Bacterial overgrowth

This is a condition seen in man and dogs where bacteria in the small intestine proliferate to abnormally high levels and subsequently interfere with intestinal function. There are few documented cases in dogs, in contrast to the extensive literature written on the subject in man. In the few canine reports Pseudomonas aeruginosa (), Bacteroides (), E. coli, Enterococcus, and Clostridium spp. () have been implicated. Clostridium difficile has been reported as causing chronic diarrhoea in dogs ().

The German Shepherd dog appears to be more frequently represented than other breeds, although any breed can be affected. There is no sex predisposition. The aetiology is complex and not fully understood but may involve one of the following; (1) increased gastric pH (above 5) results in ingested bacteria surviving passage through the stomach to the small intestine; (2) extensive use of broad spectrum antibiotics may allow one population of bacteria to survive while others are suppressed; (3) reduced intestinal motility such as in stagnant loop syndrome, paralytic ileus and intestinal obstruction may allow bacteria to proliferate (); and (4) impaired immunity has also been implicated (). Bacterial overgrowth is often present in dogs with exocrine pancreatic insufficiency, due to the presence of unabsorbed food in the small intestine.

Proliferating bacteria may produce protease enzymes which damage the microvillus structure of the small intestine and destroy the brush border enzymes and carrier proteins (). They may also increase lysosomal fragility and lead to loss of mitochondria within enterocytes. Histologically there are rarely any morphological changes present, only biochemical changes. It is also possible that bacteria may deconjugate bile salts leading to steatorrhoea and the presence of bile products in the large intestine. Unabsorbed fats are converted to hydroxy fatty acids which induce large intestinal secretory diarrhoea (). Carbohydrates may be broken down to oligosaccharides which have an osmotic effect in the large intestine ().

Anaerobic bacteria are thought to cause more damage than aerobic bacteria. The former organisms appear to break down the glycocalyx layer which protects the mucosa. This allows luminal contents to damage the brush border proteins and intestinal function ().

Clinical diagnosis

Clinical signs exhibited by dogs with bacterial overgrowth include chronic intermittent diarrhoea which may or may not be associated with weight-loss and polyphagia. Depressed appetite, coprophagia and natulance may also be detected. Culture of the faeces is not normally of diagnostic value, and it is often necessary to culture intestinal juice or determine serum folate and vitamin B12 levels. Many bacteria have the ability to synthesize folate and bind B12 in the small intestine. These changes can be measured in the serum to reflect changes in the intestine. Elevated serum folate levels and reduced serum B12 levels are indicative of bacterial overgrowth. Xylose absorption may be abnormal in some cases due to bacterial utilization of the monosaccharide (), but is equally likely to be in the normal range. Urine nitrosonapthal test may also be positive ().

Histological examination of intestinal biopsy samples rarely reveals any gross change although some infiltration with lymphocytes and plasma cells may occur. Assay of brush border enzymes will reveal a fall in alkaline phosphatase activity.

Bacterial overgrowth: Treatment

An attempt should be made to determine the underlying cause, which should be corrected, but in the majority of cases this is never found. Fortunately the changes described above are rarely permanent and following suitable antibiotic treatment normal small intestinal function can be restored. Oxytetracycline 10mg/kg tid for up to 30 days is required or tylosin 10mg/kg tid or metronidazole 20mg/kg/day for the same period may be used (Table Drugs employed for the treatment of chronie small intestinal disease). It is not unusual for relapses to occur, if the underlying cause has not been controlled.

Table Drugs employed for the treatment of chronie small intestinal disease

Tylosin (Tylan) 10mg/kg tid for baeterial overgrowth
20-40/kg inflammatory bowel disease
Oxytetracycline (Terramycin) 10mg/kg tid bacterial overgrowth
Metronidazole (Flagyl) 25mg/kg bid
Prednisolone 1-3mg/kg/day
Azathioprine (Imuran) 0.3mg/kg/day for cats
1-2mg/kg/day for dogs