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Exocrine Pancreatic Insufficiency
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GASTROENTEROLOGY: THE 1990s
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EXOCRINE PANCREATIC INSUFFICIENCY Roger M. Batt, BVSc, MSc, PhD, MRCVS
Exocrine pancreatic insufficiency (EPI) represents one of many diseases of the gastrointestinal tract that can result in chronic malabsorption, which may be defined as interference with either the degradative or absorptive phases in the handling of one or more ingested nutrients. EPI can have particularly serious clinical consequences because the pancreas plays a crucial role in the initial stages of degradation of the major dietary constituents, but the spectrum and nonspecific nature of the clinical signs means that diagnosis without specific supportive laboratory data can be extremely misleading. Until relatively recently, laboratory diagnosis was not straightforward, and in some circumstances, apparently supportive data could be misleading, a problem that is likely to have contributed to an overestimation in the occurrence of EPI and a failure to recognize the importance of small intestinal disease. This situation has changed markedly during the 1980s, which has seen important advances in the procedures available for the investigation of dogs with clinical signs suggestive of malabsorption. There have been parallel advances in the understanding of the pathophysiology of diseases causing malabsorption, and together, these developments have had a major impact on the management of these conditions. PATHOLOGY AND CLINICAL SIGNS
EPI in dogs is typically due to pancreatic acinar atrophy. 1• 25 It occurs in many purebreed and mixbred dogs but is most common in From the Department of Small Animal Medicine and Surgery, The Royal Veterinary College, University of London, North Mymms, Hertfordshire, England
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German Shepherds, in which there appears to be a genetic predisposition to the disease. 22• 48 Pancreatic hypoplasia is a relatively rare cause of EPI in dogs but is indistinguishable from acinar atrophy ih many respects, including the histopathologic appearance of the pancreas. Until the etiology of both conditions is known, the possibility that they could be different manifestations of the same disease cannot be excluded. However, this possibility seems unlikely, because hypoplasia represents an apparent failure of acinar development, so that these animals are severely affected as puppies and consequently fail to grow normally. In contrast, acinar atrophy typically presents in fully grown adults between 1 and 5 years of age or older, 48 indicating that there is a loss of acinar tissue from a previously functionally normal pancreas. This is supported by a study that monitored the development of acinar atrophy in a German Shepherd. 50 Chronic pancreatitis is also a rare cause of EPI in dogs, and the progressive loss of endocrine cells in addition to exocrine tissue means that EPI in these animals is likely to be accompanied by diabetes mellitus. Further potential causes of EPI include failure to stimulate pancreatic secretion secondary to severe intestinal disease, as reported, for example, in a Samoyed, 10 and congenital deficiencies of brush border enteropeptidase and individual pancreatic enzymes, which have been reported rarely in children but have yet to be described in small animals. EPI is relatively uncommon in cats but, when it occurs, is most commonly due to chronic pancreatitis. 38 Clinical signs depend on the duration, nature, and severity of EPI, but typically include polyphagia, weight loss, and large output of semiformed feces. Severe, watery diarrhea may occur intermittently as a direct consequence of passage of malabsorbed dietary constituents along the intestinal tract, and also as a result of secondary alterations in the transmucosal flux of fluid. Coprophagia and pica also may be reported in many cases, and soine dogs may present with vomiting. However, an important feature is that affected animals typically are not depressed or lethargic and show no signs of systemic disease, excepting diabetes mellitus in the rare cases associated with chronic pancreatitis. The problem for the clinician is that similar clinical signs can occur in association with other conditions (chronic small intestinal disease potentially causes the most confusion) emphasizing the need for reliable diagnostic procedures.
DIAGNOSIS
The investigation of dogs with suspected malabsorption has evolved rapidly during the last decade as new tests have been validated and the need to perform multiple tests has diminished. The development of a sensitive and specific test for EPI by the assay for serum trypsin-like immunoreactivity (TLI) emphasizes this change in approach, because it is now possible to make this diagnosis simply by
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the assay of a single blood sample. This compares with a previous need to rely on more impractical or less specific procedures, including quantitative fat absorption, assay of fecal trypsin, and the bentiromide test, as discussed in detail later. The TLI test now represents an important component of a simplified approach that can be applied to the investigation of dogs with chronic diarrhea or other signs suggestive of chronic malabsorption (Table 1). Baseline Investigations
Preliminary steps in the investigation of animals with clinical signs suggestive of malabsorption should include taking a detailed history, performing a thorough physical examination, and routine examination of feces, blood, and urine to determine whether disease of the large intestine, intestinal pathogens, or systemic disorders may be responsible for weight loss or diarrhea. Feces should be examined for parasitesincluding Giardia, coccidia, hookworms, and whipworms-and potentially pathogenic bacteria-including Salmonella and Campylobacter. The presence of unsplit fat (triglyceride: EPI), split fat (fatty acid: small intestinal disease), undigested muscle fibers, or starch in the feces may provide some indirect evidence for malabsorption, but such findings have become less important and are not needed to support a diagnosis of EPI. Findings that may contribute to subsequent investigations of small intestinal disease include eosinophilia, perhaps reflecting parasitism or eosinophilic gastroenteritis, neutrophilia in inflammatory bowel disease, lymphopenia in immunodeficiency or lymphangiectasia, and pari.hypoproteinemia in protein-losing enteropathy. In addition, radiography may be helpful if neoplasia or obstruction are suspected. Following these baseline investigations, attention should be focussed on the possibility that the clinical signs may be a direct or Table 1. SUMMARY OF INVESTIGATION OF bOGS WITH SUSPECTED MALABSORPTION Steps
Investigation
1. Diagnosis of intestinal parasites and pathogens, systemic diseases, large bowel disease, and partial intestinal obstruction.
Detailed history and physical examination, fecal examination, hematology, blood biochemistry, urinalysis, colonoscopy, and radiography.
2. Diagnosis of exocrine pancreatic insufficiency.
Assay serum trypsin-like immunoreactivity.
3. Initial evaluation of small intestinal disease.
Assay serum folate and cobalamin. Indirect assessment of intestinal function and permeability. Hydrogen breath test.
4. Subsequent evaluation of small intestinal disease.
Endoscopic examination of small bowel. Histologic examination of biopsy specimens. Quantitative culture of duodenal juice.
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indirect consequence of EPI or small intestinal disease. Because EPI is a well-defined disease that can have secondary effects on the small intestine, it is always important to assess exocrine pancreatic function either to diagnose EPI or to eliminate this diagnosis prior to the investigation of small intestinal disease. Specific Investigations for EPI
The diagnosis of EPI depends on the demonstration of a deficiency of pancreatic enzymes, and many approaches have been explored in the past. Indirect information may be derived by documenting malabsorption of starch or lipid, but because absorption of these also depends on extrapancreatic factors, including intestinal function, these procedures have proven either impractical or inadequate to support a definitive diagnosis of EPI. Indeed, assay of serum triglyceride after administering fat orally barely distinguishes between EPI and healthy dogs; 39 consequently, there is likely to be considerable overlap comparing dogs with EPI and small intestinal disease. Assay of pancreatic enzymes represents a more direct ilpproach to the . diagnosis of EPI, and a common test is based on the estimation of fecal proteolytic activityfor example, by digestion of gelatin on X-ray film or by assay of trypsin with specific substrates-but the results can be very misleading. The main problem is that fecal concentrations may be low in animals that do not have EPI, because residual fecal activity depends on many factors, including speed of transit through the gut. Such misleading results have undoubtedly contributed to a failure to recognize small intestinal disease, which now appears to be much more common than had been appreciated previously. Multiple fecal collections, pooled 3day samples, or pancreatic stimulation by dietary supplementation with soybean may minimize this difficulty, 14• 24• 52 but do not overcome a further problem that fecal proteolytic activity can be normal in dogs with EPJ.S4 Accurate diagnosis rriay be made by assay of stimulated output of pancreatic enzymes in the lumen of the proximal small intestine, 36 and this approach is considered the gold standard for the diagnosis of EPI in humans. However, this relatively cumbersome approach is not necessary to demonstrate the severe loss of pancreatic enzymes typically seen in dogs with EPI. To avoid duodenal intubation, in vivo assay has been achieved in the dog by oral administration of the chymotrypsin substrate bentiromide (N-benzoyl-L-tyrosyl-p-amino-benzoic acid, BTPABA) and subsequent assay of PABA in blood or urine: low levels of PABA indicate EPI. 14• 17• 34• 45• 54 This test was considered to be an important advance in the diagnosis of EPI for the first few years after its introduction, although its use remained largely restricted to referral centers and institutes, because it is relatively impractical for widespread application. However, some problems in interpretation emerged, because it was found that there could be overlap between results from
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dogs with EPI and those with small intestinal disease. 10• 34• 54 This may be due to inadequate absorption of free PABA, or secondary compromise of pancreatic function, perhaps as a result of impaired release of pancreatic secretagogues from damaged intestine. The need for a sensitive, specific, and practical test for EPI therefore remained a relatively high priority in canine gastroenterology towards the beginning of the decade, and attention was directed towards the assay of circulating concentrations of pancreatic enzymes. The principle behind such an approach is that depletion of pancreatic mass in EPI results in decreased leakage of pancreatic enzymes and hence a low concentration in the blood. However, the choice of enzyme has proved to be critical, because it is now apparent that some putative pancreatic enzymes have considerable extra-pancreatic components that make them unsuitable for the diagnosis of EPI. In particular, the finding that plasma activities of lipase, amylase, and isoamylase are not significantly altered or remain within the control range following pancreatectomy in dogs43 has provided compelling evidence that the pancreas is not the sole source of these enzymes in the circulation, so that their assay is unlikely to be a useful approach to the detection of EPI. This conclusion for amylase and lipase is in agreement with previous studies of pancreatectomised dogs, 30• 31 and the finding that activity of a putative pancreatic isoenzyme is disproportionately high in dogs with EPF6 supports the suggestion that a pancreas specific isoamylase is rarely present in the serum of normal dogs. 44 In contrast, assay of canine serum TLI has proved to be a highly sensitive and specific test for the diagnosis of EPI in dogs. Serum TLI quantitates trypsinogen that normally leaks from the pancreas into the blood and, because trypsinogen is exclusively pancreatic in origin, assay of a single fasting blood sample provides an indirect assessment of functional pancreatic tissue. In dogs with EPI, functional exocrine tissue is severely depleted, hence, serum TLI concentrations are abnormally low (<2.5 j.Lmol!L: control5.2-34.0 j.Lmol/L), clearly distinguishing EPI from other causes of malabsorption (Fig. 1).53· 54 In agreement, pancreatectomised dogs have low TLI concentrations comparable to those in dogs with naturally occurring EPI. 43 The sensitivity and specifity of the TLI test for the diagnosis of EPI have been documented as 100%, 54 and the only apparent difficulty in interpretation arises for results that are higher than those observed in dogs with EPI (>2.5 j.Lmol!L) but below normal levels (<5.2 j.Lmol/L). The author's experience is that this may be observed in a very small proportion of samples submitted for analysis, and it is recommended that these dogs should be treated for EPI and retested in 1 to 2 months, at which time the result rarely remains within this indeterminate or gray zone. A falling result is most likely to be indicative of a progressive loss of functional acinar tissue, as has now been documented in a sequential study of acinar atrophy in a dog. 50 Serum TLI fell from the gray zone to a concentration indicative of EPI prior to the onset of clinical signs in this dog, at which time gross and histologic examination of the pancreas showed no abnormalities, but electron microscopic
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HEALTHY CONTROL DOGS
SMALL INTESTINAL DISEASE
Figure 1. Serum trypsin-like immunoreactivity in 100 clinically normal dogs, 50 dogs with some intestinal disease, and 25 dogs with exocrine pancreatic insufficiency. (From Williams DA, Batt AM: Sensitivity and specificity of radioimmunoassay of serum trypsin-like immunoreactivity for the diagnosis of canine exocrine pancreatic insufficiency. J Am Vet Med Assoc 192:195-201, 1988; with permission.)
