Proximal enteritis, also known as anterior enteritis or duodenitis-proximal jejunitis (DPJ), is inflammation of the duodenum and upper jejunum. It produces a functional stasis of the affected intestine (ileus) and hypersecretion of fluid into the lumen of that intestine. This leads to large volumes of gastric reflux, dehydration, low blood pressure, and potentially shock. Although the exact cause is not yet definitively known, proximal enteritis requires considerable supportive care.

Signs include acute onset of moderate to severe pain, large volumes of gastric reflux (4–20 L per decompression) which is usually orange-brown and fetid, distended small intestine (up to 5–7 cm in diameter) palpable on rectal examination, fever, depression, increased heart rate (>60 bpm), increased respiratory rate, prolonged CRT, and darkened mucous membranes. After gastric decompression, the horse may show signs of malaise and act lethargic, but pain level usually improves.

Abdominocentesis usually reveals a yellow, turbid fluid with an increased white blood cell count (usually 5,000–10,000 cells/microliter) and protein level (>3.5 g/dl), although the fluid may be serosanginous in severe cases. A chemistry panel will often show electrolyte abnormalities (hypokalemia, hyponatremia, hypochloremia) due to electrolyte loss into the lumen of the intestine. Leukocytes may be normal, increased, or decreased. PCV and total protein are usually both increased due to fluid loss, and the horse displays a prerenal azotemia. On the chemistry panel, liver enzymes such as GGT, ALP, AST are increased, likely due to ascending infection from the common bile duct, endotoxin absorption, and hypoperfusion. A metabolic acidosis with a high anion gap is often seen due to loss of bicarbonate in gastric reflux and an increase in lactic acid in the blood, secondary to hypovolemia and decreased tissue perfusion.

Signs and symptoms of enteritis are highly variable and vary based on the specific cause and other factors such as individual variance and stage of disease.

Symptoms may include abdominal pain, cramping, diarrhoea, dehydration, fever, nausea, vomiting and weight loss.

Uterine tears often occur a few days post parturition. They can lead to peritonitis and require surgical intervention to fix. Uterine torsions can occur in the third trimester, and while some cases may be corrected if the horse in anesthetized and rolled, others require surgical correction.

Strictly speaking, colic refers only to signs originating from the gastrointestinal tract of the horse. Signs of colic may be caused by problems other than the GI-tract e.g. problems in the liver, ovaries, spleen, urogenital system, testicular torsion, pleuritis, and pleuropneumonia. Diseases which sometimes cause symptoms which appear similar to colic include uterine contractions, laminitis, and exertional rhabdomyolysis. Colic pain secondary to kidney disease is rare.

Enteritis is inflammation of the small intestine. It is most commonly caused by food or drink contaminated with pathogenic microbes. but may have other causes such as NSAIDs, cocaine, radiation therapy as well as autoimmune conditions like Crohn's disease and coeliac disease. Symptoms include abdominal pain, cramping, diarrhoea, dehydration, and fever. Related diseases include inflammation of the stomach (gastritis) and large intestine (colitis).

Duodenitis, jejunitis and ileitis are subtypes of enteritis which are only localised to a specific part of the small intestine. Inflammation of both the stomach and small intestine is referred to as gastroenteritis. Inflammation of related organs of the gastrointestinal system are:

- gastritis

- gastroenteritis

- colitis

- enterocolitis

Specific types of enterocolitis include:

- necrotizing enterocolitis (most common in premature infants)

- pseudomembranous enterocolitis (also called "Pseudomembranous colitis")

Enterocolitis or coloenteritis is an inflammation of the digestive tract, involving enteritis of the small intestine and colitis of the colon. It may be caused by various infections, with bacteria, viruses, fungi, parasites, or other causes. Common clinical manifestations of enterocolitis are frequent diarrheal defecations, with or without nausea, vomiting, abdominal pain, fever, chills, alteration of general condition. General manifestations are given by the dissemination of the infectious agent or its toxins throughout the body, or – most frequently – by significant losses of water and minerals, the consequence of diarrhea and vomiting.

