Colitis is inflammation of the colon. Acute cases are medical emergencies as the horse rapidly loses fluid, protein, and electrolytes into the gut, leading to severe dehydration which can result in hypovolemic shock and death. Horses generally present with signs of colic before developing profuse, watery, fetid diarrhea.

Both infectious and non-infectious causes for colitis exist. In the adult horse, "Salmonella", "Clostridium difficile", and "Neorickettsia risticii" (the causative agent of Potomac Horse Fever) are common causes of colitis. Antibiotics, which may lead to an altered and unhealthy microbiota, sand, grain overload, and toxins such as arsenic and cantharidin can also lead to colitis. Unfortunately, only 20–30% of acute colitis cases are able to be definitively diagnosed. NSAIDs can cause slower-onset of colitis, usually in the right dorsal colon (see Right dorsal colitis).

Treatment involves administration of large volumes of intravenous fluids, which can become very costly. Antibiotics are often given if deemed appropriate based on the presumed underlying cause and the horse's CBC results. Therapy to help prevent endotoxemia and improve blood protein levels (plasma or synthetic colloid administration) may also be used if budgetary constraints allow. Other therapies include probiotics and anti-inflammatory medication. Horses that are not eating well may also require parenteral nutrition. Horses usually require 3–6 days of treatment before clinical signs improve.

Due to the risk of endotoxemia, laminitis is a potential complication for horses suffering from colitis, and may become the primary cause for euthanasia. Horses are also at increased risk of thrombophlebitis.

Horses form ulcers in the stomach fairly commonly, a disease called equine gastric ulcer syndrome. Risk factors include confinement, infrequent feedings, a high proportion of concentrate feeds, such as grains, excessive non-steroidal anti-inflammatory drug use, and the stress of shipping and showing. Gastric ulceration has also been associated with the consumption of cantharidin beetles in alfalfa hay which are very caustic when chewed and ingested. Most ulcers are treatable with medications that inhibit the acid producing cells of the stomach. Antacids are less effective in horses than in humans, because horses produce stomach acid almost constantly, while humans produce acid mainly when eating. Dietary management is critical. Bleeding ulcers leading to stomach rupture are rare.

Cholesterol gallstone formation risk factors include age, female sex, family history, race, pregnancy, parity, obesity, birth control, diabetes mellitus, cirrhosis, prolonged fasting, rapid weight loss, total parenteral nutrition, ileal disease and impaired gallbladder emptying.

Patients that have gallstones and biliary colic are at increased risk for complications, including cholecystitis. Complications from gallstone disease is 0.3% per year and therefore prophylactic cholecystectomy are rarely indicated unless part of a special population that includes porcelain gallbladder, individuals eligible for organ transplant, diabetics and those with sickle cell anemia.

Infants who are colicky do just as well as their non colicky peers with respect to temperament at one year of age.

The cause of colic is generally unknown. Fewer than 5% of infants who cry excessively turn out to have an underlying organic disease, such as constipation, gastroesophageal reflux disease, lactose intolerance, anal fissures, subdural hematomas, or infantile migraine. Babies fed cow's milk have been shown to develop antibody responses to the bovine protein, causing colic. Studies performed showed conflicting evidence about the role of cow's milk allergy. While previously believed to be related to gas pains, this does not appear to be the case. Another theory holds that colic is related to hyperperistalsis of the digestive tube (increased level of activity of contraction and relaxation). The evidence that the use of anticholinergic agents improve colic symptoms supports this hypothesis.

Psychological and social factors have been proposed as a cause, but there is no evidence. Studies performed don't support the theory that maternal (or paternal) personality or anxiety causes colic, nor that it is a consequence of a difficult temperament of the baby, but families with colicky children may eventually develop anxiety, fatigue and problems with family functioning as a result. There is some evidence that cigarette smoke may increase the risk. It seems unrelated to breast or bottle feeding with rates similar in both groups. Reflux does not appear to be related to colic.