examination revealed obvious pathologic changes within acinar cells, including dilatation of the rough endoplasmic reticulum and extensive fusion of zymogen granules. The dog developed clinical signs of EPI within a month, and subsequent gross and histologic examination of the pancreas revealed typical features of acinar atrophy. These findings indicate that a subnormal TLI within the gray zone may predict the development of acinar atrophy and also suggest that the clinical signs at that time may not be due to EPI but to disease elsewhere, for example, affecting the small intestine. Nutritional deficiencies are known to cause acinar atrophy in other species, including amino acid imbalance and copper deficiency in the rat and proteincalorie malnutrition in humans, 2• 16• 29 and the possibility that canine acinar atrophy could represent a specific nutritional deficiency secon-
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dary to small intestinal disease should be considered. Particular attention also needs to be directed towards the cause of the extensive fusion of zymogen granules during the development of acinar atrophy, 50 which could occur as a consequence of interference with the normal fusion of these granules with the apical acinar cell membrane and could have a nutritional basis. 28• 33 Further work is indicated to determine the relative importance of genetic and environmental factors in the development of • the disease. A TLI result that falls within the gray zone on initial testing but is no longer subnormal on repeat sampling may indicate some recovery of previously compromised acinar function . This could reflect some reversal of a developing acinar atrophy, perhaps as a rate-limiting nutrient becomes more available, or could be due to recovery of pancreatic function following an episode of pancreatitis. In such cases, continued treatment for EPI may not be necessary, depending on response to withdrawal of pancreatic enzyme supplementation. The apparently low prevalence of EPI in the cat compared to the dog has not driven the same need for a sensitive and specific test in this species, although little difficulty would be anticipated in developing a TLI test for the cat. The bentiromide test does not appear to work well in cats because there is a wide variation in normal animals.23• 37 Therefore, at present, the assay of fecal trypsin activity by the use of specific substrates remains the most reliable approach to the diagnosis of EPI in cats.
Investigation of Small Intestinal Disease Once a diagnosis of EPI has been excluded, attention should be directed towards the investigation of small intestinal disease in dogs with chronic diarrhea or other signs suggestive of chronic malabsorption. This can be achieved in two further steps which are briefly considered here and are summarized in Table 1. Initial evaluation involves indirect assessment of intestinal disorders and can include assays of serum folate and cobalamin, function and permeability tests, and the hydrogen breath test, whereas the subsequent step is more invasive and involves examination of intestinal mucosa and duodenal juice. Some of these procedures are also applicable to cats, but at present, there is less comprehensive information than for dogs. The folate and cobalamin assays are widely available and have proved particularly helpful in dogs, but control ranges will vary with different assays, which need to be validated for small animals. 12 The principle is that disease of the proximal or distal small intestine can result in reduced serum concentrations of folate or cobalamin respectively, reflecting the sites for the normal absorption of these vitamins in dogs. 3• 4• 13 Furthermore, proximal small intestinal bacterial overgrowth (SIBO), which is emerging as a particularly important condition in many breeds of dog, can be detected by finding either an increased serum folate or reduced serum cobalamin concentration, because many enteric bacteria can synthesize folate, which is subsequently absorbed
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in the jejunum, but bind cobalamin, making it unavailable for transport. 5• 7• 11 It is important to appreciate that normal serum concentrations of these vitamins do not exclude the possibility of small intestinal disorders, because alterations depend on the nature, extent, and duration of a mucosal abnormality and also on the type and numbers of organisms present in small intestinal bacterial overgrowth. In addition, because EPI can affect serum folate and cobalamin concentrations for reasons discussed later, it is essential to eliminate this diagnosis before interpretation of these vitamin results. At present, the ability to perform function and permeability tests is predominantly restricted to specialist centers, but such oral tests can be particularly useful and are evolving from the concept of using a single sugar such as xylose to assess intestinal function, to the application of different test probes to the objective assessment of intestinal damage. 19• 20• 21 • 35 Hydrogen breath tests are relatively simple to perform and can detect carbohydrate malabsorption and 518046• 47 but are likely to be relatively restricted in application. Endoscopic examination of the mucosa and histologic examination of intestinal biopsy specimens represents the last step in the diagnostic procedure and may assist in determining that an animal has small intestinal disease. However, it must be appreciated that there may be minimal or no obvious morphologic abnormalities in certain disorders despite considerable interference with intestinal function, and that histologic descriptions alone provide little information on mechanisms of damage to the mucosa and therefore may make a relatively small contribution to a rational approach to treatment. 6 As the understanding of intestinal diseases advances, there will be increased opportunities for identification of the underlying cause of damage, resulting in a more rational approach to management of individual cases.
PATHOPHYSIOLOGIC CONSEQUENCES OF PANCREATIC INSUFFICIENCY
Other research that was performed on dogs with EPI during the 1980s provided insight into the pathophysiologic changes in the small intestine of affected animals, and the findings have had an important impact on the successful management of the disease. In addition to a lack of pancreatic enzymes resulting in interference with degradation of the major dietary constituents, there are secondary changes that can also have important functional consequences in the small intestine and are relevant to the diagnosis and management of EPI. These include a decreased synthesis of protein by enterocytes, which may affect absorptive function; bacterial overgrowth in the proximal intestine (SIBO); and malabsorption of vitamins, including cobalamin. The pancreas has a considerable functional reserve, and EPI is thought to represent a loss of more than 90% of the secretory capacity of the pancreas, which is supported in dogs by extrapolation of the
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relationship between serum TLI and pancreatic weight following partial pancreatic resection. 41 The resultant deficiencies of a-amylase, proteolytic enzymes, and lipase result in relatively severe malabsorption of starch, protein, and triglycerides, respectively. This impaired ability to digest the main dietary constituents undoubtedly makes a major contribution to the clinical signs of weight loss and diarrhea. However, intestinal function also may be impaired in animals with EPI, and this may make a further contribution to the clinical signs. Abnormally low protein synthesis by the small intestine has been reported in dogs with EPI. 55 This may be due to a combination of factors, including malnutrition and loss of the trophic influence of pancreatic secretions on the small intestine. A reduction in the synthesis of specific brush border carrier proteins by enterocytes could be particularly important and could result in reduced absorption of water-soluble molecules such as monosaccharides: this could contribute to the apparent reduction in xylose absorption reported in dogs with EPP 0• 34 and is compatible with abnormal transport of sugars and amino acids reported in humans. Effects on the digestive enzymes in the brush border depend on the net balance between decreased synthesis and the decreased degradation that occurs in the absence of intraluminal pancreatic proteases. Indeed, in dogs without an accompanying anaerobic overgrowth, disaccharidase activities are increased, not decreased, because these enzymes are exposed at the brush border surface and would normally be cleaved by pancreatic proteases.56 There is evidence that the decrease in protein synthesis is reversible following the treatment of EPI with oral pancreatic supplementation, underlining the need for a high-quality source of dietary protein as the management of these cases.55 These functional problems in animals with EPI may be further compounded by bacterial overgrowth in the proximal small intestine, which has been documented in approximately 70% of cases in a series of dogs with EPI. 56 There was relatively severe brush border damage in those cases with anaerobic overgrowth, which was manifest by villus atrophy in some cases and by reduced disaccharidase activities. 56 In addition to these direct effects on the mucosa, both aerobic and anaerobic overgrowth could directly contribute to the malabsorption of carbohydrate in animals with EPI and could cause secretory diarrhea by deconjugation of bile salts and hydroxylation of fatty acids. 27 Malabsorption and hence deficiencies of fat-soluble vitamins are to be expected in EPI, but reduced concentrations of cobalamin (vitamin B12} in serum also have been reported in a .high proportion of affected dogs. 13• 22 Indeed, it is now apparent that the canine pancreas plays a major role in the normal absorption of cobalamin in dogs by secretion of an intrinsic factor that acts as ligand, permitting attachment of cobalamin to specific receptors in the microvillar membrane of ileal enterocytes. 8• 9 Malabsorption of cobalamin in EPI therefore may be related to defective secretion of pancreatic intrinsic factor, 42 although other possible mechanisms include interference with proteolysis of R binders, small intestinal bacterial overgrowth, and abnormally low pH in the lumen of the small intestine.
