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'''Antibiotic responsive diarrhoea''' (ARD) describes a clinical syndrome which is associated with alterations in the population of enteric bacterial flora and in the response of the host immune system to these bacteria.  It may occur independently of any other apparent pathological process ('''idiopathic''') but it occurs commonly with a number of intestinal diseases ('''secondary''').  The term 'antibiotic responsive diarrhoea' has replaced the previous description of '''small intestinal bacterial overgrowth''' (SIBO) due to uncertainty over the level at which enteric bacteria could be said to be present in excessive numbers and because an increased number of bacteria is not always the cause of the clinical syndrome described.   

'''Antibiotic responsive diarrhoea''' (ARD) describes a clinical syndrome which is associated with alterations in the population of enteric bacterial flora and in the response of the host immune system to these bacteria.  It may occur independently of any other apparent pathological process ('''idiopathic''') but it occurs commonly with a number of intestinal diseases ('''secondary''').  The term 'antibiotic responsive diarrhoea' has replaced the previous description of '''small intestinal bacterial overgrowth''' (SIBO) due to uncertainty over the level at which enteric bacteria could be said to be present in excessive numbers and because an increased number of bacteria is not always the cause of the clinical syndrome described.   

The mucosal immune system of the host and the enteric bacterial flora interact constantly in the gastro-intestinal (GI) tract.  The host must remain tolerant of the enteric flora but must still be able to recognise and respond to potentially pathogenic organisms.  These apparently contradictory tasks are resolved by the ability of the immune system to 'tolerate' certain antigens if these are presented to macrophages and dendritic cells in an appropriate manner.   

The mucosal immune system of the host and the enteric bacterial flora interact constantly in the gastro-intestinal (GI) tract.  The host must remain tolerant of the enteric flora but must still be able to recognise and respond to potentially pathogenic organisms.  These apparently contradictory tasks are resolved by the ability of the immune system to 'tolerate' certain antigens if these are presented to macrophages and dendritic cells in an appropriate manner.  The evidence available so far suggests that ARD is partly due to an alteration in size and dynamics of the intestinal bacterial population and partly due to a failure of the normal tolerance mechanisms.

In cases of idiopathic ARD, the only consistent clinical finding is responsiveness to antimicrobial therapy.  This is found commonly, but not exclusively, in young [[Canine Breeds - WikiNormals #Pastoral Group|German Shepherd Dogs]] and it has been suggested that this is associated with a deficiency in [[IgA]] or with another form of immune dysregulation in this breed.  However, the true underlying mechanism could be far more complex and numerous other hypotheses have been proposed.  In cases of secondary ARD, there is usually an underlying intestinal disorder, of which the most common are:

In cases of idiopathic ARD, the only consistent clinical finding is responsiveness to antimicrobial therapy.  This is found commonly, but not exclusively, in young [[Canine Breeds - WikiNormals #Pastoral Group|German Shepherd dogs]] and it has been suggested that this is associated with a deficiency in [[IgA]] or with another form of immune dysregulation in this breed.  In cases of secondary ARD, there is usually an underlying intestinal disorder, of which the most common are:

*Increased concentrations of small intestinal substrates resulting from failure of host digestion or absorption

*Increased concentrations of small intestinal substrates resulting from failure of host digestion or absorption

**[[Lymphangiectasia]] leads to increased luminal concentrations of fat and protein.

**[[Lymphangiectasia]] leads to increased luminal concentrations of fat and protein.

**[[Exocrine Pancreatic Insufficiency]] results in an inability to digest fat, protein and carbohydrate, leaving these substrates in the intestinal lumen.

**[[Exocrine Pancreatic Insufficiency]] results in an inability to digest fat, protein and carbohydrate, leaving these substrates in the intestinal lumen.

**[[Villous Atrophy]] leads to the loss of digestive enzymes on the brush borders of enterocytes.

**[[Villous atrophy with intact/hypertrophic crypt glands|Villous atrophy]] leads to the loss of digestive enzymes on the brush borders of enterocytes.

**[[Extrahepatic Biliary Obstruction]] leads to an inability to digest and absorb fat because bile salts do not pass into the intestine.

**[[Biliary Tract - Obstruction|Extrahepatic Biliary Obstruction]] leads to an inability to digest and absorb fat because bile salts do not pass into the intestine.

**Congenital deficiencies of brush border enzymes are very rare in animals.

**Congenital deficiencies of brush border enzymes are very rare in animals.

*Altered GI motility causing changes in the population density of enteric microflora

*Altered GI motility causing changes in the population density of enteric microflora

**Anatomical disorders which may be congenital or acquired (as with surgical removal of the ileo-caeco-colic junction allowing reflux of bacteria from the large to the small intestine.)

**Anatomical disorders which may be congenital or acquired (as with surgical removal of the ileo-caeco-colic junction allowing reflux of bacteria from the large to the small intestine.)

*Reduction in the concentration of factors that usually act to limit bacterial population growth

*Reduction in the concentration of factors that usually act to limit bacterial population growth

**Failure to produce gastric acid (achlorhydria) is rare in small animals, even with atrophic gastritis.

**Failure to produce gastric acid (achlorhydria) is rare in small animals, even with atrophic gastritis. Gastric acid production may be suppressed by [[Gastroprotective Drugs|drugs that inhibit secretion]], such as ranitidine and omeprazole

The consequences of ARD are numerous and these are only beginning to be explored fully.  They include:

The consequences of ARD are numerous and these are only beginning to be explored fully.  They include:

*Interference with fluid and nutritional absorption due to dysfunction of the enzymes located at the microvillous brush border.  Depending on the cause of the ARD, this may worsen any concurrent maldigestion or malabsorption.

*Interference with fluid and nutritional absorption due to dysfunction of the enzymes located at the microvillous brush border.  Depending on the cause of the ARD, this may worsen any concurrent or underlying maldigestion or malabsorption.

*Disturbances in mucosal permeability.

*Disturbances in mucosal permeability.

*Deconjugation of bile acids which reduces the ease with which they are removed from the circulation by the liver during enterohepatic recirculation.  Hepatic bile acid synthesis must therefore increase to compensate.  Deconjugated bile acids also irritate the colonic mucosa causing colitis and diarrhoea.

*Deconjugation of bile acids which reduces the ease with which they are removed from the circulation by the liver during enterohepatic recirculation.  Hepatic bile acid synthesis must therefore increase to compensate.  Deconjugated bile acids also irritate the colonic mucosa causing colitis and diarrhoea.

*Hydroxylation of fatty acids, which like deconjugated bile acids, are irritant to the colonic mucosa and cause colitis and diarrhoea.

*Hydroxylation of fatty acids, which like deconjugated bile acids, are irritant to the colonic mucosa and cause colitis and diarrhoea.

==Signalment==

==Signalment==

Idiopathic ARD is common in young German Shepherd Dogs but secondary ARD may occur in any breed or age of dog depending on the underlying cause.  ARD is thought to occur in cats but it is not well characterised in this species.  It is suggested that the condition is much more common than previously suspected.

Idiopathic ARD is common in young German Shepherd dogs but secondary ARD may occur in any breed or age of dog depending on the underlying cause.  ARD is thought to occur in cats but it is not well characterised in this species.  It is suggested that the condition is much more common than previously suspected.

==Diagnosis==

==Diagnosis==