Ileus is a cause of colic in horses due to functional obstruction of the intestines. It most commonly seen in horses postoperatively, especially following colic surgery. Horses experiencing ileus are at risk for gastric rupture due to rapid reflux build-up, and require intense medical management with frequent nasogastric intubation. Ileus may increase adhesion formation, because intestinal segments have more prolonged contact and intestinal distention causes serosal injury and ischemia. It is usually treated with aggressive fluid support, prokinetics, and anti-inflammatories.

Biliary pain is most frequently caused by obstruction of the common bile duct or the cystic duct by a gallstone. However, the presence of gallstones is a frequent incidental finding and does not always necessitate treatment, in the absence of identifiable disease. Furthermore, biliary pain may be associated with functional disorders of the biliary tract, so called acalculous biliary pain (pain without stones), and can even be found in patients post-cholecystectomy (removal of the gallbladder), possibly as a consequence of dysfunction of the biliary tree and the sphincter of Oddi. Acute episodes of biliary pain may be induced or exacerbated by certain foods, most commonly those high in fat.

Colic (from Greek κολικός "kolikos", "relative to the colon") or cholic is a form of pain that starts and stops abruptly. It occurs due to muscular contractions of a hollow tube (colon, ureter, gall bladder, etc.) in an attempt to relieve an obstruction by forcing content out. It may be accompanied by vomiting and sweating. Types include:

- Baby colic, a condition, usually in infants, characterized by incessant crying

- Renal colic, a pain in the flank, characteristic of kidney stones

- Biliary colic, blockage by a gallstone of the common bile duct or cystic duct

- Horse colic, a potentially fatal condition experienced by horses, caused by intestinal displacement or blockage

- Devon colic, an affliction caused by lead poisoning

- Painter's colic or lead poisoning

The differential diagnoses of acute abdomen include but are not limited to:

1. Acute appendicitis

2. Acute peptic ulcer and its complications

3. Acute cholecystitis

4. Acute pancreatitis

5. Acute intestinal ischemia (see section below)

6. Acute diverticulitis

7. Ectopic pregnancy with tubal rupture

8. Ovarian torsion

9. Acute peritonitis (including hollow viscus perforation)

10. Acute ureteric colic

11. Bowel volvulus

12. Bowel obstruction

13. Acute pyelonephritis

14. Adrenal crisis

15. Biliary colic

16. Abdominal aortic aneurysm

17. Familial Mediterranean fever

18. Hemoperitoneum

19. Ruptured spleen

20. Kidney stone

21. Sickle cell anaemia

Cholecystitis occurs when the gallbladder becomes inflamed. Gallstones are the most common cause of gallbladder inflammation but it can also occur due to blockage from a tumor or scarring of the bile duct. The greatest risk factor for cholecystitis is gallstones. Risk factors for gallstones include female sex, increasing age, pregnancy, oral contraceptives, obesity, diabetes mellitus, ethnicity (Native North American), rapid weight loss.

The inflammation of cholecystitis can lead to adhesions between the gallbladder and other parts of the gastrointestinal tract, most commonly the duodenum. These adhesions can lead to the formation of direct connections between the gallbladder and gastrointestinal tract, called fistulas. With these direct connections, gallstones can pass from the gallbladder to the intestines. Gallstones can get trapped in the gastrointestinal tract, most commonly at the connection between the small and large intestines (ileocecal valve). When a gallstone gets trapped, it can lead to an intestinal obstruction, called gallstone ileus, leading to abdominal pain, vomiting, constipation, and abdominal distension.

Paralysis of the intestine is often termed paralytic ileus, in which the intestinal paralysis need not be complete, but it must be sufficient to prohibit the passage of food through the intestine and lead to intestinal blockage. Paralytic ileus is a common side effect of some types of surgery, commonly called postsurgical ileus. It can also result from certain drugs and from various injuries and illnesses, such as acute pancreatitis. Paralytic ileus causes constipation and bloating. On listening to the abdomen with a stethoscope, no bowel sounds are heard because the bowel is inactive.