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The significance of these findings to the clinician is twofold. Firstly, they emphasis the need to diagnose EPI prior to the interpretation of tests that might otherwise indicate primary small intestinal disease. Secondly, they suggest additional measures that may need to be taken to manage EPI successfully.
TREATMENT Many of the principles that underlie successful management of EPI have been derived from information and experience based on dogs, but some of these also should be relevant to treatment of the rare cases of EPI in cats. Specific treatment of EPI involves replacement therapy by the addition of pancreatic extract to the diet, and a good or partial response has been documented in over 80% of treated dogs. 22 Uncoated powder or tablets (eg, Pancrex Veterinary Powder®, Paynes and Byrne; Viokase®, A.H. Robins: 1 tsp/10 kg body weight) or fresh pancreas (approximately 100 g of chopped bovine or porcine pancreas, which can be stored frozen at - 20°C and freshly thawed) are preferable to enteric-coated preparations and capsules. 22, 32, 49 Diarrhea should resolve within a few days, and weight gain of 0.5 to 1.0 kg/week can be expected, but moderation of increased appetite should not be anticipated in more than approximately 50% of treated cases.22 Replacement therapy theoretically needs to be lifelong, but in some dogs it may be possible to withdraw pancreatic supplementation, 51 in which case dietary management may prove a particularly important factor. There may be a variety of reasons for a poor response to pancreatic replacement therapy, but in the first instance, SIBO should be suspected and treated with oral antibiotic for at least a month (eg, 10-20 mg/kg body weight oxytetracycline, repeated every 8 hours; 10 mg/kg metronidazole every 12 hours; 10 mg/kg tylosin every 8 hours) . Overgrowth secondary to experimentally-induced EPI has been reversed by pancreatic supplementation alone, 40 but there may be additional factors that result in persistence of overgrowth in the naturally occurring disease, because there appears to be a positive response in a high proportion of cases to which antibiotic has been administered. 22 Inactivation of exogenous pancreatic enzymes, especially lipase, also may be relevant to a poor response to pancreatic supplementation alone. Consequently, it may be helpful to administer H2-receptor blockers such as cimetidine (5-10 mg/kg Tagamet®; SmithKline Beecham) or ranitidine (2 mg/kg Zantac®; Glaxo) orally 20 minutes before a meal to inhibit acid secretion and minimize degradation of enzymes in the pancreatic extract. Supplementation with acid-resistant fungal enzymes is a logical long-term approach, and a fungal lipase has been shown to be successful in correcting fat malabsorption in experimental canine EPI. 18 Oral multivitamin supplements containing fat-soluble vitamins should be considered as supportive therapy, but because cobalamin malabsorption is not likely to resolve with pancreatic replacement therapy, 42 cobalamin needs to be given parenterally (eg, 500 f.Lg/month) .
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Dietary modification can play an important supportive role in the management of EPI and should be considered, particularly in animals that fail to demonstrate an adequate clinical response to pancreatic replacement therapy alone. This next section summarizes the rationale for dietary management of EPI with a low-fat, high-quality protein diet, which could comprise rice as the main carbohydrate source, fed in small divided meals. Fat malabsorption is a major problem, particularly owing to lipase deficiency, but potentially also owing to bacterial deconjugation of bile salts in SIBO, which is present in approximately two thirds of dogs with EPI; therefore, diets containing poorly digestible fat should not be fed. Furthermore, because bacterial metabolism of unabsorbed fat to hydroxy fatty acids can stimulate secretion in the lower small intestine and colon, 27 it may be necessary to restrict dietary fat, particularly in animals with persistent watery diarrhea. Carbohydrate malabsorption is a consequence of a-amylase deficiency but can also be due to bacterial overgrowth and decreased transport of monosaccharides by malfunctioning enterocytes. The latter in particular may contribute to the clinical signs by causing an osmotic diarrhea and accompanying problems including abdominal discomfort and flatulence. Consequently, attention to the type of carbohydrate fed and perhaps restriction of carbohydrate in animals with watery diarrhea may be valuable until nutritional status improves and mucosal damage is repaired. Breath hydrogen excretion in normal dogs has been shown to be considerably increased when corn or wheat flour are added to the diet, indicative of incomplete absorption of carbohydrate, whereas there was no such increase in breath hydrogen when rice was fed. 46 Such differences are likely to be particularly relevant considering the malabsorption of carbohydrate in animals with pancreatic insufficiency, in which case rice may be preferable to cereal as a source of carbohydrate. Whereas malabsorption of fat and carbohydrate can contribute to malnutrition and hence severe loss of weight, gastrointestinal peptidases appear to be able to compensate for pancreatic protease deficiencies in EPI. 15 A high-quality protein diet is therefore an important consideration in the management of EPI. Because the lack of pancreatic proteases results in the bombardment of the small intestine by antigenic macromolecules, there is greater potential for these to cross the mucosa and interact with the immune system, so that dietary sensitivities are a potential secondary consequence of EPI. In such circumstances, feeding an exclusion diet of a selected protein source could be helpful to exclude proteins that have damaging immunologic consequences. SUMMARY
EPI in dogs represents a well-defined condition that can now be diagnosed simply by the analysis of a single serum sample for TLI. A
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low TLI concentration represents a highly sensitive and specific test for EPI and may also predict the development of disease before the onset of clinical signs. A lack of pancreatic enzymes results in interference with degradation of the major dietary constituents, and there are secondary changes in the small intestine including a decreased synthesis of enterocyte proteins; bacterial overgrowth in the proximal intestine (SIBO); and malabsorption of vitamins, including cobalamin. Management with uncoated pancreatic extract and a low-fat, high-quality protein diet fed in small, divided meals should be effective in most cases. In animals showing a poor response, additional treatment may be necessary with long-term oral antibiotic for SIBO and H2-receptor blockers before a meal to inhibit acid secretion and minimize degradation of pancreatic extract. Diagnosis of the relatively rare cases of EPI in cats is best achieved by analysis of fecal trypsin by the use of specific substrates until a TLI test becomes readily available, and management should follow similar principles to those established for dogs. The major question for the future is the underlying cause of pancreatic acinar atrophy in dogs, particularly the relative importance of genetic and environmental factors. This information may allow detection and elimination of a genetic abnormality by selective breeding or prophylactic treatment that would prevent the development of the disease. References 1. Anderson NV, Low DG: Juvenile atrophy of the canine pancreas. Anim Hosp 1:101-
109, 1965 2. Barbezat GO, Hansen JDL: The exocrine pancreas and protein-calorie malnutrition. Pediatrics 42:77-92, 1968 3. Batt RM, Bush BM, Peters TJ: Subcellular biochemical studies of a naturally occurring enteropathy in the dog resembling chronic tropical sprue in human beings. Am J Vet Res 44:1492-1496, 1983 4. Batt RM, Carter MW, McLean L: Morphological and biochemical studies of a naturally occurring enteropathy in the Irish setter dog: a comparison with coeliac disease in man. Res Vet Sci 37:339-346, 1984 5. Batt RM, Carter MW, Peters TJ: Biochemical changes in the jejunal mucosa of dogs with a naturally occurring enteropathy associated with bacterial overgrowth. Gut 25:816-823, 1984 6. Batt RM, Hall EJ: Chronic enteropathies in the dog. J Small Anim Pract 30:3-12, 1989 7. Batt RM, Hall EJ, McLean L, et al: Small intestinal bacterial overgrowth and enhanced intestinal permeability in healthy Beagles. Am J Vet Res 53:1935-1940, 1992 8. Batt RM, Horadagoda NU: Gastric and pancreatic intrinsic factor-mediated absorption of cobalamin in the dog. Am J Physiol 257:G344-G349, 1989 9. Batt RM, Horadagoda NU, McLean L, et a!: Identification and characterization of a pancreatic intrinsiC factor in the dog. Am J Physiol 256:G517-G523, 1989 10. Batt RM, Mann LC: Specificity of the BT-PABA test for the diagnosis of exocrine pancreatic insufficiency in the dog. Vet Rec 108:303-307, 1981 11. Batt RM, McLean L: Comparison of the biochemical changes in the jejunal mucosa of dogs with aerobic and anaerobic bacterial overgrowth. Gastroenterology 93:986993, 1987 . 12. Batt RM, McLean L, Rutgers HC, et al: Validation of a radioassay for the determination of serum folate and cobalamin concentrations in dogs. J Small Anim Pract 32:221224, 1991
