Many gastrointestinal (GI) diseases were historically characterised by their histological appearance.^1-3 But before endoscopy was readily available, surgical GI biopsy was often considered the last step in the investigation. This was partly because surgical biopsy carries a real risk of dehiscence, with a complication rate of up to 12% reported,⁴ but also partly because some diseases do not manifest histological changes.

In recent years, endoscopic biopsy has become an important part of the investigative process in patients with gastrointestinal (GI) disease and, with a very low complication rate, has almost become the method of first choice for investigating GI disease. Its immense value as a diagnostic tool will be explored in the next lecture, but it must not be forgotten that endoscopic biopsy is not without its limitations:

 It cannot routinely reach the whole GI tract; patchy/focal disease may be missed

 Biopsy specimens are often of poor quality; they can be small, fragmented and hard to interpret

 Full-thickness tissue samples may be required for a reliable diagnosis in some diseases, e.g., alimentary lymphoma, lymphangiectasia

And whether biopsy specimens are taken endoscopically or surgically, some diseases cannot be diagnosed by biopsy as there are no histological changes seen. Histopathology can assess morphological but not functional disease, i.e., it cannot diagnose:

 Motility disorders

 Hypersecretory disorders, e.g., enterotoxigenic diarrhoea, antibiotic-responsive diarrhoea

 Brush border enzyme deficiencies

 Type 1 dietary hypersensitivities

The ready availability of endoscopic biopsy has almost made other methods of investigating GI disease appear redundant, whereas they may actually offer some crucial information. This lecture will review those methods.

History

The taking of a good history remains the cornerstone of any initial investigation of disease, including GI disease. It can help decide whether a patient is regurgitating or truly vomiting, or whether it has small or large intestinal problems. Ultimately it may direct where biopsy specimens are collected from. However, the nature of the clinical signs may help not only to localise the disease but may also provide evidence of its chronicity and severity. The presence of blood in vomit or stool is usually highly significant.

Physical Examination

Findings on physical examination of patients with GI disease may be minimal because most of the GI tract is unavailable for examination, or non-specific, e.g., weight loss. It is only possible to observe changes in the mouth and palpate most of the intestine through the abdominal wall and the rectum by digital examination. Cytology is occasionally a useful adjunctive technique after rectal examination. Obvious masses, foreign bodies, and intussusception will quickly lead to more invasive investigations, but detection of more subtle changes or just pain will often lead to investigation by radiographic and ultrasound examination.

Minimum Data Base

Routine haematology and serum biochemistry are frequently unremarkable in animals with primary GI disease. However, these negative results are helpful in ruling out endocrine and metabolic causes of GI signs. Normal results indicate that hypoadrenocorticism, renal failure and liver disease are not the cause of GI signs.

Non-specific haematological changes may be found in primary GI disease:

 Increased PCV may indicate dehydration

 Decreased PCV may suggest GI bleeding

 Microcytic iron-deficiency anaemia may suggest chronic GI blood loss

 An inflammatory leukogram suggests infectious/inflammatory disease

 Leukopenia (neutropenia, lymphopenia) can occur in parvovirus infection of dogs and cats

 Lymphopenia may occur in lymphangiectasia

Serum biochemical changes are also often non-specific:

 Electrolyte abnormalities due to anorexia, GI obstruction and diarrhoea; hypokalaemia is a common sequel to GI disease

 Pre-renal azotaemia in dehydration

 Panhypoproteinaemia in protein-losing enteropathies

 Hypocholesterolaemia in malabsorption

 Hypocalcaemia and hypomagnesaemia in protein-losing enteropathies

 Raised liver enzymes secondary to a compromised intestinal mucosal barrier

Faecal Examination

The search for intestinal parasites is an important part of GI investigations. Roundworms are rarely a significant cause of GI disease, but hookworms, whipworms and protozoa (Cryptosporidia, Giardia, Tritrichomonas) are important and potentially treatable. Searching for specific enteric viruses such as parvovirus by ELISA is feasible in acute GI disease, but screening by electron microscopy is rarely available.

Stool culture may identify Salmonella and Campylobacter whilst their speciation and the identification of pathogenic E. coli require molecular techniques. Culture is of value in searching for these potential pathogens in acute gastroenteritis, but their isolation from chronic disease may not be of significance as they can be carried by clinically healthy pets. Similarly identification of bacterial toxins does not always correlate with disease. These issues are discussed elsewhere in these Proceedings.

Testing for faecal occult blood loss is a very sensitive but non-specific test as meat ingestion causes false positives. If the patient can be fed a vegetarian diet for at least three days the test may have some value, but often evidence of iron-deficiency anaemia is more reliable.

Adjunctive Lab Tests

ACTH Stimulation Test

Atypical Addison's disease may cause GI signs in the absence of electrolyte abnormalities. An ACTH stimulation test should be carried out in cases of chronic GI disease when simple investigations (above) have failed to reveal a diagnosis.

Thyroxine (T4)

Serum concentrations of T4 should be measured in any older cat with GI signs as hyperthyroidism is common and commonly causes GI signs.

Trypsin-like Immunoreactivity (TLI)

Signs of malabsorption (diarrhoea, weight loss and polyphagia) can be caused by both small intestinal disease and exocrine pancreatic insufficiency (EPI). A serum TLI test should be performed to rule out EPI before investigating chronic intestinal diarrhoea.

Radiography

Patients with acute GI disease should always have plain survey abdominal radiographs taken. Radiographs are also often helpful in patients with chronic vomiting before biopsy is considered. In particular radiographs often enable identification of surgical disease. The diagnostic yield from plain abdominal radiographs in patients with chronic intestinal disease is more limited and is rarely significantly enhanced by contrast studies. Indeed the utility of endoscopy has largely superseded contrast radiographs even for investigating gastric and colonic disease. Barium follow-through studies are time-consuming, messy and often performed poorly as well as being difficult to interpret. Use of barium-impregnated polyethylene spheres (BIPS) seems rarely to add to the yield from accurate interpretation of plain radiographs.

