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Antibiotic-responsive enteropathy (ARE) in the dog

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Category: Schede
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  • Disciplina: Gastroenterologia
  • Specie: Cane

Since a few years the term “antibiotic-responsive enteropathy” (ARE) has substituted the term “small intestinal bacterial overgrowth” (SIBO). ARE describes an antibiotic-responsive chronic inflammatory enteropathy the pathophysiology of which is, to date, not entirely known. SIBO, on the other hand, describes a condition in which the increased number of intestinal bacteria is causative of the disease. The term SIBO can therefore be used in all those cases in which a disease or a primary clinical condition is responsible for an increased number of bacteria, with consequent associated clinical signs. Exocrine pancreatic insufficiency is a typical example of this; in spite of it being a primary disorder, it is always the cause of secondary SIBO. Finnish authors have further classified ARE on the basis of the antimicrobial used, thus defining an enteropathy which responds to tylosin as TRD  (tylosin-responsive diarrhoea). In the cat, to date, no scientific evidence on the existence of ARE has been reported.

 

PATHOPHYSIOLOGY

The pathophysiology of ARE is not yet entirely known. To date, no uniformity of opinion exists with regard to the extent, make-up and physiological behaviour of the intestinal microbiota in the dog.

In veterinary medicine the definition of bacterial overgrowth, especially when this is responsible for clinical signs, is complex. In man, secondary SIBO is associated with the presence of intestinal blind pouches, idiopathic intestinal dysmotility, prolonged achlorhydria, stenosis, etc., all conditions which have not been shown to be causal of SIBO in veterinary medicine. In the dog, exocrine pancreatic insufficiency is perhaps the main cause of SIBO; the deficiency of pancreatic secretions and the consequent presence of undigested food results in a favourable growth medium for the resident microbial flora and for the subsequent dismicrobism. Partial intestinal obstructions, of whichever origin, are also a cause of SIBO. In the above mentioned conditions, an increase in the number of bacteria is causal of the clinical signs, while in the course of idiopathic ARE this cause-and-effect relationship is still to be proven. The current hypothesis is that in the course of ARE the number of intestinal bacteria may vary considerably, depending on the presence of concurrent and predisposing factors, and apparently rather than the number it is the type of bacteria which has an impact on the pathogenesis of the enteropathy.

In terms of pathophysiology, the most widely accepted hypothesis considers the interaction between microbial flora, altered in number and make-up, and the local immune system. In the German Shepherd, one the of the breeds most affected by ARE, various studies have been carried out in order to show a deficiency in the production, distribution, or in the efficiency of mucosal IgA. Some studies have reported an altered mucosal permeability and a defect in enterocyte brush-border enzymes, however it is not entirely clear if these abnormalities are primary or secondary. The serum IgA deficiency shown in the German Shepherd1,2 does also not seem to be entirely associated with IgA deficiency at intestinal level, and the faecal IgA3,4 test has given variable results in the various studies made. Finally, some recent studies5,6  in the German Shepherd have identified genetic abnormalities correlated with the expression of IgA, but unfortunately such abnormalities are also present in patients without clinical signs of ARE. This finding makes the eventual genetic test poorly specific and the abnormality presumably a breed-linked phenomenon. Finally, the IgA deficiency could simply be an epiphenomenon correlated to a deficient interaction between the microbial flora and the innate immune system dealing with bacteria.

TLR (toll-like receptor) abnormalities and their interaction with the microbial flora have been the object of many studies on the pathophysiology of IBD; in ARE as well, this interaction could play a role in its pathophysiology. In the course of ARE, an increased expression of cytokines in the lamina propria has been reported7,8 in the absence of an increased number of CD4 cells. Such studies have shown that antimicrobial therapies cause a reduction in the number of cytokines but not a decrease in the number of bacteria, underlying the fact that the therapeutic effect might depend on the immunomodulatory activity of antibiotics.

All the above findings support the hypothesis that the dysregulation of the immune system may play a key role also in the pathophysiology of ARE. The border between IBD and ARE is rather thin, and presumably more than one overlap exists between these two disorders. Other, different hypotheses on the pathophysiology of ARE have been reported; one of these takes into consideration the presence of specific pathogens, such as Helicobacter-like bacteria or microorganisms belonging to the genus Escherichia Coli, which may abnormally interact with the immune system or cause perinatal infections. Another hypothesis, considered credible by the author, takes into consideration the “neonatal memory” of the resident microbial flora. According to this theory, the dysbiosis which is present in the course of ARE finds its origin in the neonatal period, for unexplained reasons; this qualitative and quantitative alteration is rebalanced with antibiotic therapy, with an improvement of the clinical signs, however, in view of this concept of neonatal memory, following the suspension of the therapy the dysbiosis regenerates itself. Whichever the underlying pathophysiology, in the presence of ARE the pathogenic action of bacteria can be based on various mechanisms: competition for nutrients, damage to the enterocyte brush-border, deconjugation of bile salts and consequent malabsorption of fats, and hydroxylation of fatty acids and of their derivatives with consequent hyperstimulation of colonic secretions. All these mechanisms contribute to altering the function of the intestinal mucosa, thus triggering the inflammatory process, which is in turn responsible for the clinical signs.

