The most common symptoms are diarrhea, abdominal pain, weight loss, and joint pains. The joint pains may be due to migratory non-deforming arthritis, which may occur many years before any digestive tract symptoms develop; they tend to involve the large joints but can occur in any pattern and tend not to damage the joint surface to the point that the joint becomes deformed. Fever and chills occur in a small proportion of people.

In its more advanced form, malabsorption (insufficient absorption of nutrients from the diet) leads to wasting and the enlargement of lymph nodes in the abdomen. Neurological symptoms (discussed below) are more common in those with the severe form of the abdominal disease. Chronic malabsorptive diarrhea leads to the poor absorption of fat, causing steatorrhea (fatty, offensive stool), flatulence, and abdominal distension. Protein-losing enteropathy may also occur, causing depletion of albumin, a blood protein, which may lead to peripheral edema caused by the lowered oncotic pressures.

Hyperpigmentation of the skin occurs in almost half; some also have skin nodules. Various eye problems, such as uveitis, may occur; this is typically associated with deteriorating vision and pain in the affected eye. Endocarditis (infection of the heart valve) has been reported in a small number of cases, sometimes in people with no other symptoms of Whipple's disease; this is typically noticed as breathlessness and leg swelling due to fluid accumulation as the heart is unable to pump fluid through the body.

Of those affected by Whipple's disease, 10–40% of people have problems related to the involvement of the brain; the symptoms relate to the part of the brain that is affected. The most common problems are dementia, memory loss, confusion, and decreased level of consciousness. Eye movement disturbances and myorhythmia (rapidly repetitive movements of the muscles) of the face, together referred to as "oculomasticatory myorhythmia", are highly characteristic for Whipple's disease. Weakness and poor coordination of part of the body, headaches, seizures, as well as a number of more uncommon neurological features, are present in some cases.

Whipple's disease is a rare, systemic infectious disease caused by the bacterium "Tropheryma whipplei". First described by George Hoyt Whipple in 1907 and commonly considered a gastrointestinal disorder, Whipple's disease primarily causes malabsorption but may affect any part of the body including the heart, brain, joints, skin, lungs and the eyes. Weight loss, diarrhea, joint pain, and arthritis are common presenting symptoms, but the presentation can be highly variable and approximately 15% of patients do not have these classic signs and symptoms.

Whipple's disease is significantly more common in men, with 87% of the patients being male. When recognized and treated, Whipple's disease can usually be cured with long-term antibiotic therapy; if the disease is left untreated, it is ultimately fatal.

In spite of Crohn's and UC being very different diseases, both may present with any of the following symptoms: abdominal pain, vomiting, diarrhea, rectal bleeding, severe internal cramps/muscle spasms in the region of the pelvis and weight loss. Anemia is the most prevalent extraintestinal complication of inflammatory bowel disease. Associated complaints or diseases include arthritis, pyoderma gangrenosum, primary sclerosing cholangitis, and non-thyroidal illness syndrome (NTIS). Associations with deep vein thrombosis (DVT) and bronchiolitis obliterans organizing pneumonia (BOOP) have also been reported. Diagnosis is generally by assessment of inflammatory markers in stool followed by colonoscopy with biopsy of pathological lesions.

Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine. Crohn's disease and ulcerative colitis are the principal types of inflammatory bowel disease. It is important to note that not only does Crohn's disease affect the small intestine and large intestine, it can also affect the mouth, esophagus, stomach and the anus whereas ulcerative colitis primarily affects the colon and the rectum.

Sclerosing mesenteritis may present with no or nearly no signs or symptoms, but many people have chronic and severe pain in the abdomen as the most common chief complaint. Other people have chronic problems with bowel movements, resulting in diarrhea, bloating, gas, and cramping which can range from severe to mild.

The disorder is identified by histopathology showing fat necrosis, fibrosis and chronic inflammation of the small intestine. Examination of the mesentery may indicate a solitary mass, but diffuse mesentery thickening is common.