Ultrasound

Complementary to radiography, ultrasonography is ideally performed before GI biopsy is undertaken, in order to identify the site that requires sampling. It may even be possible to reach a cytological diagnosis by ultrasound-guided fine needle aspirates. Ultrasonography may allow the detection of:

 GI foreign bodies (both radiodense and radiolucent)

 Gastric ulceration

 Intussusceptions

 Vascular disease

 Thickened bowel wall and loss of layering consistent with a neoplastic process

 Thickened bowel wall in chronic infectious bowel diseases such as pythiosis

 Thickened bowel wall consistent with idiopathic inflammatory bowel disease (IBD), although evidence of thickening has been questioned recently⁵

Intestinal Function Tests

Many attempts have been made to develop indirect laboratory tests of intestinal function because of the potential difficulties and risks of GI biopsy. Regrettably few have stood the test of time and the following tests have either fallen from favour or have not been found to be applicable in general practice:

 Glucose, starch and fat tolerance tests

 D-xylose absorption test

 Breath hydrogen

 Urinary metabolites, e.g., indican, p-nitrosonaphthol

 Unconjugated bile acids

 Intestinal permeability

Currently only the measurement of serum folate and cobalamin concentrations and faecal alpha₁ antiprotease excretion have shown value in general practice.

Serum Folate & Cobalamin

Folate is absorbed in the proximal small intestine whilst cobalamin (bound to intrinsic factor) is taken up in the ileum. Thus either low serum folate or low cobalamin concentrations reflect proximal or distal small intestinal disease respectively; diffuse disease may result in low serum concentrations of both vitamins.⁶ Abnormalities do not render a specific diagnosis and are poor indicators of the condition often called small intestinal bacterial overgrowth, but are of value in the investigation of chronic GI disease as they are:

 Markers of small intestinal damage

 Indicators of the chronicity and/or severity of GI disease

 Potential markers of infiltrative bowel disease, i.e., IBD or lymphoma

 Indicators of a need to supplement

Faecal Alpha₁ Antiprotease Excretion

Ascites caused by severe protein-losing enteropathies can be readily detected by the presence of hypoalbuminaemia in the absence of proteinuria or liver dysfunction. More difficult to detect is milder GI protein loss as increased hepatic synthesis may compensate. A test to detect faecal protein-loss would be useful to identify these milder/earlier cases and to monitor response to treatment. Historically ⁵¹chromium radiolabelled albumin was used to track albumin loss into the faeces but is no longer practicable. Measurement of the faecal loss of an endogenous serum protein that isn't degraded offers an alternative test.⁷ Alpha₁ antiprotease is a similar size to albumin and is therefore lost at the same rate into the GI tract. However, as an antiprotease, it is not degraded and can now be measured by ELISA. Three fresh faecal samples must be tested and currently the test is only available in the USA. The test is not applicable if there is either spontaneous GI bleeding or if samples are collected by digital evacuation of the rectum.

Allergy Tests

Measurement of antigen-specific serum IgE is now accepted as a useful alternative to intradermal skin tests in atopy, but similar serological tests for food allergy are problematic. Healthy individuals often have circulating food-specific antibodies, and results do not correlate with the results of exclusion diet trials.

Exclusion Diet Trial

The only reliable method to diagnose a dietary sensitivity is to perform an exclusion diet trial. It may be difficult to choose a completely novel diet for the trial because of the range of foods often fed to pets, so use of a hydrolysed diet may be helpful. However, absolute proof of a food allergy by showing a relapse on feeding the normal diet or challenge with specific foods is rarely obtained.

Empirical Treatment

Either because biopsy is ultimately declined by the owner for economic reasons or is considered too risky, empirical treatment is often attempted, and if successful may yield a tentative diagnosis:

Empirical treatment is not ideal from an academic perspective, and the unjustified use of antibiotics or steroids cannot be recommended. However a response to treatment in conjunction with the type of evidence suggested in this paper, often gives a reasonable tentative diagnosis without having to resort to biopsy.

References

1.  Finco DR, Duncan JR, Schall WD, Hooper BE, Chandler FW, Keating KA. Chronic enteric disease and hypoproteinemia in 9 dogs. Journal of the American Veterinary Medical Association 1973;163:262-271.

2.  Hill FWG, Kelly DF. Naturally occurring intestinal malabsorption in dogs. American Journal of Digestive Diseases 1974;19:649-665

3.  Batt RM, Hall EJ. Chronic enteropathies in the dog. Journal of Small Animal Practice 1989;30:3-12.

4.  Shales CJ, Warren J, Anderson DM, Baines SJ, White RAS. Complications following full-thickness small intestinal biopsy in 66 dogs: a retrospective study. Journal of Small Animal Practice 2005;46:317-321.

5.  Rudorf H, van Schaik G, O'Brien RT, Brown PJ, Barr FJ, Hall EJ. Ultrasonographic evaluation of the thickness of the small intestinal wall in dogs with inflammatory bowel disease. Journal of Small Animal Practice 2005;46: 322-326.

6.  Batt RM, Morgan JO. Role of serum folate and vitamin-B12 in the differentiation of small intestinal abnormalities in the dog. Research in Veterinary Science 1982;32:17-22.

7.  Murphy KF, German AJ, Ruaux CG, Steiner JM, Williams DA, Hall EJ. Fecal alpha(1)-proteinase inhibitor concentration in dogs with chronic gastrointestinal disease. Veterinary Clinical Pathology 2003;32:67-72.