 

CLINICAL SIGNS

ARE is mostly found in large, young-aged dogs. The age issue is questionable, as many dogs treated for years with antibiotic therapy (perhaps not confessed by the owner) and which come for a physical examination in old age may be affected with ARE. The German Shepherd is the breed which is traditionally associated with ARE. In publications, no reports have been made on the presence of ARE in small-sized dogs. In general, not being this disorder well defined, it cannot be excluded from the differential diagnosis just on the basis of epidemiological data and on the clinical history. The main clinical sign consists in chronic and relapsing diarrhoea of the small bowel. In some patients colitis is the main presenting sign. Many patients present polyphagia and/or coprophagia, however disorexia or anorexia are not infrequent, especially in the presence of hypocobalaminaemia. Weight loss or stunted growth may be present in the more severely affected patients. In secondary SIBO the clinical signs are more variable and depend on the primary cause: extreme emaciation and polyphagia may for example be present in the course of EPI, as against ingravescent vomiting in the presence of total or partial obstructive conditions.

 

DIAGNOSIS

A key factor in the diagnostic workup which allows to suspect the presence of ARE is the clinical history. A good clinical and symptomatic response to antibiotic therapy and especially a rapid relapse of symptoms following the suspension of the drug therapy are indicative of the presence of ARE. Secondary SIBO may also show a clear improvement following antibiotic therapy, however in this case clinical manifestations vary depending on the primary cause.

The definitive diagnosis of ARE is not easily established; to date there isn’t a single test or a combination of tests with the sensitivity and specificity necessary to allow the differentiation with other chronic enteropathies. Various laboratory tests have been proposed over the years to achieve this end.

Duodenal juice culture, which is widely accepted in human medicine, has shown some limitations in veterinary medicine. First, in view of the existing uncertainties about what is the normal number of enteric bacteria. In addition, patient variability may also have an impact on the concept of normality: the age, diet, environmental and hygienic conditions. Culture tests are of difficult execution and depend on many pre-analytic and analytic procedural variables, such as method of sample collection, culture and data assessment methodologies. All the concepts uncritically taken from human medicine are currently being questioned. The idea that the proximal bowel is sterile has currently been abandoned and most authors consider a duodenal bacterial concentration of 1 x 109 cfu/ml as normal. Faecal culture tests are not useful and are not used in the diagnosis of ARE. Serum TLI (Trypsin-like immunoreactivity) is necessary to exclude the presence of pancreatic failure, which, as previously mentioned, is one of the main primary disorders causing SIBO.

The combined measurement of folate and cobalamin is indicative of ARE when the concentration of the first is elevated and the concentration of the second is reduced. This combination correlates with an increased production of folate by disbiotic bacteria and the reduction of vitamin B12 in view of its reduced intestinal absorption and the increased consumption by the same bacteria. Unfortunately, this combination is rarely present, and in a retrospective study on SIBO it was reported in only 5% of the subjects.9 Notwithstanding the limited sensitivity, this test should always be carried out as it can be useful for the therapeutic approach; in the presence of hypocobalaminaemia supplementation is mandatory.

Although theoretically valid, as bacteria are the only producers of hydrogen, the exhaled hydrogen breath test is not reliable in the diagnosis of ARE. This because other causes of carbohydrate malabsorption may be responsible for an increased number of hydrogen-producing bacteria; in addition, the analytical methodology is hardly standardizable.