It often mimics other abdominal diseases such as pancreatic or disseminated cancer. CT scanning is important for making the initial diagnosis.

Idiopathic sclerosing mesenteritis (ISM) or mesenteric panniculitus is a rare disease of the small intestine, characterized by chronic inflammation and eventual fibrosis of the mesentery. It has also been called mesenteric lipodystrophy, or retractile mesenteritis.

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.

EE is rarely symptomatic and is considered a subclinical condition. However, adults may have mild symptoms or malabsorption such as altered stool consistency, increased stool frequency and weight loss.

The current gold standard diagnostic test for EE is intestinal biopsy and histological analysis. Histological changes observed include:

- Villous blunting

- Crypt hypertrophy

- Villous fusion

- Mucosal inflammation

However, this procedure is considered too invasive, complex and expensive to be implemented as standard of care. As a result, there are various research efforts underway to identify biomarkers associated with EE, which could serve as less invasive, yet representative, tools to screen for and identify EE from stool samples.

In an effort to identify simple, accurate diagnostic tests for EE, the Bill and Melinda Gates Foundation (BMGF) has established an EE biomarkers consortium as part of their Global Grand Challenges initiative (specifically, the Discover Biomarkers of Gut Function challenge).

So far, various biomarkers have been selected and studied based on the current understanding of EE pathophysiology:

- Gut permeability/barrier function

- Dual sugar permeability (lactose-to-mannitol ratio)

- Intestinal inflammation

- Alpha-1 anti-trypsin

- Neopterin

- Myeloperoxidase

- Exocrine (hormonal) markers

- Bacterial translocation markers

- Endotoxin core antibody

- Markers of systemic inflammation

- Alpha-1 glycoprotein

- C-reactive protein (CRP)

It is postulated that the limited of understanding of EE is partially due to the paucity of reliable biomarkers, making it difficult for researchers to track the epidemiology of the condition and assess the efficacy of interventions.

human intestinal spirochetosis, also intestinal spirochetes, colonic spirochetosis and colonic spirochetes, is an infection of the colonic-type mucosa with spirochete microorganisms.

No clear association exists with complaints. However, potential associations exist with include watery diarrhea and abdominal pain, which may be seen with blood; these findings are not specific, i.e. may be seen due to number of other causes.

Pneumatosis intestinalis (also called intestinal pneumatosis, pneumatosis cystoides intestinalis, or pneumatosis coli) is of an intestine, that is, gas cysts in the bowel wall. As a radiological sign it is highly suggestive for necrotizing enterocolitis. This is in contrast to gas in the intestinal lumen (which is relieved by flatulence). In newborns, pneumatosis intestinalis is considered diagnostic for necrotizing enterocolitis, and the air is produced by bacteria in the bowel wall. The pathogenesis of pneumatosis intestinalis is poorly understood and is likely multifactorial. PI itself is not a disease, but rather a clinical sign. In some cases, PI is an incidental finding, whereas in others, it portends a life-threatening intra-abdominal condition.

Intestinal Connective tissue abnormality may cause Intestinal Desmosis The absence of the tendinous plexus layer was first described in 1998 by Meier-Ruge. Desmosis is implicated in disturbed gut motility.

Normal peristalsis depends upon the interaction between muscles, nerve cells and tendinous connective tissue. A malfunction of any of these leads to intestinal motility disorders.

Desmosis may be congenital (aplastic form) or acquired (atrophic form).

The "aplastic" form is rare. Typical clinical findings are hypoperistalsis, and pseudo-obstruction. These are found in premature infants, associated with low birth weight. The "atrophic" form is more frequent. Inflammation of the muscularis propria releases enzymes including collagenases which destroy the connective tissue of the bowel wall. Primarily newborns and small children are affected, although this manifestation can also be found in adults. The most common location is the colon with a necrotizing enterocolitis as well as Crohn Disease and diverticulitis. If the taenia are also affected, the disease is defined as complete atrophic desmosis, all other forms without involvement of the taenia are referred to as incomplete. Clinically, patients demonstrate chronic constipation.