Among the causal agents of acute enterocolitis are:

- bacteria: "Salmonella", "Shigella", "Escherichia coli", "Campylobacter" etc.;

- viruses: enteroviruses, rotaviruses, Norwalk virus, adenoviruses;

- fungi: candidiasis, especially in immunosuppressed patients or who have previously received prolonged antibiotic treatment;

- parasites: "Giardia lamblia" (with high frequency of infestation in the population, but not always with clinical manifestations), "Balantidium coli", "Blastocystis homnis", "Cryptosporidium" (diarrhea in people with immunosuppression), "Entamoeba histolytica" (produces the amebian dysentery, common in tropical areas).

CNE is a necrotizing inflammation of the small bowel (especially the jejunum but also the ileum). Clinical results may vary from mild diarrhea to a life-threatening sequence of severe abdominal pain, vomiting, bloody stool, ulceration of the small intestine with leakage (perforation) into the peritoneal cavity and possible death within a single day due to peritonitis. Many patients exhibit meteorism. Treatment involves suppressing the toxin-producing organisms with antibiotics such as penicillin G or metronidazole. About half of seriously ill patients require surgery for perforation, persistent intestinal obstruction, or failure to respond to the antibiotics. An investigational toxoid vaccine has been used successfully in some developing countries but is not available outside of research.

All the factors collectively causing CNE are generally only present in the hinterlands of New Guinea and parts of Africa, Latin America, and Asia. These factors include protein deprivation (causing inadequate synthesis of trypsin protease (an enzyme), to which the toxin is very sensitive), poor food hygiene, episodic meat feasting, staple diets containing trypsin inhibitors (sweet potatoes), and infection by "Ascaris" parasites which secrete a trypsin inhibitor. In New Guinea (origin of the term "pigbel"), the disease is usually spread through contaminated meat (especially pork) and perhaps by peanuts. (CNE was also diagnosed in post World War II Germany, where it was known as "Darmbrand" or "fire bowels").

Common symptoms of "Staphylococcus aureus" food poisoning include: a rapid onset which is usually 1–6 hours, nausea, explosive vomiting for up to 24 hours, abdominal cramps/pain, headache, weakness, diarrhea and usually a subnormal body temperature. Symptoms usually start one to six hours after eating and last less than 12 hours. The duration of some cases may take two or more days to fully resolve.

People who have been treated with radiotherapy for pelvic and other abdominal cancers frequently develop gastrointestinal symptoms. These include:

- rectal bleeding

- diarrhea and steatorrhea

- other defecation disorders including fecal urgency and incontinence.

- nutritional deficiencies and weight loss

- abdominal pain and bloating

- nausea, vomiting and fatigue

Gastrointestinal symptoms are often found together with those in other systems including genitourinary disorders and sexual dysfunction. The burden of symptoms substantially impairs the patients' quality of life.

Nausea, vomiting, fatigue and diarrhea may happen early during the course of radiotherapy. Radiation enteropathy represents the longer-term, chronic effects which may be found after a latent period most commonly of 6 months to 3 years after the end of treatment. In some cases, it does not become a problem for 20-30 years after successful curative therapy.

Early radiation enteropathy is very common during or immediately after the course of radiotherapy. This involves cell death, mucosal inflammation and epithelial barrier dysfunction. This injury is termed mucositis and results in symptoms of nausea, vomiting, fatigue, diarrhea and abdominal pain. It recovers within a few weeks or months.

Staphylococcal enteritis is an inflammation that is usually caused by eating or drinking substances contaminated with staph enterotoxin. The toxin, not the bacterium, settles in the small intestine and causes inflammation and swelling. This in turn can cause abdominal pain, cramping, dehydration, diarrhea and fever.

"Staphylococcus aureus" is a Gram-positive, facultative anaerobe, coccal (round shaped) bacteria that appears in grape-like clusters that can thrive in high salt and low water activity habitats. "S. aureus" bacteria can live on the skin which is one of the primary modes of transmission. "S. aureus" can cause a range of illnesses from minor skin infections to Staphylococcus aureus food poisoning enteritis. Since humans are the primary source, cross-contamination is the most common way the microorganism is introduced into foods. Foods at high risks are those prepared in large quantities.