The measurement of unconjugated bile acids (SUCA) is not reliable, and is not used.10 Intestinal permeability tests, such as the measurement of cr-EDTA or sugar absorption tests, may result altered in the course of ARE, however these are neither specific nor pathognomonic. In addition, these are difficult tests to carry out and have currently been abandoned. Markers of bacterial metabolism, such as p-nitrosonaphthol or glycolic acid, could be useful in the course of ARE and SIBO, however it is not clear if an increased number of bacteria is always present; to date, no studies have confirmed the reliability of these tests. The assay of orally introduced metabolites, such as para-aminobenzoic acid (PABA), has been considered, however the resulting information is non-specific.10

The diagnostic approach to a chronic enteropathy often requires the execution of an endoscopic  (Figs. 1 and 2) and subsequently of an histopathological examination; unfortunately, no pathognomonic histological alterations are known for ARE and hence the resulting information is non-specific and may even be misleading, as it is also usually compatible with the diagnosis of IBD. In the course of ARE the histological examination is characterized by mild alterations and modifications of the enterocyte brush-border (Fig. 3).

In conclusion, in order to establish a final diagnosis of ARE it is necessary to eliminate all other known causes of chronic enteropathy, to eliminate the primary causes of secondary SIBO, to confirm an adequate clinical response to antibiotic therapy and to have a relapse of symptoms following its suspension.

 

TREATMENT

No cure is available for idiopathic ARE, however antibiotic therapy allows the control of clinical signs. The more commonly used active molecules to achieve this goal are oxytetracycline, metronidazole and tylosin. Oxytetracycline is administered ad the dose of 10-20 mg/kg per os every 8-12 hours. Oxytetracycline is excreted via the bile, it enters the enterohepatic circulation and reaches high penetration levels in the bowels and bile. As its therapeutic action takes place at endoluminal level, when possible oxytetracycline should be administered orally. This drug should not be administered to subjects without permanent dentition, as the colour of the teeth may be altered. In addition, some authors11 have reported the rapid development of antibiotic resistance presumably mediated by bacterial plasmids. The efficacy of oxytetracycline is not linked to its direct antimicrobial activity, as it does not decrease the total number of bacteria; the hypothesis is that it acts by exerting a selective pressure on the flora, favouring the predominance of the less harmful bacteria. It is also not clear if the benefit observed at the enterocyte brush-border is caused by a direct effect of the drug or if it is secondary to bacterial inhibition. An additional hypothesis is that oxytetracycline may also have an immune-modulatory action. In addition, this active molecule may itself be the cause of gastrointestinal clinical signs such as vomiting and diarrhoea.

Metronidazoleis used at the dose of 10-15 mg/kg BID or TID per os. Its immune-modulatory action is known, and presumably contributes to its efficacy in the course of both ARE and IBD. In vitro studies have shown the mutagenic properties of metronidazole;12 although no conclusive data on the mutagenic potential of the drug in vivo are available, its continued use for long periods of time should be avoided. Tylosin is a macrolide antibiotic, mostly active against Gram-positive bacteria, against some Gram-negative bacteria and some microorganisms such as Mycoplasma and Clamydia spp. It has been hypothesized that tylosin may have an immune-modulatory effect, even if the exact mechanism of action has not been identified. The bactericidal dose of tylosin is of 25 mg/kg BID per os;  in the course of ARE it is used at the dose of 15-20 mg/kg BID or TID per os. The therapeutic action of tylosin has been especially studied by Scandinavian authors13,14,15,16  who, as previously mentioned, defined this tylosin-responsive enteropathy as TRD. The hypothesis of these authors on the efficacy of this molecule does not take into consideration the direct antimicrobial action of the drug, but rather the fact that tylosin, also at low dosage, prevents bacteria from reaching the enterocytes by means of a sort of direct mechanical action16,17 which prevents adhesion to mucosal cells. In a communication at the ACVIM 2010 congress it has been reported that tylosin may be effective at a dose of 5 mg/kg SID per os, hence at a dose quite inferior to the antimicrobial dose, and therefore acting as a prebiotic.

Whichever the active molecule used, the therapy must be continued for at least 4-6 weeks. In the presence of a partial initial response the drug may be changed. If the antibiotic therapy is administered for too short of a period the risk of relapse is greater. Only in rare cases can the medical therapy be suspended with the patient then maintaining good clinical conditions for an extended period of time before relapsing; in most cases, instead, the suspension of therapy is followed by relapse within a short period of time. In these patients it is however possible to reduce the frequency of daily administrations, passing even to a weekly regime, until achieving the minimum effective dose. Some authors prefer a reduction in the dose while maintaining unchanged the frequency of administration; the opinion of the author is however that this modality favours the development of antibiotic resistance. This said, antimicrobial resistance and the selection of bacterial strains difficult to eradicate are potential risks, which are always present when treating ARE. These risks are present not only in the patient undergoing therapy but also for humans and for animal public health. It is for this reason that in the course of ARE prolonged therapies making use of the most modern and effective antimicrobial molecules should be strictly avoided.

