nutrients. Lactase, an enzyme required for digestion of milk, becomes deficient. When this occurs, undigested lactose passing into the large intestine causes diarrhea. The diagnosis is made by identifying the virus in feces using laboratory tests. Treatment is generally supportive.

This type of diarrhea is highly contagious. In situations where multiple foals are housed on the same farm, sick foals should be isolated in the stall or barn in which the foal originally became ill or moved to a special isolation facility. Strict hygiene and disinfection practices should be followed, including use of disposable gloves and foot covers, handwashing, and disinfection of stalls and equipment with compounds that meet Environmental Protection Agency standards. Because stalls with dirt floors are difficult to adequately clean and disinfect, removal of the top layers of dirt may be required. Fecal material of sick foals removed from stalls should not be spread on pastures that are used for horses and foals. Care should be taken with all equipment that may contact the manure of ill foals so that it is either thoroughly cleaned and disinfected prior to use with healthy foals or is used only for care of ill foals. For example, a wheelbarrow and pitchfork used for cleaning stalls of ill foals should never be used to feed foals and should never be used to clean the stalls of healthy foals unless it is thoroughly cleaned and disinfected.

A vaccine is available for pregnant mares to help protect their foals. The vaccine causes the mares to produce antibodies to rotavirus. These protective antibodies are passed to their foals during suckling of the first milk (colostrum) from the mare. Upon arriving at a farm, new horses and residents that have been traveling should be isolated from other horses for at least 7 days to reduce the risk of infection.

Miscellaneous Causes of Diarrhea in Foals

Nutritional diarrhea can result from overfeeding (for example, when a foal is reunited with the mare after a period of separation) and improper nutrition (as when orphan foals are being fed calf milk, a replacer formula, or sucrose). Lactose intolerance in foals is rare and can be determined by a lactose tolerance challenge test. Diarrhea can also develop when foals consume indigestible substances such as roughage, sand, dirt, or rocks.

For More Information

quiz link

Test your knowledge

Take a Quiz!