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13. Batt RM, Morgan JO: Role of serum folate and vitamin B12 concentrations in the differentiation of small intestinal abnormalities in the dog. Res Vet Sci 32:17-22, 1982 14. Burrows CF, Merritt AM, Chiapella AM: Determination of fecal fat and trypsin output in the evaluation of chronic canine diarrhoea. JAm Vet Med Assoc 177:11281131, 1980 15. Curtis KJ, Gaines HD, Kim YS: Protein digestion and absorption in rats with pancreatic duct occlusion. Gastroenterology 74:1271-1276, 1978 16. Fell BF, King TP, Davies NT, et a!: Pancreatic atrophy in copper-deficient rats: histochemical and ultrastructural evidence of a selective effect on acinar cells. Histochem J 14:665-680, 1982 17. Freudiger U, Bigler B: The diagnosis of chronic exocrine pancreatic insufficiency by the PABA test. Klintier-Prax 22:73-79, 1977 18. Griffin SM, Alderson D, Famdon JR: Acid-resistant lipase as replacement therapy in chronic pancreatic exocrine insufficiency: a study in dogs. Gut 30:1012-1015, 1989 19. Hall EJ, Batt RM: Enhanced intestinal permeability to 51 Cr-labeled EDTA in dogs with small intestinal disease. JAm Vet Med Assoc 196:91-95, 1990 20. Hall EJ, Batt RM: Differential sugar absorption for the assessment of canine intestinal permeability: The cellobiose/mannitol test in gluten-sensitive enteropathy of Irish setters. Res Vet Sci 51:83-87, 1991 21. Hall EJ, Batt RM, Brown A: Assessment of canine intestinal permeability using 51 chromium-labelled ethylenediaminetetra-acetate. Am J Vet Res 50:2069-2074, 1989 22. Hall EJ, Bond PM, McLean C, et a!: A survey of the diagnosis and treatment of canine exocrine pancreatic insufficiency. J Small Anim Pract 32:613-619, 1991 23. Hawkins EC, Meric SM, Washabau RJ, eta!: Digestion of bentiromide and absorption of xylose in healthy cats and absorption of xylose in cats with infiltrative intestinal disease. Am J Vet Res 47:567-569, 1986 24. Hill FWG: Malabsorption syndrome in the dog: A study of thirty-eight cases. J Small Anim Pract: 13:575-594, 1972 25. Hill FWG, Osborne AD, Kidder DE. Pancreatic degenerative atrophy in dogs. J Comp Pathol 81:321-330, 1971 26. Jacobs RM, Hall RL, Rogers WA: lsoamylases in clinically normal and diseased dogs. Vet Clin Pathol 11:26-32, 1982 27. King CE, Toskes PP: Small intestine bacterial overgrowth. Gastroenterology 76:10351055, 1979 28. Koike H, Steer ML, Meldolesi J: Pancreatic effects of ethionine: Blockade of exocytosis and appearance of crinophagy and autophagy precede cellular necrosis. Am J Physiol 242:G297-G307, 1982 29. Mizunuma T, Kawamura S, Kishino Y: Effects of injecting excess arginine on rat pancreas. J Nutr 114:467-471, 1984 30. Nothman MM, Callow AD: Investigations on the origin of amylase in serum and urine. Gastroenterology 60:82-89, 1971 31. Nothman MM, Pratt TO, Benotti J: The effect of ligation of the pancreatic ducts and of pancreatectomy after duct ligation on serum lipase. J Clin Lab Med 33:833-840, 1948 32. Pidgeon G, Strombeck DR: Evaluation of treatment for pancreatic exocrine insufficiency in dogs with ligated pancreatic ducts. Am J Vet Res 43:461-464, 1982 33. Powers RE, Saluja AK, Houlihan MJ, eta!: Diminished agonist-stimulated inositol triphosphate generation blocks stimuli-secretion coupling in mouse pancreatic acini during diet-induced experimental pancreatitis. J Clin Invest 77:1668-1674, 1986 34. Rogers WA, Stradley RP, Sherding RG, eta!: Simultaneous evaluation of pancreatic exocrine function and intestinal absorptive function in dogs with chronic diarrhoea. JAm Vet Med Assoc 177:1128-1131, 1980 35. Rutgers HC, Hall EJ, Serensen SH, et a!: Differential sugar absorption for the assessment of diet-sensitive intestinal disease in dogs. In Proceedings of the Annual Congress of the British Small Animal Veterinary Association, Birmingham, UK, 1992, p184 36. Sateri H: Investigations on the exocrine pancreatic function in dogs suffering from chronic exocrine pancreatic insufficiency. Acta Vet Scand (Suppl) 53:1-86, 1975 37. Sherding RG, Stradley RP, Rogers WA, eta!: Bentiromide: xylose test in healthy cats. Am J Vet Res 43:2272-2273, 1982
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38. Sheridan V: Pancreatic deficiency in the cat. Vet Rec 96:229, 1975 39. Simpson JW, Doxey DL: Quantitative assessment of fat absorption and its diagnostic value in exocrine pancreatic insufficiency. Res Vet Sci 35:249-251, 1983 40. Simpson KW, Batt RM, Jones D, eta!: Effects of exocrine pancreatic insufficiency and replacement therapy on the bacterial flora of the duodenum in dogs. Am J Vet Res 51:203-206, 1990 41. Simpson KW, Johnstone JMS, Bell PRF, et al: Pancreatic function following partial pancreatectomy and anastomosis of the pancreatic duct to the stomach or duodenum in dogs. Res Vet Sci 52:97-104, 1992 42. Simpson KW, Morton DB, Batt RM: Effect of exocrine pancreatic insufficiency on cobalamin absorption in the dog. Am J Vet Res 50:1233-1236, 1989 43. Simpson KW, Simpson JW, Lake S, et a!: Effect of pancreatectomy on plasma activities of amylase, isoamylase, lipase, and trypsin-like immunoreactivity in dogs. Res Vet Sci 51:78-82, 1991 44. Stickle JE, Carlton WW, Boon GD: Isoamylases in clinically normal dogs. Am J Vet Res 41 :506-509, 1980 45. Strombeck DR: New method for evaluation of chymotrypsin deficiency in dogs. J Am Vet Med Assoc 173:1319-1323, 1978 46. Washabau RJ, Strombeck DR, Buffington CA, et a!: Evaluation of intestinal carbohydrate malabsorption in the dog by pulmonary hydrogen gas excretion. Am J Vet Res 47:1402-1406, 1986 47. Washabau RJ, Strombeck DR, Buffington CA, et al: Use of pulmonary hydrogen gas excretion to detect carbohydrate malabsorption in dogs. JAm Vet Med Assoc 189:674679, 1986 48. Westermarck E: The hereditary nature of canine pancreatic degenerative atrophy in the German shepherd dog. Acta Vet Scand 21 :389-394, 1980 49. Westermarck E: Treatment of pancreatic degenerative atrophy with raw pancreas homogenate and various enzyme preparations. J Vet Med A34:728-733, 1987 50. Westermarck E, Batt RM, Wiberg M, et al: The development of pancreatic acinar atrophy (PAA) in a German shepherd dog. In Proceedings of the lOth Annual Veterinary Forum of the American College of Veterinary Internal Medicine, San Diego, 1992, p 813 51. Westermarck E, Rimaila-Parnanen E: Two unusual cases of canine exocrine panceatic insufficiency. J Small Anim Pract 30:32-34, 1989 52. Westermarck E, Sandholm M: Faecal hydrolase activity as determined by radial enzyme diffusion: a new method for detecting pancreatic dysfunction in the dog. Res Vet Sci 28:341- 346, 1980 53. Williams DA, Batt RM: Diagnosis of canine exocrine pancreatic insufficiency by the assay of serum trypsin-like immunoreactivity. J Small Anim Pract 24:583-588, 1983 54. Williams DA, Batt RM: Sensitivity and specificity of radioimmunoassay of serum trypsin-like immunoreactivity for the diagnosis of canine exocrine pancreatic insufficiency. JAm Vet Med Assoc 192:195-201, 1988 55. Williams DA, Batt RM, McLean L: Reversible impairment of protein synthesis may contribute to jejunal abnormalities in exocrine pancreatic insufficiency. Clin Sci 68(suppl 11):37, 1985 56. Williams DA, Batt RM, McLean L: Bacterial overgrowth in the duodenum of dogs with exocrine pancreatic insufficiency. JAm Vet Med Assoc 191:201-206, 1987
Address reprint requests to Roger M. Batt, BVSc, MSc, PhD, MRCVS Department of Small Animal Medicine and Surgery The Royal Veterinary College Hawkshead Lane North Mymms Hatfield, Hertfordshire AL9 7TA England
GASTROENTEROLOGY: THE 1990s
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EXOCRINE PANCREATIC INSUFFICIENCY Roger M. Batt, BVSc, MSc, PhD, MRCVS
Exocrine pancreatic insufficiency (EPI) represents one of many diseases of the gastrointestinal tract that can result in chronic malabsorption, which may be defined as interference with either the degradative or absorptive phases in the handling of one or more ingested nutrients. EPI can have particularly serious clinical consequences because the pancreas plays a crucial role in the initial stages of degradation of the major dietary constituents, but the spectrum and nonspecific nature of the clinical signs means that diagnosis without specific supportive laboratory data can be extremely misleading. Until relatively recently, laboratory diagnosis was not straightforward, and in some circumstances, apparently supportive data could be misleading, a problem that is likely to have contributed to an overestimation in the occurrence of EPI and a failure to recognize the importance of small intestinal disease. This situation has changed markedly during the 1980s, which has seen important advances in the procedures available for the investigation of dogs with clinical signs suggestive of malabsorption. There have been parallel advances in the understanding of the pathophysiology of diseases causing malabsorption, and together, these developments have had a major impact on the management of these conditions. PATHOLOGY AND CLINICAL SIGNS
EPI in dogs is typically due to pancreatic acinar atrophy. 1• 25 It occurs in many purebreed and mixbred dogs but is most common in From the Department of Small Animal Medicine and Surgery, The Royal Veterinary College, University of London, North Mymms, Hertfordshire, England
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German Shepherds, in which there appears to be a genetic predisposition to the disease. 22• 48 Pancreatic hypoplasia is a relatively rare cause of EPI in dogs but is indistinguishable from acinar atrophy ih many respects, including the histopathologic appearance of the pancreas. Until the etiology of both conditions is known, the possibility that they could be different manifestations of the same disease cannot be excluded. However, this possibility seems unlikely, because hypoplasia represents an apparent failure of acinar development, so that these animals are severely affected as puppies and consequently fail to grow normally. In contrast, acinar atrophy typically presents in fully grown adults between 1 and 5 years of age or older, 48 indicating that there is a loss of acinar tissue from a previously functionally normal pancreas. This is supported by a study that monitored the development of acinar atrophy in a German Shepherd. 50 Chronic pancreatitis is also a rare cause of EPI in dogs, and the progressive loss of endocrine cells in addition to exocrine tissue means that EPI in these animals is likely to be accompanied by diabetes mellitus. Further potential causes of EPI include failure to stimulate pancreatic secretion secondary to severe intestinal disease, as reported, for example, in a Samoyed, 10 and congenital deficiencies of brush border enteropeptidase and individual pancreatic enzymes, which have been reported rarely in children but have yet to be described in small animals. EPI is relatively uncommon in cats but, when it occurs, is most commonly due to chronic pancreatitis. 38 Clinical signs depend on the duration, nature, and severity of EPI, but typically include polyphagia, weight loss, and large output of semiformed feces. Severe, watery diarrhea may occur intermittently as a direct consequence of passage of malabsorbed dietary constituents along the intestinal tract, and also as a result of secondary alterations in the transmucosal flux of fluid. Coprophagia and pica also may be reported in many cases, and soine dogs may present with vomiting. However, an important feature is that affected animals typically are not depressed or lethargic and show no signs of systemic disease, excepting diabetes mellitus in the rare cases associated with chronic pancreatitis. The problem for the clinician is that similar clinical signs can occur in association with other conditions (chronic small intestinal disease potentially causes the most confusion) emphasizing the need for reliable diagnostic procedures.