Some subjects improve with ageing, presumably in view of reduced caloric consumption or for the complete maturation of the immune system. In all cases, and also for reasons of antimicrobial resistance and hence for public health concerns, in patients maintaining good clinical conditions for extended periods of time the suspension of therapy should be attempted.

The therapeutic management of subjects with ARE may also benefit from the use of pro- and pre-biotics, although the real efficacy of these has not been proven. Diet is a relevant factor in all enteropathies; in the course of ARE a highly digestible low-fat diet or a commercial or home-made hypoallergenic diet are advisable. In the presence of hypocobalaminaemia associated with ARE, supplementation is necessary with the dosages and modalities previously described for IBD.

 

Suggested readings

  1. Batt RM, Barnes A, Rutgers HC, et al. Relative IgA deficiency and small intestinal bacterial overgrowth in German shepherd dogs. Res Vet Sci 1991;50:106–11.
  2.  German AJ, Hall EJ, Day MJ. Measurement of IgG, IgM and IgA concentrations in canine serum, saliva, tears and bile. Vet Immunol Immunopathol 1998;64:107–21.
  3. Peters IR, Calvert EL, Hall EJ, et al. Measurement of immunoglobulin concentrations in the feces of healthy dogs. Clin Diagn Lab Immunol 2004;11:841–8.
  4. Littler RM, Batt RM, Lloyd DH. Total and relative deficiency of gut mucosal IgA in German shepherd dogs demonstrated by faecal analysis. Vet Rec 2006;158:334–41.
  5.  Peters IR, Helps CR, Calvert EL, et al. Identification of four allelic variants of the dog IGHA gene. Immunogenetics 2004;56:254–60.
  6.  Peters IR, Helps CR, Lait PL, et al. Detection of allelic variants of the canine IGHA gene by fluorescence resonance energy transfer melting temperature examination. J Immunol Methods 2005;304:60–7.
  7. German AJ, Helps CR, Hall EJ, et al. Cytokine mRNA expression in mucosal biopsies from German Shepherd dogs with small intestinal enteropathies. Dig Dis Sci 2000;45:7–17.
  8. Peters IR, Helps CR, Calvert EL, et al. Cytokine mRNA quantification in duodenal mucosa from dogs with chronic enteropathies by real-time reverse transcriptase polymerase chain reaction. J Vet Intern Med 2005;19:644–53.
  9.  Rutgers HC, Batt RM, Elwood CM, et al. Small intestinal bacterial overgrowth in dogs with chronic intestinal disease. J Am Vet Med Assoc 1995;206:187–93.
  10. German AJ, Day MJ, Ruaux CG, et al. Comparison of direct and indirect tests for small intestinal bacterial overgrowth and antibiotic-responsive diarrhea in dogs. J Vet Intern Med 2003;17:33–43.
  11.  Marks SL. Editorial: small intestinal bacterial overgrowth in dogs—less common than you think? J Vet Intern Med 2003;17:5–7.
  12. Mudry MD, Carballo M, Labal de Vinuesa M, Gonzalez Cid M, Larripa I. Mutagenic bioassay of certain pharmacological drugs: III. Metronidazole (MTZ).   Mutat Res 1994 Mar 1;305(2):127-32.
  13. Kilpinen S, Spillmann T, Syrjä P, Skrzypczak T, Louhelainen M, Westermarck E. Effect of tylosin on dogs with suspected tylosin-responsive diarrhea: a placebo-controlled, randomized, double-blinded, prospective clinical trial. Acta Vet Scand 2011 Apr 14;53:26.
  14. Suchodolski JS, Dowd SE, Westermarck E, Steiner JM, Wolcott RD, Spillmann T, Harmoinen JA. The effect of the macrolide antibiotic tylosin on microbial diversity in the canine small intestine as demonstrated by massive parallel 16S rRNA gene sequencing. BMC Microbiol 2009 Oct 2;9:210.
  15. Westermarck E, Frias R, Skrzypczak T. Effect of diet and tylosin on chronic diarrhea in beagles. J Vet Intern Med 2005 Nov-Dec;19(6):822-7.
  16. Westermarck E, Skrzypczak T, Harmoinen J, Steiner JM, Ruaux CG, Williams DA, Eerola E, Sundbäck P, Rinkinen M. Tylosin-responsive chronic diarrhea in dogs. J Vet Intern Med 2005 Mar-Apr;19(2):177-86.
  17. Chugh TD, Babaa E, Burns G, Shuhaiber H. Effect of sublethal concentration of antibiotics on the adherence of Staphylococcus epidermidis to eukaryotic cells. Chemotherapy 1989;35:113–118.