The Clinical Problem
Chronic gastroenteric disease is a major clinical problem in small animal medicine, and in both dogs and cats is characterized by vomiting, diarrhoea, and weight loss of at least several weeks' duration. Three distinct clinical entities are defined in the dog (and are less well characterized in cats) on the basis of response to therapy, but in reality these form a clinical spectrum with overlapping immunopathogenesis. These entities are (1) idiopathic inflammatory bowel disease (IBD) responsive to immunosuppression; (2) food-responsive diarrhoea (including dietary hypersensitivity, food allergy) responsive to dietary manipulation; and (3) antibiotic-responsive diarrhoea (ARD) previously known as small intestinal bacterial overgrowth (SIBO). The overlap in these disorders has been demonstrated by recent studies that show successful management of dogs with features of IBD by dietary management alone. There are also a number of canine breed-associated chronic enteropathies that fall into this spectrum, including those intestinal diseases of the Irish setter (gluten-sensitive enteropathy), soft-coated wheaten terrier (nephropathy-enteropathy), basenji (small intestinal lymphoproliferative disease), and Norwegian Lundehund (diarrhoeal syndrome).
The two major inflammatory bowel diseases of man are Crohn's disease (CD) and ulcerative colitis (UC). These are distinct from the small animal disorders, as they primarily affect the lower intestine and involve a granulomatous (CD) or mixed (UC) inflammatory response. However, even if the clinical and pathological features of human and small animal IBD differ, it is believed that these disorders share elements of immunopathogenesis including: (1) genetic susceptibility, (2) impaired mucosal barrier function, (3) exposure to antigens from the luminal microbiota, (4) altered Toll-like receptor (TLR) expression, and (5) reduced immunoregulation leading to enhanced immune responses to microbial antigen.
There are many issues surrounding the canine and feline chronic enteropathies, including definition of the clinical diseases and assessment of their severity. These questions are discussed in an ACVIM Consensus Statement produced by members of the WSAVA Gastrointestinal Standardization Group.1 It has been proposed that disease severity (and response to therapy) might be classified by scoring indices that take into account a range of clinical, laboratory, and pathological features. A Canine Inflammatory Bowel Disease Activity Index (CIBDAI) and equivalent feline index are both reported.2
Issues Related to Endoscopic Biopsy
In recent years, the use of endoscopy in the investigation of canine and feline IBD has become routine. One of the tasks of the WSAVA GI Group was to develop a standard approach to performing and recording the endoscopic examination and the associated collection of mucosal biopsy samples.
Although there are clear benefits to collection of full-thickness surgical biopsy samples of intestine,3 there are practical limitations to this technique in many cases. There has been great frustration arising from the common finding of poor correlation between the clinical severity of disease, visual abnormalities on endoscopic examination, and histopathological changes in mucosal tissue samples. One possible reason underlying the lack of correlation between visual endoscopic findings and histopathology may be the absence of a standardized approach to the review of endoscopic biopsy samples. This may impede not only diagnosis in individual cases, but also the ability to perform collaborative multicentre research studies where application of uniform diagnostic criteria is important.
The issues with histopathological interpretation were clearly demonstrated in a simple study reported by Willard et al.4 Fourteen slides of endoscopic intestinal biopsy tissue were circulated to five different pathologists in four separate institutions, and there was poor agreement in interpretation.
The WSAVA Standardization Group
The issues described above underpinned the formation of the WSAVA Gastrointestinal Standardization Group in 2004. The group had a broad remit in developing standards for all stages of the medical investigation of GI disease in dogs and cats. The group included a mixture of internal medicine and pathology diplomates, and the first phase of the work was concluded in 2008. A major focus was the development of standards for the microscopical interpretation of endoscopic biopsy material.5
One simple study performed by the group addressed the question of how many tissue samples should be collected by the endoscopist to permit a histopathological diagnosis to be made.6 Such tissue samples may be graded as adequate or inadequate for diagnostic purpose. This study involved one pathologist assessing samples from 99 dogs and 51 cats and the most important finding was that a minimum of six endoscopic biopsies is required to detect lesions present in a tissue with 99% confidence. A recent study has addressed the question of whether ileal mucosal biopsies should be collected with duodenal and colonic samples in dogs with clinical evidence of small and large intestinal diarrhoea.7 In this study, only 8 of 30 cases had identical pathology in duodenal and ileal samples, and in only 3 of these 8 cases was the severity of pathology the same in each location. These findings support the routine collection of ileal samples in such dogs. A further recent study has confirmed these findings.8 Collection of concurrent duodenal and ileal samples also maximizes the likelihood of making a diagnosis of feline small T-cell alimentary lymphoma.9
The aim of the WSAVA GI Group was to develop simple histopathological standards, defined as text and pictorial templates, which might be applied by diagnostic histopathologists in any country, with the use of only light microscopy and HE-stained sections. The pictorial templates were produced at the level of the x40 objective, being that most likely to be used to make the histological diagnosis. For each of the four anatomical levels assessed (gastric fundus and antrum, duodenum and colon) the first task was to define the normal histological appearance of these tissues for the dog and cat. Once normality had been established, for each tissue a series of microarchitectural and inflammatory changes were defined and the severity of each graded on a three-point scale (mild, moderate, and marked). The architectural changes vary with tissue, but the fundamental inflammatory processes involve assessment of the level of infiltration of the epithelium and lamina propria by neutrophils, eosinophils, lymphocytes, and plasma cells. In order to simplify the recording of these findings and to ensure that each is assessed in a methodical fashion, the group also developed a standardized histopathology reporting form in 'check-box' format for each of the four anatomical areas. These forms are particularly useful for multicentre research studies where the grading schemes can be converted to numerical points for statistical analysis.