DIAGNOSIS
The investigation of dogs with suspected malabsorption has evolved rapidly during the last decade as new tests have been validated and the need to perform multiple tests has diminished. The development of a sensitive and specific test for EPI by the assay for serum trypsin-like immunoreactivity (TLI) emphasizes this change in approach, because it is now possible to make this diagnosis simply by
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the assay of a single blood sample. This compares with a previous need to rely on more impractical or less specific procedures, including quantitative fat absorption, assay of fecal trypsin, and the bentiromide test, as discussed in detail later. The TLI test now represents an important component of a simplified approach that can be applied to the investigation of dogs with chronic diarrhea or other signs suggestive of chronic malabsorption (Table 1). Baseline Investigations
Preliminary steps in the investigation of animals with clinical signs suggestive of malabsorption should include taking a detailed history, performing a thorough physical examination, and routine examination of feces, blood, and urine to determine whether disease of the large intestine, intestinal pathogens, or systemic disorders may be responsible for weight loss or diarrhea. Feces should be examined for parasitesincluding Giardia, coccidia, hookworms, and whipworms-and potentially pathogenic bacteria-including Salmonella and Campylobacter. The presence of unsplit fat (triglyceride: EPI), split fat (fatty acid: small intestinal disease), undigested muscle fibers, or starch in the feces may provide some indirect evidence for malabsorption, but such findings have become less important and are not needed to support a diagnosis of EPI. Findings that may contribute to subsequent investigations of small intestinal disease include eosinophilia, perhaps reflecting parasitism or eosinophilic gastroenteritis, neutrophilia in inflammatory bowel disease, lymphopenia in immunodeficiency or lymphangiectasia, and pari.hypoproteinemia in protein-losing enteropathy. In addition, radiography may be helpful if neoplasia or obstruction are suspected. Following these baseline investigations, attention should be focussed on the possibility that the clinical signs may be a direct or Table 1. SUMMARY OF INVESTIGATION OF bOGS WITH SUSPECTED MALABSORPTION Steps
Investigation
1. Diagnosis of intestinal parasites and pathogens, systemic diseases, large bowel disease, and partial intestinal obstruction.
Detailed history and physical examination, fecal examination, hematology, blood biochemistry, urinalysis, colonoscopy, and radiography.
2. Diagnosis of exocrine pancreatic insufficiency.
Assay serum trypsin-like immunoreactivity.
3. Initial evaluation of small intestinal disease.
Assay serum folate and cobalamin. Indirect assessment of intestinal function and permeability. Hydrogen breath test.
4. Subsequent evaluation of small intestinal disease.
Endoscopic examination of small bowel. Histologic examination of biopsy specimens. Quantitative culture of duodenal juice.
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indirect consequence of EPI or small intestinal disease. Because EPI is a well-defined disease that can have secondary effects on the small intestine, it is always important to assess exocrine pancreatic function either to diagnose EPI or to eliminate this diagnosis prior to the investigation of small intestinal disease. Specific Investigations for EPI
The diagnosis of EPI depends on the demonstration of a deficiency of pancreatic enzymes, and many approaches have been explored in the past. Indirect information may be derived by documenting malabsorption of starch or lipid, but because absorption of these also depends on extrapancreatic factors, including intestinal function, these procedures have proven either impractical or inadequate to support a definitive diagnosis of EPI. Indeed, assay of serum triglyceride after administering fat orally barely distinguishes between EPI and healthy dogs; 39 consequently, there is likely to be considerable overlap comparing dogs with EPI and small intestinal disease. Assay of pancreatic enzymes represents a more direct ilpproach to the . diagnosis of EPI, and a common test is based on the estimation of fecal proteolytic activityfor example, by digestion of gelatin on X-ray film or by assay of trypsin with specific substrates-but the results can be very misleading. The main problem is that fecal concentrations may be low in animals that do not have EPI, because residual fecal activity depends on many factors, including speed of transit through the gut. Such misleading results have undoubtedly contributed to a failure to recognize small intestinal disease, which now appears to be much more common than had been appreciated previously. Multiple fecal collections, pooled 3day samples, or pancreatic stimulation by dietary supplementation with soybean may minimize this difficulty, 14• 24• 52 but do not overcome a further problem that fecal proteolytic activity can be normal in dogs with EPJ.S4 Accurate diagnosis rriay be made by assay of stimulated output of pancreatic enzymes in the lumen of the proximal small intestine, 36 and this approach is considered the gold standard for the diagnosis of EPI in humans. However, this relatively cumbersome approach is not necessary to demonstrate the severe loss of pancreatic enzymes typically seen in dogs with EPI. To avoid duodenal intubation, in vivo assay has been achieved in the dog by oral administration of the chymotrypsin substrate bentiromide (N-benzoyl-L-tyrosyl-p-amino-benzoic acid, BTPABA) and subsequent assay of PABA in blood or urine: low levels of PABA indicate EPI. 14• 17• 34• 45• 54 This test was considered to be an important advance in the diagnosis of EPI for the first few years after its introduction, although its use remained largely restricted to referral centers and institutes, because it is relatively impractical for widespread application. However, some problems in interpretation emerged, because it was found that there could be overlap between results from
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dogs with EPI and those with small intestinal disease. 10• 34• 54 This may be due to inadequate absorption of free PABA, or secondary compromise of pancreatic function, perhaps as a result of impaired release of pancreatic secretagogues from damaged intestine. The need for a sensitive, specific, and practical test for EPI therefore remained a relatively high priority in canine gastroenterology towards the beginning of the decade, and attention was directed towards the assay of circulating concentrations of pancreatic enzymes. The principle behind such an approach is that depletion of pancreatic mass in EPI results in decreased leakage of pancreatic enzymes and hence a low concentration in the blood. However, the choice of enzyme has proved to be critical, because it is now apparent that some putative pancreatic enzymes have considerable extra-pancreatic components that make them unsuitable for the diagnosis of EPI. In particular, the finding that plasma activities of lipase, amylase, and isoamylase are not significantly altered or remain within the control range following pancreatectomy in dogs43 has provided compelling evidence that the pancreas is not the sole source of these enzymes in the circulation, so that their assay is unlikely to be a useful approach to the detection of EPI. This conclusion for amylase and lipase is in agreement with previous studies of pancreatectomised dogs, 30• 31 and the finding that activity of a putative pancreatic isoenzyme is disproportionately high in dogs with EPF6 supports the suggestion that a pancreas specific isoamylase is rarely present in the serum of normal dogs. 44 In contrast, assay of canine serum TLI has proved to be a highly sensitive and specific test for the diagnosis of EPI in dogs. Serum TLI quantitates trypsinogen that normally leaks from the pancreas into the blood and, because trypsinogen is exclusively pancreatic in origin, assay of a single fasting blood sample provides an indirect assessment of functional pancreatic tissue. In dogs with EPI, functional exocrine tissue is severely depleted, hence, serum TLI concentrations are abnormally low (<2.5 j.Lmol!L: control5.2-34.0 j.Lmol/L), clearly distinguishing EPI from other causes of malabsorption (Fig. 1).53· 54 In agreement, pancreatectomised dogs have low TLI concentrations comparable to those in dogs with naturally occurring EPI. 43 The sensitivity and specifity of the TLI test for the diagnosis of EPI have been documented as 100%, 54 and the only apparent difficulty in interpretation arises for results that are higher than those observed in dogs with EPI (>2.5 j.Lmol!L) but below normal levels (<5.2 j.Lmol/L). The author's experience is that this may be observed in a very small proportion of samples submitted for analysis, and it is recommended that these dogs should be treated for EPI and retested in 1 to 2 months, at which time the result rarely remains within this indeterminate or gray zone. A falling result is most likely to be indicative of a progressive loss of functional acinar tissue, as has now been documented in a sequential study of acinar atrophy in a dog. 50 Serum TLI fell from the gray zone to a concentration indicative of EPI prior to the onset of clinical signs in this dog, at which time gross and histologic examination of the pancreas showed no abnormalities, but electron microscopic
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HEALTHY CONTROL DOGS
SMALL INTESTINAL DISEASE
Figure 1. Serum trypsin-like immunoreactivity in 100 clinically normal dogs, 50 dogs with some intestinal disease, and 25 dogs with exocrine pancreatic insufficiency. (From Williams DA, Batt AM: Sensitivity and specificity of radioimmunoassay of serum trypsin-like immunoreactivity for the diagnosis of canine exocrine pancreatic insufficiency. J Am Vet Med Assoc 192:195-201, 1988; with permission.)