Staphylococcus aureus is a true food poisoning organism. It produces a heat stable enterotoxin when allowed to grow for several hours in foods such as cream-filled baked goods, poultry meat, gravies, eggs, meat salads, puddings and vegetables. It is important to note that the toxins may be present in dangerous amounts in foods that have no signs of spoilage, such as a bad smell, any off color, odor, or textural or flavor change.

Enteritis is the inflammation of the small intestine. It is generally caused by eating or drinking substances that are contaminated with bacteria or viruses. The bacterium and/or toxin settles in the small intestine and cause inflammation and swelling. This in turn can cause abdominal pain, cramping, diarrhea, fever, and dehydration. There are other types of enteritis, the types include: bacterial gastroenteritis, "Campylobacter" enteritis, "E. coli" enteritis, radiation enteritis, "Salmonella" enteritis and "Shigella" enteritis.

Enteropathy refers to any pathology of the intestine. Although enteritis specifically refers to an inflammation of the intestine, and is thus a more specific term than "enteropathy", the two phrases are sometimes used interchangeably.

Complications include toxic megacolon, dehydration and sepsis. Such complications generally occur in young children (< 1 year of age) and immunocompromised people. A chronic course of the disease is possible; this disease process is likely to develop without a distinct acute phase. Chronic campylobacteriosis features a long period of sub-febrile temperature and asthenia; eye damage, arthritis, endocarditis may develop if infection is untreated.

Occasional deaths occur in young, previously healthy individuals because of blood volume depletion (due to dehydration), and in persons who are elderly or immunocompromised.

Some individuals (1–2 in 100,000 cases) develop Guillain–Barré syndrome, in which the nerves that join the spinal cord and brain to the rest of the body are damaged, sometimes permanently. This occurs only with infection of "C. jejuni" and "C. upsaliensis".

The mortality of the flock is unaffected, but a certain proportion of birds (1 to 10 percent) show decreased body weights ("runts") and elevated feed conversion. This leads to reduced uniformity of the flock.

The prodromal symptoms are fever, headache, and myalgia, which can be severe, lasting as long as 24 hours. After 1–5 days, typically, these are followed by diarrhea (as many as 10 watery, frequently bloody, bowel movements per day) or dysentery, cramps, abdominal pain, and fever as high as 40 °C (104 °F). In most people, the illness lasts for 2–10 days. It is classified as invasive/inflammatory diarrhea, also described as bloody diarrhea or dysentery.

There are other diseases showing similar symptoms. For instance, abdominal pain and tenderness may be very localized, mimicking acute appendicitis. Furthermore, "Helicobacter pylori" is closely related to Campylobacter and causes peptic ulcer disease.

Specific types of enteropathy include:

- Enteropathy-associated T-cell lymphoma

- Environmental enteropathy

- Eosinophilic enteropathy

- Gluten-sensitive enteropathy (which can progress to coeliac disease)

- Coeliac disease

- Human immunodeficiency virus (HIV) HIV Enteropathy

- Immunodysregulation polyendocrinopathy and enteropathy, X-linked (see FOXP3)

- Protein-losing enteropathy

- Radiation enteropathy

- Tropical enteropathy

If the condition also involves the stomach, it is known as "gastroenteropathy".

In pigs, porcine proliferative enteropathy is a diarrheal disease.

Runting-stunting syndrome in broilers is a syndrome described in broilers since the 1940s, but often with specific etiological appellations (viral enteritis, malabsorption syndrome, brittle bone disease, infectious proventriculitis, helicopter disease and pale bird syndrome). It consists of stunted growth in birds, which is clearly visible in the second month of growth (30–42 days).