Having proposed this scheme, the WSAVA group then wished to test its validity. A recent study reports one part of a large trial of the applicability of the scheme.10 Four pathologists independently evaluated duodenal biopsies from 62 dogs and 25 cats using the pictorial and text templates. The slides were derived from 7 different institutions in four countries. The results were disappointing, with relatively poor correlation obtained for most parameters. This was particularly the case for the identification of neutrophils and eosinophils, and it was proposed that this largely related to the quality of sectioning and HE staining which markedly differed between laboratories. It was recognized that standards in laboratory processing may be a further consideration in the evaluation of endoscopic tissue biopsies.
The dataset from this WSAVA study was recently reanalyzed in an attempt to select specific histological criteria that were minimally divergent between the four study pathologists. These selected criteria were then applied to a small pilot study of endoscopic samples from 15 cases of canine IBD and 5 control normal dogs by two veterinary pathologists. The study has identified three gastric, five duodenal, and five colonic criteria that are of greatest value in distinguishing between diseased and normal tissue.11
Although the WSAVA standards may be imperfect at this time, they do represent an advance and may be particularly important in the research setting to enable comparison of studies between institutions and the performance of multicentre clinical trials.
The CGS Project
The groundwork laid by the WSAVA GI Standardization Group has now been taken up by the Comparative Gastroenterology Society. Recognizing the faults with the WSAVA scheme, a new project has been started to evaluate the simplified scoring scheme described above (based on fewer selected criteria). Accordingly, gastric, duodenal, and colonic biopsy samples are being collected from 24 healthy dogs and 24 dogs with IBD (8 mild, 8 moderate, and 8 severe on clinical scoring). These will all be stained in one laboratory and then reviewed by 8 independent pathologists. The results of this ongoing investigation may help to refine further the current histopathological standards.
A preliminary report has evaluated the potential for the technique of confocal laser endomicroscopy to evaluate the cellular and subcellular morphology of the canine gastrointestinal tract in real time.12
1. Washabau RJ, Day MJ, Willard MD, et al. Endoscopic, biopsy, and histopathologic guidelines for the evaluation of gastrointestinal inflammation in companion animals. J Vet Intern Med. 2010;24:10–26.
2. Jergens AE, Schreiner CA, Frank DE, et al. A scoring index for disease activity in canine inflammatory bowel disease. J Vet Intern Med. 2003;17:291–297.
3. Kleinschmidt S, Harder J, Nolte I, et al. Chronic inflammatory and non-inflammatory diseases of the gastrointestinal tract in cats: diagnostic advantages of full-thickness intestinal and extraintestinal biopsies. J Feline Med Surg. 2010;12:97–103.
4. Willard MD, Jergens AE, Duncan RB, et al. Interobserver variation among histopathologic evaluations of intestinal tissues from dogs and cats. J Am Vet Med Assoc. 2002;220:1177–1182.
5. Day MJ, Bilzer T, Mansell J, et al. Histopathological standards for the diagnosis of gastrointestinal inflammation in endoscopic biopsy samples from the dog and cat: a report from the World Small Animal Veterinary Association gastrointestinal standardization group. J Comp Pathol. 2008;138:S1–S43.
6. Willard M, Mansell J, Fosgate G, et al. Effect of sample quality upon the sensitivity of endoscopic biopsy for detecting gastric and duodenal lesions in dogs and cats. J Vet Intern Med. 2008;22:1084–1089.
7. Casamian-Sorrosal D, Willard MD, Murray JK, et al. Comparison of histopathological findings in endoscopic biopsies from the duodenum and ileum of dogs with enteropathy. J Vet Intern Med. 2010;24:80–83.
8. Procoli F, Motskula PF, Keyte SV, et al. Comparison of histopathologic findings in duodenal and ileal endoscopic biopsies in dogs with chronic small intestinal enteropathies. J Vet Intern Med.2013;27:268–274.
9. Scott KD, Zoran DL, Mansell J, et al. Utility of endoscopic biopsies of the duodenum and ileum for diagnosis of inflammatory bowel disease and small cell lymphoma in cats. J Vet Intern Med. 2011;25:1253–1257.
10. Willard MD, Moore GE, Denton BD, et al. Effect of tissue processing on assessment of endoscopic intestinal biopsies in dogs and cats. J Vet Intern Med. 2010;24:84–89.
11. Jergens AE, Evans RB, Ackermann M, et al. Design of a simplified histopathologic model for gastrointestinal inflammation in dogs. Vet Pathol. 2014;51:946–950.
12. Sharman MJ, Bacci B, Whittem T, et al. In vivo histologically equivalent evaluation of gastric mucosal topologic morphology in dogs by using confocal endomicroscopy. J Vet Intern Med. 2014;28:799–808.