examination revealed obvious pathologic changes within acinar cells, including dilatation of the rough endoplasmic reticulum and extensive fusion of zymogen granules. The dog developed clinical signs of EPI within a month, and subsequent gross and histologic examination of the pancreas revealed typical features of acinar atrophy. These findings indicate that a subnormal TLI within the gray zone may predict the development of acinar atrophy and also suggest that the clinical signs at that time may not be due to EPI but to disease elsewhere, for example, affecting the small intestine. Nutritional deficiencies are known to cause acinar atrophy in other species, including amino acid imbalance and copper deficiency in the rat and proteincalorie malnutrition in humans, 2• 16• 29 and the possibility that canine acinar atrophy could represent a specific nutritional deficiency secon-
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dary to small intestinal disease should be considered. Particular attention also needs to be directed towards the cause of the extensive fusion of zymogen granules during the development of acinar atrophy, 50 which could occur as a consequence of interference with the normal fusion of these granules with the apical acinar cell membrane and could have a nutritional basis. 28• 33 Further work is indicated to determine the relative importance of genetic and environmental factors in the development of • the disease. A TLI result that falls within the gray zone on initial testing but is no longer subnormal on repeat sampling may indicate some recovery of previously compromised acinar function . This could reflect some reversal of a developing acinar atrophy, perhaps as a rate-limiting nutrient becomes more available, or could be due to recovery of pancreatic function following an episode of pancreatitis. In such cases, continued treatment for EPI may not be necessary, depending on response to withdrawal of pancreatic enzyme supplementation. The apparently low prevalence of EPI in the cat compared to the dog has not driven the same need for a sensitive and specific test in this species, although little difficulty would be anticipated in developing a TLI test for the cat. The bentiromide test does not appear to work well in cats because there is a wide variation in normal animals.23• 37 Therefore, at present, the assay of fecal trypsin activity by the use of specific substrates remains the most reliable approach to the diagnosis of EPI in cats.
Investigation of Small Intestinal Disease Once a diagnosis of EPI has been excluded, attention should be directed towards the investigation of small intestinal disease in dogs with chronic diarrhea or other signs suggestive of chronic malabsorption. This can be achieved in two further steps which are briefly considered here and are summarized in Table 1. Initial evaluation involves indirect assessment of intestinal disorders and can include assays of serum folate and cobalamin, function and permeability tests, and the hydrogen breath test, whereas the subsequent step is more invasive and involves examination of intestinal mucosa and duodenal juice. Some of these procedures are also applicable to cats, but at present, there is less comprehensive information than for dogs. The folate and cobalamin assays are widely available and have proved particularly helpful in dogs, but control ranges will vary with different assays, which need to be validated for small animals. 12 The principle is that disease of the proximal or distal small intestine can result in reduced serum concentrations of folate or cobalamin respectively, reflecting the sites for the normal absorption of these vitamins in dogs. 3• 4• 13 Furthermore, proximal small intestinal bacterial overgrowth (SIBO), which is emerging as a particularly important condition in many breeds of dog, can be detected by finding either an increased serum folate or reduced serum cobalamin concentration, because many enteric bacteria can synthesize folate, which is subsequently absorbed
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in the jejunum, but bind cobalamin, making it unavailable for transport. 5• 7• 11 It is important to appreciate that normal serum concentrations of these vitamins do not exclude the possibility of small intestinal disorders, because alterations depend on the nature, extent, and duration of a mucosal abnormality and also on the type and numbers of organisms present in small intestinal bacterial overgrowth. In addition, because EPI can affect serum folate and cobalamin concentrations for reasons discussed later, it is essential to eliminate this diagnosis before interpretation of these vitamin results. At present, the ability to perform function and permeability tests is predominantly restricted to specialist centers, but such oral tests can be particularly useful and are evolving from the concept of using a single sugar such as xylose to assess intestinal function, to the application of different test probes to the objective assessment of intestinal damage. 19• 20• 21 • 35 Hydrogen breath tests are relatively simple to perform and can detect carbohydrate malabsorption and 518046• 47 but are likely to be relatively restricted in application. Endoscopic examination of the mucosa and histologic examination of intestinal biopsy specimens represents the last step in the diagnostic procedure and may assist in determining that an animal has small intestinal disease. However, it must be appreciated that there may be minimal or no obvious morphologic abnormalities in certain disorders despite considerable interference with intestinal function, and that histologic descriptions alone provide little information on mechanisms of damage to the mucosa and therefore may make a relatively small contribution to a rational approach to treatment. 6 As the understanding of intestinal diseases advances, there will be increased opportunities for identification of the underlying cause of damage, resulting in a more rational approach to management of individual cases.
PATHOPHYSIOLOGIC CONSEQUENCES OF PANCREATIC INSUFFICIENCY
Other research that was performed on dogs with EPI during the 1980s provided insight into the pathophysiologic changes in the small intestine of affected animals, and the findings have had an important impact on the successful management of the disease. In addition to a lack of pancreatic enzymes resulting in interference with degradation of the major dietary constituents, there are secondary changes that can also have important functional consequences in the small intestine and are relevant to the diagnosis and management of EPI. These include a decreased synthesis of protein by enterocytes, which may affect absorptive function; bacterial overgrowth in the proximal intestine (SIBO); and malabsorption of vitamins, including cobalamin. The pancreas has a considerable functional reserve, and EPI is thought to represent a loss of more than 90% of the secretory capacity of the pancreas, which is supported in dogs by extrapolation of the
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relationship between serum TLI and pancreatic weight following partial pancreatic resection. 41 The resultant deficiencies of a-amylase, proteolytic enzymes, and lipase result in relatively severe malabsorption of starch, protein, and triglycerides, respectively. This impaired ability to digest the main dietary constituents undoubtedly makes a major contribution to the clinical signs of weight loss and diarrhea. However, intestinal function also may be impaired in animals with EPI, and this may make a further contribution to the clinical signs. Abnormally low protein synthesis by the small intestine has been reported in dogs with EPI. 55 This may be due to a combination of factors, including malnutrition and loss of the trophic influence of pancreatic secretions on the small intestine. A reduction in the synthesis of specific brush border carrier proteins by enterocytes could be particularly important and could result in reduced absorption of water-soluble molecules such as monosaccharides: this could contribute to the apparent reduction in xylose absorption reported in dogs with EPP 0• 34 and is compatible with abnormal transport of sugars and amino acids reported in humans. Effects on the digestive enzymes in the brush border depend on the net balance between decreased synthesis and the decreased degradation that occurs in the absence of intraluminal pancreatic proteases. Indeed, in dogs without an accompanying anaerobic overgrowth, disaccharidase activities are increased, not decreased, because these enzymes are exposed at the brush border surface and would normally be cleaved by pancreatic proteases.56 There is evidence that the decrease in protein synthesis is reversible following the treatment of EPI with oral pancreatic supplementation, underlining the need for a high-quality source of dietary protein as the management of these cases.55 These functional problems in animals with EPI may be further compounded by bacterial overgrowth in the proximal small intestine, which has been documented in approximately 70% of cases in a series of dogs with EPI. 56 There was relatively severe brush border damage in those cases with anaerobic overgrowth, which was manifest by villus atrophy in some cases and by reduced disaccharidase activities. 56 In addition to these direct effects on the mucosa, both aerobic and anaerobic overgrowth could directly contribute to the malabsorption of carbohydrate in animals with EPI and could cause secretory diarrhea by deconjugation of bile salts and hydroxylation of fatty acids. 27 Malabsorption and hence deficiencies of fat-soluble vitamins are to be expected in EPI, but reduced concentrations of cobalamin (vitamin B12} in serum also have been reported in a .high proportion of affected dogs. 13• 22 Indeed, it is now apparent that the canine pancreas plays a major role in the normal absorption of cobalamin in dogs by secretion of an intrinsic factor that acts as ligand, permitting attachment of cobalamin to specific receptors in the microvillar membrane of ileal enterocytes. 8• 9 Malabsorption of cobalamin in EPI therefore may be related to defective secretion of pancreatic intrinsic factor, 42 although other possible mechanisms include interference with proteolysis of R binders, small intestinal bacterial overgrowth, and abnormally low pH in the lumen of the small intestine.