As proposed by Giuseppe Martucciello, microscopic diagnosis requires laparoscopic intestinal full-thickness biopsies from colon. Histological findings are absence of the tendinous plexus layer and connective tissue fibers in longitudinal and circular muscle layer.

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.

Intestinal failure is decreased intestinal function such that nutrients, water, and electrolytes are not sufficiently absorbed. Short bowel syndrome is when there is less than of working bowel and is the most common cause of intestinal failure.

Erythema nodosum is a form of panniculitis characterised by tender red nodules, 1–10 cm, associated with systemic symptoms including fever, malaise, and joint pain. Nodules may become bluish-purple, yellowing, and green, and subside over a period of 2–6 weeks without ulcerating or scarring. Erythema nodosum is associated with infections, including Hepatitis C, EBV and tuberculosis, Crohn's disease and sarcoidosis, pregnancy, medications including sulfonamides, and some cancers, including Non-Hodgkin lymphoma and pancreatic cancer.

The symptoms of short bowel syndrome can include:

- Abdominal pain

- Diarrhea and steatorrhea (oily, bulky stool, which can be malodorous)

- Fluid depletion

- Weight loss and malnutrition

- Fatigue

Persons with short bowel syndrome may have complications caused by malabsorption of vitamins and minerals, such as deficiencies in vitamins A, D, E, K, B (folic acid), and B, calcium, magnesium, iron, and zinc. These may appear as anemia, hyperkeratosis (scaling of the skin), easy bruising, muscle spasms, poor blood clotting, and bone pain.

Bilateral hilar lymphadenopathy is a bilateral enlargement of the lymph nodes of pulmonary hila. It is a radiographic term that describes the enlargement of mediastinal lymph nodes and is most commonly identified by a chest x-ray.

The symptoms can vary but include weight loss, diarrhea, vomiting, and anorexia.

The following are causes of BHL:

- Sarcoidosis

- Infection

- Tuberculosis

- Fungal infection

- Mycoplasma

- Intestinal Lipodystrophy (Whipple's disease)

- Malignancy

- Lymphoma

- Carcinoma

- Mediastinal tumors

- Inorganic dust disease

- Silicosis

- Berylliosis

- Extrinsic allergic alveolitis

- Such as bird fancier's lung

- Less common causes also exist:

- Eosinophilic granulomatosis with polyangiitis

- Human immunodeficiency virus

- Extrinsic allergic alveolitis

- Adult-onset Still's disease

Panniculitis can also be classified based on the presence or absence of systemic symptoms. Panniculitis without systemic disease can be a result of trauma or cold; Panniculitis with systemic disease can be caused:

- by connective tissue disorders such as lupus erythematosus or scleroderma;

- by lymphoproliferative disease such as lymphoma or histiocytosis;

- by pancreatitis or pancreatic cancer;

- by sarcoidosis with cutaneous involvement (seen in up to 20 percent)

- and by many other causes.

- Alpha 1-antitrypsin deficiency, also, is a major cause of Panniculitis.

Microvillus inclusion disease, also known as Davidson's disease, congenital microvillus atrophy and, less specifically, microvillus atrophy (note: microvillus is often misspelled as microvillous), is a rare genetic disorder of the small intestine that is inherited in an autosomal recessive pattern.

DPJ is most commonly seen in the Southeastern US, although cases have been reported throughout the United States and Canada, as well as sporadically in the United Kingdom and Europe. Horses in the Southeastern US tend to have a more severe form of the disease relative to other locations. Age, breed, and gender appear to have no effect on disease prevalence.

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.

The veterinarian will typically perform a series of tests such as blood tests and imaging studies. The most definitive way to confirm/rule out intestinal tumors is to perform a medical procedure called endoscopy to visualize the organ and do a tissue biopsy.