Adult flukes are known to be quite harmless, as they do not attack on the host tissue. It is the immature flukes which are most damaging as they get attached to the intestinal wall, literally and actively sloughing off of the tissue. This necrosis is indicated by haemorrhage in faeces, which in turn is a sign of severe enteritis. Under such condition the animals become anorexic and lethargic. It is often accompanied by pronounced diarrhoea, dehydration, oedema, polydipsia, anaemia, listlessness and weight loss. In sheep profuse diarrhoea usually develops two to four weeks after initial infection. If infection is not properly attended death can ensue within 20 days, and in a farm mortality can be very high. In fact there are intermittent reports of mortality as high as 80% among sheep and cattle. Sometimes chronic form is also seen with severe emaciation, anaemia, rough coat, mucosal oedema, thickened duodenum and oedema in the sub maxillary space. The terminally sick animals lie prostrate on the ground, completely emaciated until they die. In buffalos, severe haemorrhage was found to be associated with liver cirrhosis and nodular hepatitis.

Amphistomiasis or paramphistomiasis (alternatively spelled amphistomosis or paramphistomosis) is a parasitic disease of livestock animals, more commonly of cattle and sheep, and humans caused by immature helminthic flatworms belonging to the order Echinostomida. The term amphistomiasis is used for broader connotation implying the disease inflicted by members of Echinostomida including the family Paramphistomidae/Gastrodiscidae (to be precise, the species "Gastrodiscoides hominis"); whereas paramphistomiasis is restricted to that of the members of the family Paramphistomatidae only. "G. discoides" and "Watsonius watsoni" are responsible for the disease in humans, while most paramphistomes are responsible in livestock animals, and some wild mammals. In livestock industry the disease causes heavy economic backlashes due to poor production of milk, meat and wool.

Not all acute diseases or injuries are severe, and vice versa. For example, a mild stubbed toe is an acute injury. Similarly, many acute upper respiratory infections and acute gastroenteritis cases in adults are mild and usually resolve within a few days or weeks.

The term "acute" is also included in the definition of several diseases, such as severe acute respiratory syndrome, acute leukemia, acute myocardial infarction, and acute hepatitis. This is often to distinguish diseases from their chronic forms, such as chronic leukemia, or to highlight the sudden onset of the disease, such as acute myocardial infarct.

In medicine, describing a disease as acute denotes that it is of short and, as a corollary of that, of recent . The quantitation of how much time constitutes "short" and "recent" varies by disease and by context, but the core denotation of "acute" is always qualitatively in contrast with "chronic", which denotes long-lasting disease (for example, in acute leukemia and chronic leukemia). In addition, "acute" also often connotes two other meanings: onset and , such as in acute myocardial infarction (EMI), where suddenness and severity are both established aspects of the meaning. It thus often connotes that the condition is fulminant (as in the EMI example), but not always (as in acute rhinitis, which is usually synonymous with the common cold). The one thing that acute MI and acute rhinitis have in common is that they are not chronic. They can happen again (as in recurrent pneumonia, that is, multiple acute pneumonia episodes), but they are not the same ongoing for months or years (unlike chronic obstructive pulmonary disease, which is).

A noncount sense of "acute disease" refers to the acute phase, that is, a short course, of any disease entity. For example, in an article on ulcerative enteritis in poultry, the author says, "in acute disease there may be increased mortality without any obvious signs", referring to the acute form or phase of ulcerative enteritis.

The portal of entry is the gastrointestinal tract. The organism is acquired usually by insufficiently cooked pork or contaminated water, meat, or milk. Acute "Y. enterocolitica" infections usually lead to mild self-limiting enterocolitis or terminal ileitis and adenitis in humans. Symptoms may include watery or bloody diarrhea and fever, resembling appendicitis or salmonellosis or shigellosis. After oral uptake, "Yersinia" species replicate in the terminal ileum and invade Peyer's patches. From here they can disseminate further to mesenteric lymph nodes causing lymphadenopathy. This condition can be confused with appendicitis, so is called pseudoappendicitis. In immunosuppressed individuals, they can disseminate from the gut to the liver and spleen and form abscesses. Because "Yersinia" species are siderophilic (iron-loving) bacteria, people with hereditary hemochromatosis (a disease resulting in high body iron levels) are more susceptible to infection with "Yersinia" (and other siderophilic bacteria). In fact, the most common contaminant of stored blood is "Y. enterocolitica". See yersiniosis for further details.