Note for Pet Owners

This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

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Bacterial overgrowth is a condition in which abnormally large numbers of bacteria (108 to 1010 organisms per gram of intestinal contents) colonize the lumen of the small intestine, having serious effects on small intestine function resulting in intermittent
diarrhoea and sometimes weight loss. Bacteria involved include the anaerobic spp. Clostridia are the most important, but Bacteroides, Lactobacilli, Enterobacter and coliforms are also involved.

There are many causes of bacterial overgrowth but the most important are:

Breed Occurrence
German Shepherd Dogs seem to be predisposed to develop bacterial overgrowth. The reason is not known, but it may be related to low IgA concentrations in the small intestine. IgA is an important immunoglobulin which forms part of the local defense mechanism against infectious agents in the intestinal lumen.

In some cases bacterial overgrowth occurs as a complication in exocrine pancreatic deficiency or in juvenile pancreatic atrophy.

Chronic intermittent
diarrhoea, sometimes with weight loss. Steatorrhoea may be present and increased appetite in animals with energy deficiency. Some individuals may have bacterial overgrowth but show no external signs.

Affects of bacterial overgrowth on the intestine
Large numbers of bacteria cause various changes in the small intestine including:

  1. morphological changes - hypertrophy of crypt cells, blunting of villi, inflammatory cell infiltration
  2. functional changes - altered brush border enzymes, reduced brush border density, decreased carbohydrate and amino acid uptake, protein loss, decreased enterokinase concentration.
  3. deconjugation of bile acids - this is achieved by large numbers of Bacteroides, Clostridia, Lactobacilli and cocci in the gut lumen. The deconjugated bile acids are unable to form micelles so fat digestion and absorption is impaired leading to steatorrhoea. In addition they affect brush border enzymes and lysosomal enzymes.
  4. fat absorption - in metabolizing nutrients bacteria produce short-chained fatty acids which are absorbed very quickly across the small intestine, taking fluid with them, but medium and long-chained fatty acids (produced from the hydrolysis of triglycerides) inhibit the absorption of fluids in both the small and large intestine. This results in diarrhoea. In bacterial overgrowth fatty acids are metabolized (by lipases) to form hydroxy fatty acids which inhibit absorption and stimulate fluid secretion. I addition fatty acids can damage the intestine causing villus shortening and brush border damage - again interfering with normal absorption.
  5. carbohydrate metabolism - in bacterial overgrowth carbohydrates are broken down and rapidly absorbed and do not contribute to diarrhoea.
  6. protein metabolism - in bacterial overgrowth protein is metabolized and protein malnutrition can result . The proteins that are deaminated release ammonia which can be absorbed across the gut wall and converted to urea. In severely affected cases hypoproteinaemia can result with a loss of body mass. This contributes to the weight loss seen in some dogs.
  7. vitamins
    Vitamin B12 (cobalamin) is used by bacteria and so the absorption of cobalamin is reduced in bacterial overgrowth. Depletion of cobalamin may take several months as there are large stores in the body .Nevertheless this is the basis for the laboratory test which involves measuring serum cobalamin, and is now widely used in the assessment of animals with chronic diarrhoea.

    Another vitamin (Folate) is manufactured by intestinal bacteria - thus absorbed concentrations increase in the presence of bacterial overgrowth. Serum measurement of Folate is also a useful tool in assessing chronic diarrhoea cases.

All these changes affect normal nutrition and result in diarrhoea.

Confirmation is achieved by quantitative culture of duodenal fluid for both aerobic and anaerobic bacteria. The fluid is aspirated via endoscopy or duodenotomy. Affected dogs have more than 105 colony forming units per ml of duodenal fluid.

In addition, the results of the following tests are useful indicators of the presence of bacterial overgrowth:


  • Correct underlying primary cause if there is one.
  • Tetracyclines at a dose rate of 20mg/kg body weight every 8 hours for 10-14 day course - which has to be repeated if the condition recurs.

Recommended further reading reference "Small Animal Gastroenterology" by Strombeck & Guilford. 


Updated October 2013