A temporary paralysis of a portion of the intestines occurs typically after abdominal surgery. Since the intestinal content of this portion is unable to move forward, food or drink should be avoided until peristaltic sound is heard, by auscultation (use of a stethoscope) of the area where this portion lies. Intestinal atony or paralysis may be caused by inhibitory neural reflexes, inflammation or other implication of neurohumoral peptides.

The clinical course of biliary sludge can do one of three things: (1) it can resolve completely, (2) wax and wane, or (3) progress to gallstones. If the biliary sludge has a cause (e.g. pregnancy), it oftentimes is resolved when the underlying cause is removed.

The prevalence of biliary sludge is low in the general population. It has been reported that the prevalence ranges from 0-0.20% in men and 0.18-0.27% in women. However, in patients with certain conditions, the prevalence may be higher.

Acute abdomen is occasionally used synonymously with peritonitis. While this is not entirely incorrect, peritonitis is the more specific term, referring to inflammation of the peritoneum. It manifests on physical examination as rebound tenderness, or pain upon "removal" of pressure more than on "application" of pressure to the abdomen. Peritonitis may result from several of the above diseases, notably appendicitis and pancreatitis. While rebound tenderness is commonly associated with peritonitis, the most specific finding is rigidity.

Intestinal decompression by tube placement in a small stoma can also be used to reduce distension and pressure within the gut. The stoma may be a gastrostomy, jejunostomy, ileostomy or cecostomy, and may also be used to feed, in the case of gastrostomy and jejunostomy, or flush the intestines.

Colostomy or ileostomy can bypass affected parts if they are distal to (come after) the stoma. For instance, if only the large colon is affected, an ileostomy may be helpful. Either of these ostomies are typically placed at or a few centimeters below the patients belly button per doctor recommendation based on the affected area of the intestines as well as concerns for patient comfort and future physical growth for children.

The total removal of the colon, called a colectomy or resection of affected parts of the colon may be needed if part of the gut dies (for instance toxic megacolon), or if there is a localised area of dysmotility.

Gastric and colonic pacemakers have been tried. These are strips placed along the colon or stomach which create an electric discharge intended to cause the muscle to contract in a controlled manner.

A potential solution, albeit radical, is a multi-organ transplant. The operation involved transplanting the pancreas, stomach, duodenum, small intestine, and liver, and was performed by Doctor Kareem Abu-Elmagd on Gretchen Miller, the subject of the Discovery Channel program "Surgery Saved My Life".

In primary chronic intestinal pseudo-obstruction (the majority of chronic cases), the condition may be caused by an injury to the smooth muscle (myopathic) or the nervous system (neuropathic) of the gastrointestinal tract.

In some cases there appears to be a genetic association. One form has been associated with DXYS154.

Secondary chronic intestinal pseudo-obstruction can occur as a consequence of a number of other conditions, including Kawasaki disease, Parkinson's disease, Chagas' disease, Hirschsprung's disease, intestinal hypoganglionosis, collagen vascular diseases, mitochondrial disease, endocrine disorders and use of certain medications. The term may be used synonymously with enteric neuropathy if a neurological cause is suspected.

It is a serious medical disorder and the mortality rate can be as high as 30%. The high mortality rate is likely a measure that this syndrome is seen in critically ill patients, rather than this syndrome being in itself lethal, although it can also present in otherwise healthy individuals (especially if the disorder was induced by pharmacologic agents). Drug induced megacolon (i.e. from Clozapine) has been associated with mortality as high as 27.5%.

Equine enteroliths are found by walking pastures or turning over manure compost piles to find small enteroliths, during necroscopy, and increasingly, during surgery for colic. Therefore, the incidence of asymptomatic enteroliths is unknown.

Equine enteroliths typically are smoothly spherical or tetrahedral, consist mostly of the mineral struvite (ammonium magnesium phosphate), and have concentric rings of mineral precipitated around a nidus.