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The significance of these findings to the clinician is twofold. Firstly, they emphasis the need to diagnose EPI prior to the interpretation of tests that might otherwise indicate primary small intestinal disease. Secondly, they suggest additional measures that may need to be taken to manage EPI successfully.
TREATMENT Many of the principles that underlie successful management of EPI have been derived from information and experience based on dogs, but some of these also should be relevant to treatment of the rare cases of EPI in cats. Specific treatment of EPI involves replacement therapy by the addition of pancreatic extract to the diet, and a good or partial response has been documented in over 80% of treated dogs. 22 Uncoated powder or tablets (eg, Pancrex Veterinary Powder®, Paynes and Byrne; Viokase®, A.H. Robins: 1 tsp/10 kg body weight) or fresh pancreas (approximately 100 g of chopped bovine or porcine pancreas, which can be stored frozen at - 20°C and freshly thawed) are preferable to enteric-coated preparations and capsules. 22, 32, 49 Diarrhea should resolve within a few days, and weight gain of 0.5 to 1.0 kg/week can be expected, but moderation of increased appetite should not be anticipated in more than approximately 50% of treated cases.22 Replacement therapy theoretically needs to be lifelong, but in some dogs it may be possible to withdraw pancreatic supplementation, 51 in which case dietary management may prove a particularly important factor. There may be a variety of reasons for a poor response to pancreatic replacement therapy, but in the first instance, SIBO should be suspected and treated with oral antibiotic for at least a month (eg, 10-20 mg/kg body weight oxytetracycline, repeated every 8 hours; 10 mg/kg metronidazole every 12 hours; 10 mg/kg tylosin every 8 hours) . Overgrowth secondary to experimentally-induced EPI has been reversed by pancreatic supplementation alone, 40 but there may be additional factors that result in persistence of overgrowth in the naturally occurring disease, because there appears to be a positive response in a high proportion of cases to which antibiotic has been administered. 22 Inactivation of exogenous pancreatic enzymes, especially lipase, also may be relevant to a poor response to pancreatic supplementation alone. Consequently, it may be helpful to administer H2-receptor blockers such as cimetidine (5-10 mg/kg Tagamet®; SmithKline Beecham) or ranitidine (2 mg/kg Zantac®; Glaxo) orally 20 minutes before a meal to inhibit acid secretion and minimize degradation of enzymes in the pancreatic extract. Supplementation with acid-resistant fungal enzymes is a logical long-term approach, and a fungal lipase has been shown to be successful in correcting fat malabsorption in experimental canine EPI. 18 Oral multivitamin supplements containing fat-soluble vitamins should be considered as supportive therapy, but because cobalamin malabsorption is not likely to resolve with pancreatic replacement therapy, 42 cobalamin needs to be given parenterally (eg, 500 f.Lg/month) .
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Dietary modification can play an important supportive role in the management of EPI and should be considered, particularly in animals that fail to demonstrate an adequate clinical response to pancreatic replacement therapy alone. This next section summarizes the rationale for dietary management of EPI with a low-fat, high-quality protein diet, which could comprise rice as the main carbohydrate source, fed in small divided meals. Fat malabsorption is a major problem, particularly owing to lipase deficiency, but potentially also owing to bacterial deconjugation of bile salts in SIBO, which is present in approximately two thirds of dogs with EPI; therefore, diets containing poorly digestible fat should not be fed. Furthermore, because bacterial metabolism of unabsorbed fat to hydroxy fatty acids can stimulate secretion in the lower small intestine and colon, 27 it may be necessary to restrict dietary fat, particularly in animals with persistent watery diarrhea. Carbohydrate malabsorption is a consequence of a-amylase deficiency but can also be due to bacterial overgrowth and decreased transport of monosaccharides by malfunctioning enterocytes. The latter in particular may contribute to the clinical signs by causing an osmotic diarrhea and accompanying problems including abdominal discomfort and flatulence. Consequently, attention to the type of carbohydrate fed and perhaps restriction of carbohydrate in animals with watery diarrhea may be valuable until nutritional status improves and mucosal damage is repaired. Breath hydrogen excretion in normal dogs has been shown to be considerably increased when corn or wheat flour are added to the diet, indicative of incomplete absorption of carbohydrate, whereas there was no such increase in breath hydrogen when rice was fed. 46 Such differences are likely to be particularly relevant considering the malabsorption of carbohydrate in animals with pancreatic insufficiency, in which case rice may be preferable to cereal as a source of carbohydrate. Whereas malabsorption of fat and carbohydrate can contribute to malnutrition and hence severe loss of weight, gastrointestinal peptidases appear to be able to compensate for pancreatic protease deficiencies in EPI. 15 A high-quality protein diet is therefore an important consideration in the management of EPI. Because the lack of pancreatic proteases results in the bombardment of the small intestine by antigenic macromolecules, there is greater potential for these to cross the mucosa and interact with the immune system, so that dietary sensitivities are a potential secondary consequence of EPI. In such circumstances, feeding an exclusion diet of a selected protein source could be helpful to exclude proteins that have damaging immunologic consequences. SUMMARY
EPI in dogs represents a well-defined condition that can now be diagnosed simply by the analysis of a single serum sample for TLI. A
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low TLI concentration represents a highly sensitive and specific test for EPI and may also predict the development of disease before the onset of clinical signs. A lack of pancreatic enzymes results in interference with degradation of the major dietary constituents, and there are secondary changes in the small intestine including a decreased synthesis of enterocyte proteins; bacterial overgrowth in the proximal intestine (SIBO); and malabsorption of vitamins, including cobalamin. Management with uncoated pancreatic extract and a low-fat, high-quality protein diet fed in small, divided meals should be effective in most cases. In animals showing a poor response, additional treatment may be necessary with long-term oral antibiotic for SIBO and H2-receptor blockers before a meal to inhibit acid secretion and minimize degradation of pancreatic extract. Diagnosis of the relatively rare cases of EPI in cats is best achieved by analysis of fecal trypsin by the use of specific substrates until a TLI test becomes readily available, and management should follow similar principles to those established for dogs. The major question for the future is the underlying cause of pancreatic acinar atrophy in dogs, particularly the relative importance of genetic and environmental factors. This information may allow detection and elimination of a genetic abnormality by selective breeding or prophylactic treatment that would prevent the development of the disease. References 1. Anderson NV, Low DG: Juvenile atrophy of the canine pancreas. Anim Hosp 1:101-