Enteroliths in horses were reported widely in the 19th century, infrequently in the early 20th century, and now increasingly. They have also been reported in zebras: five in a zoo in California and one in a zoo in Wisconsin. Struvite enteroliths are associated with elevated pH and mineral concentrations in the lumen. In California, struvite enteroliths are associated also with a high proportion of alfalfa in the feed and less access to grass pasture. This association has been attributed to the cultivation of alfalfa on serpentine soils, resulting in high concentrations of magnesium in the alfalfa.

There is considerable research into the causes, diagnosis and treatments for FGIDs. Diet, microbiome, genetics, neuromuscular function and immunological response all interact. Heightened mast cell activation has been proposed to be a common factor among FGIDs, contributing to visceral hypersensitivity as well as epithelial, neuromuscular, and motility dysfunction.

Laparoscopic cholecystectomy has been used to treat the condition when due to dyskinesia of the gallbladder.

Symptoms may persist after cholecystectomy, and have been linked to the use of proton pump inhibitors.

Osteopathic treatment, oral magnesium supplementation with 325 mg and the use of digestive enzymes caused improvement in one case.

Biliary dyskinesia is a disorder of some component of biliary part of the digestive system in which bile physically can not move normally in the proper direction through the tubular biliary tract. It most commonly involves abnormal biliary tract peristalsis muscular coordination within the gallbladder in response to dietary stimulation of that organ to squirt the liquid bile through the common bile duct into the duodenum. Ineffective peristaltic contraction of that structure produces postprandial (after meals) right upper abdominal pain (cholecystodynia) and almost no other problem. When the dyskinesia is localized at the biliary outlet into the duodenum just as increased tonus of that outlet sphincter of Oddi, the backed-up bile can cause pancreatic injury with abdominal pain more toward the upper left side. In general, biliary dyskinesia is the disturbance in the coordination of peristaltic contraction of the biliary ducts, and/or reduction in the speed of emptying of the biliary tree into the duodenum.

Ogilvie syndrome is the acute dilation of the colon in the absence of any mechanical obstruction in severely ill patients.

Colonic pseudo-obstruction is characterized by massive dilatation of the cecum (diameter > 10 cm) and right colon on abdominal X-ray. It is a type of megacolon, sometimes referred to as "acute megacolon", to distinguish it from toxic megacolon.

The condition carries the name of the British surgeon Sir (1887–1971), who first reported it in 1948.

Renal colic typically begins in the flank and often radiates to the hypochondrium (the part of the anterior abdominal wall below the costal margins) or the groin. It is typically colicky (comes in waves) due to ureteric peristalsis, but may be constant. It is often described as one of the strongest pain sensations known.

Although this condition can be very painful, kidney stones usually cause no permanent physical damage. The experience is said to be traumatizing due to pain, and the experience of passing blood, blood clots, and pieces of the stone. Depending on the sufferer's situation, nothing more than drinking significant amounts of water may be called for; in other instances, surgery may be needed. Preventive treatment can be instituted to minimize the likelihood of recurrence.

In humans, enteroliths are rare and may be difficult to distinguish from gall stones. Their chemical composition is diverse, and rarely can a nidus be found. A differential diagnosis of an enterolith requires the enterolith, a normal gallbladder, and a diverticulum.

An enterolith typically forms within a diverticulum. An enterolith formed in a Meckel's diverticulum sometimes is known as a Meckel's enterolith. Improper use of magnesium oxide as a "long-term" laxative has been reported to cause enteroliths and/or medication bezoars.

Most enteroliths are not apparent and cause no complications. However, any complications that do occur are likely to be severe. Of these, bowel obstruction is most common, followed by ileus and perforation. Bowel obstruction and ileus typically occur when a large enterolith is expelled from a diverticulum into the lumen. Perforation typically occurs within the diverticulum.

Most human enteroliths are radiolucent on plain X-rays. They sometimes can be visualized on CT scans without contrast; presence of contrast in the lumen may reveal the enterolith as a void. Most often, they are visualized using ultrasound.

Although recent surveys of enterolith composition are lacking, one early review notes struvite (as in equines), calcium phosphate, and calcium carbonate and reports choleic acid. Deoxycholic acid and cholic acid have also been reported.