109, 1965 2. Barbezat GO, Hansen JDL: The exocrine pancreas and protein-calorie malnutrition. Pediatrics 42:77-92, 1968 3. Batt RM, Bush BM, Peters TJ: Subcellular biochemical studies of a naturally occurring enteropathy in the dog resembling chronic tropical sprue in human beings. Am J Vet Res 44:1492-1496, 1983 4. Batt RM, Carter MW, McLean L: Morphological and biochemical studies of a naturally occurring enteropathy in the Irish setter dog: a comparison with coeliac disease in man. Res Vet Sci 37:339-346, 1984 5. Batt RM, Carter MW, Peters TJ: Biochemical changes in the jejunal mucosa of dogs with a naturally occurring enteropathy associated with bacterial overgrowth. Gut 25:816-823, 1984 6. Batt RM, Hall EJ: Chronic enteropathies in the dog. J Small Anim Pract 30:3-12, 1989 7. Batt RM, Hall EJ, McLean L, et al: Small intestinal bacterial overgrowth and enhanced intestinal permeability in healthy Beagles. Am J Vet Res 53:1935-1940, 1992 8. Batt RM, Horadagoda NU: Gastric and pancreatic intrinsic factor-mediated absorption of cobalamin in the dog. Am J Physiol 257:G344-G349, 1989 9. Batt RM, Horadagoda NU, McLean L, et a!: Identification and characterization of a pancreatic intrinsiC factor in the dog. Am J Physiol 256:G517-G523, 1989 10. Batt RM, Mann LC: Specificity of the BT-PABA test for the diagnosis of exocrine pancreatic insufficiency in the dog. Vet Rec 108:303-307, 1981 11. Batt RM, McLean L: Comparison of the biochemical changes in the jejunal mucosa of dogs with aerobic and anaerobic bacterial overgrowth. Gastroenterology 93:986993, 1987 . 12. Batt RM, McLean L, Rutgers HC, et al: Validation of a radioassay for the determination of serum folate and cobalamin concentrations in dogs. J Small Anim Pract 32:221224, 1991
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13. Batt RM, Morgan JO: Role of serum folate and vitamin B12 concentrations in the differentiation of small intestinal abnormalities in the dog. Res Vet Sci 32:17-22, 1982 14. Burrows CF, Merritt AM, Chiapella AM: Determination of fecal fat and trypsin output in the evaluation of chronic canine diarrhoea. JAm Vet Med Assoc 177:11281131, 1980 15. Curtis KJ, Gaines HD, Kim YS: Protein digestion and absorption in rats with pancreatic duct occlusion. Gastroenterology 74:1271-1276, 1978 16. Fell BF, King TP, Davies NT, et a!: Pancreatic atrophy in copper-deficient rats: histochemical and ultrastructural evidence of a selective effect on acinar cells. Histochem J 14:665-680, 1982 17. Freudiger U, Bigler B: The diagnosis of chronic exocrine pancreatic insufficiency by the PABA test. Klintier-Prax 22:73-79, 1977 18. Griffin SM, Alderson D, Famdon JR: Acid-resistant lipase as replacement therapy in chronic pancreatic exocrine insufficiency: a study in dogs. Gut 30:1012-1015, 1989 19. Hall EJ, Batt RM: Enhanced intestinal permeability to 51 Cr-labeled EDTA in dogs with small intestinal disease. JAm Vet Med Assoc 196:91-95, 1990 20. Hall EJ, Batt RM: Differential sugar absorption for the assessment of canine intestinal permeability: The cellobiose/mannitol test in gluten-sensitive enteropathy of Irish setters. Res Vet Sci 51:83-87, 1991 21. Hall EJ, Batt RM, Brown A: Assessment of canine intestinal permeability using 51 chromium-labelled ethylenediaminetetra-acetate. Am J Vet Res 50:2069-2074, 1989 22. Hall EJ, Bond PM, McLean C, et a!: A survey of the diagnosis and treatment of canine exocrine pancreatic insufficiency. J Small Anim Pract 32:613-619, 1991 23. Hawkins EC, Meric SM, Washabau RJ, eta!: Digestion of bentiromide and absorption of xylose in healthy cats and absorption of xylose in cats with infiltrative intestinal disease. Am J Vet Res 47:567-569, 1986 24. Hill FWG: Malabsorption syndrome in the dog: A study of thirty-eight cases. J Small Anim Pract: 13:575-594, 1972 25. Hill FWG, Osborne AD, Kidder DE. Pancreatic degenerative atrophy in dogs. J Comp Pathol 81:321-330, 1971 26. Jacobs RM, Hall RL, Rogers WA: lsoamylases in clinically normal and diseased dogs. Vet Clin Pathol 11:26-32, 1982 27. King CE, Toskes PP: Small intestine bacterial overgrowth. Gastroenterology 76:10351055, 1979 28. Koike H, Steer ML, Meldolesi J: Pancreatic effects of ethionine: Blockade of exocytosis and appearance of crinophagy and autophagy precede cellular necrosis. Am J Physiol 242:G297-G307, 1982 29. Mizunuma T, Kawamura S, Kishino Y: Effects of injecting excess arginine on rat pancreas. J Nutr 114:467-471, 1984 30. Nothman MM, Callow AD: Investigations on the origin of amylase in serum and urine. Gastroenterology 60:82-89, 1971 31. Nothman MM, Pratt TO, Benotti J: The effect of ligation of the pancreatic ducts and of pancreatectomy after duct ligation on serum lipase. J Clin Lab Med 33:833-840, 1948 32. Pidgeon G, Strombeck DR: Evaluation of treatment for pancreatic exocrine insufficiency in dogs with ligated pancreatic ducts. Am J Vet Res 43:461-464, 1982 33. Powers RE, Saluja AK, Houlihan MJ, eta!: Diminished agonist-stimulated inositol triphosphate generation blocks stimuli-secretion coupling in mouse pancreatic acini during diet-induced experimental pancreatitis. J Clin Invest 77:1668-1674, 1986 34. Rogers WA, Stradley RP, Sherding RG, eta!: Simultaneous evaluation of pancreatic exocrine function and intestinal absorptive function in dogs with chronic diarrhoea. JAm Vet Med Assoc 177:1128-1131, 1980 35. Rutgers HC, Hall EJ, Serensen SH, et a!: Differential sugar absorption for the assessment of diet-sensitive intestinal disease in dogs. In Proceedings of the Annual Congress of the British Small Animal Veterinary Association, Birmingham, UK, 1992, p184 36. Sateri H: Investigations on the exocrine pancreatic function in dogs suffering from chronic exocrine pancreatic insufficiency. Acta Vet Scand (Suppl) 53:1-86, 1975 37. Sherding RG, Stradley RP, Rogers WA, eta!: Bentiromide: xylose test in healthy cats. Am J Vet Res 43:2272-2273, 1982
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38. Sheridan V: Pancreatic deficiency in the cat. Vet Rec 96:229, 1975 39. Simpson JW, Doxey DL: Quantitative assessment of fat absorption and its diagnostic value in exocrine pancreatic insufficiency. Res Vet Sci 35:249-251, 1983 40. Simpson KW, Batt RM, Jones D, eta!: Effects of exocrine pancreatic insufficiency and replacement therapy on the bacterial flora of the duodenum in dogs. Am J Vet Res 51:203-206, 1990 41. Simpson KW, Johnstone JMS, Bell PRF, et al: Pancreatic function following partial pancreatectomy and anastomosis of the pancreatic duct to the stomach or duodenum in dogs. Res Vet Sci 52:97-104, 1992 42. Simpson KW, Morton DB, Batt RM: Effect of exocrine pancreatic insufficiency on cobalamin absorption in the dog. Am J Vet Res 50:1233-1236, 1989 43. Simpson KW, Simpson JW, Lake S, et a!: Effect of pancreatectomy on plasma activities of amylase, isoamylase, lipase, and trypsin-like immunoreactivity in dogs. Res Vet Sci 51:78-82, 1991 44. Stickle JE, Carlton WW, Boon GD: Isoamylases in clinically normal dogs. Am J Vet Res 41 :506-509, 1980 45. Strombeck DR: New method for evaluation of chymotrypsin deficiency in dogs. J Am Vet Med Assoc 173:1319-1323, 1978 46. Washabau RJ, Strombeck DR, Buffington CA, et a!: Evaluation of intestinal carbohydrate malabsorption in the dog by pulmonary hydrogen gas excretion. Am J Vet Res 47:1402-1406, 1986 47. Washabau RJ, Strombeck DR, Buffington CA, et al: Use of pulmonary hydrogen gas excretion to detect carbohydrate malabsorption in dogs. JAm Vet Med Assoc 189:674679, 1986 48. Westermarck E: The hereditary nature of canine pancreatic degenerative atrophy in the German shepherd dog. Acta Vet Scand 21 :389-394, 1980 49. Westermarck E: Treatment of pancreatic degenerative atrophy with raw pancreas homogenate and various enzyme preparations. J Vet Med A34:728-733, 1987 50. Westermarck E, Batt RM, Wiberg M, et al: The development of pancreatic acinar atrophy (PAA) in a German shepherd dog. In Proceedings of the lOth Annual Veterinary Forum of the American College of Veterinary Internal Medicine, San Diego, 1992, p 813 51. Westermarck E, Rimaila-Parnanen E: Two unusual cases of canine exocrine panceatic insufficiency. J Small Anim Pract 30:32-34, 1989 52. Westermarck E, Sandholm M: Faecal hydrolase activity as determined by radial enzyme diffusion: a new method for detecting pancreatic dysfunction in the dog. Res Vet Sci 28:341- 346, 1980 53. Williams DA, Batt RM: Diagnosis of canine exocrine pancreatic insufficiency by the assay of serum trypsin-like immunoreactivity. J Small Anim Pract 24:583-588, 1983 54. Williams DA, Batt RM: Sensitivity and specificity of radioimmunoassay of serum trypsin-like immunoreactivity for the diagnosis of canine exocrine pancreatic insufficiency. JAm Vet Med Assoc 192:195-201, 1988 55. Williams DA, Batt RM, McLean L: Reversible impairment of protein synthesis may contribute to jejunal abnormalities in exocrine pancreatic insufficiency. Clin Sci 68(suppl 11):37, 1985 56. Williams DA, Batt RM, McLean L: Bacterial overgrowth in the duodenum of dogs with exocrine pancreatic insufficiency. JAm Vet Med Assoc 191:201-206, 1987
Address reprint requests to Roger M. Batt, BVSc, MSc, PhD, MRCVS Department of Small Animal Medicine and Surgery The Royal Veterinary College Hawkshead Lane North Mymms Hatfield, Hertfordshire AL9 7TA England