SMALL INTESTINAL BACTERIAL OVERGROWTH

Small intestinal bacterial overgrowth (SIBO) is a syndrome in which there are excessive numbers of bacteria (more than 105 organisms per milliliter of intestinal contents) in the duodenum and jejunum in a fasting state.

The normal small intestinal microflora consists of a small but stable population of aerobic and fac­ultative anaerobic bacteria. Population size is influ­enced by factors such as the host's immune system, bacterial interactions, dietary composition, and the action of normal mechanisms that help to limit bacterial overgrowth (secretion of gastric acid, the dynamic process of intestinal motility and contin­uous aborad flow of ingesta, and antibacterial fac­tors in pancreatic juice). Causes of overgrowth of bacteria may include anatomic factors such as obstruction (e.g., partial stricture, presence of a mass), segmental hypomotility, conditions associ­ated with decreased secretion of gastric acid, intes­tinal mucosal disease, immunodeficiency states, and concurrent exocrine pancreatic insufficiency, or it may result from some unidentifiable cause. A state of immunodeficiency may be one of the reasons that SIBO might be more commonly diagnosed in German shepherds and shar-peis than in other breeds. These breeds appear to have a higher incidence of IgA deficiency. The most common problems in dogs with IgA deficiency are recurrent infections and atopic dermatitis. The infections associated with IgA deficiency are gen­erally not severe or life threatening, and treatment is symptomatic.

Although any species of bacteria may be found, Escherichia coli, enterococci, and lactobacilli are more common in dogs. Obligate anaerobic species are rarely found in the proximal small intestine of the dog, whereas in cats there may be up to 10⁵ to 108 bacteria per milliliter of fluid, commonly includ­ing obligate anaerobic bacteria such as Bacteroides, Eubacterium, and Fusobacterium.

The pathophysiology of SIBO is very complex and is related to both the effects of proliferation of bacteria in the intestinal lumen and direct damage to enterocytes. Potential mechanisms include direct injurious effects on brush border enzymes and carrier proteins, secretion of enterotoxins, deconjugation of bile acids, hydroxylation of fatty acids, and competition for nutrients.

The most common clinical signs of SIBO are diarrhea and weight loss.Vomiting, flatulence, and anorexia may also occur. Diarrhea is usually of small bowel type of consistency and may be watery to soft formed and malodorous. Stools may also be lighter in color than normal, but this is a nonspecific sign. Blood and mucus are usually not present (if they are, a large bowel disorder of any type should be considered). Other clinical signs that might be present occur as a result of a primary disorder (e.g., ravenous appetite associated with exocrine pancreatic insufficiency, decreased appetite, frequent vomiting, and lethargy associated with obstruction).

Diagnosis

Establishing a definitive diagnosis of SIBO is diffi­cult in a private-practice setting. Ideally, quantitative duodenal fluid cultures should be done to deter­mine if SIBO is present. This is difficult to do properly and is also expensive. Aliquots of duode­nal fluid need to be obtained either at laparotomy or by using a sterilized endoscope to obtain sam­ples from the small intestine. A positive response to antibiotics (e.g., amoxicillin, metronidazole, tetracycline, tylosin) for 2 to 4 weeks can be used as a presumptive diagnosis of SIBO.

The best screening tests for use in private practice involve serum analysis of fasting cobal­amin (vitamin B₁₂) and folate levels, although these tests are not regarded as very sensitive, because many affected dogs do not have abnor­mal test results. Abnormalities that suggest SIBO include elevated folate levels (due to increased production by abnormal microflora) and decreased cobalamin levels (due to utiliza­tion by microflora).

A new test for SIBO is serum unconju­gated cholic acid (SUCA). Many of the species of bacteria that increase in number in SIBO have the capacity to unconjugate bile acids. Unlike the con­jugated bile acids that are normally present in the small intestinal lumen, bile acids that are unconju­gated diffuse across the intestinal mucosa into the blood. In dogs, SUCA values greater than 72 nm/L are suggestive of bacterial overgrowth or disturbance of the normal flora of the upper small intestine. The SUCA test is currently available at the Texas A&M University GI Lab.

The sample should be shipped overnight on ice, ideally on the same day on which the sample is obtained. Dogs should be fasted for 12 hours before sampling. When testing for SIBO, it is rec­ommended that serum for cobalamin, folate, and SUCA assays should be run concurrently. Trypsin­like immunoreactivity (TLI) assay should be done as well if pancreatic exocrine insufficiency has not already been ruled out.

It is not uncommon for a dog with IBD to have SIBO as well. Intestinal biopsy speci­mens from patients with SIBO are normal or may show minimal morphologic mucosal changes.

Treatment

Treatment of SIBO involves use of selected antibi­otics, and treatment time may vary from several weeks to many months. Some patients require intermittent treatment. Sometimes once-daily antibiotic administration is adequate in patients that require long-term therapy. Any identifiable causes should be removed (e.g., surgical removal of blind or stagnant loops of bowel). Antibiotics that have broad-spectrum effect that includes anaerobic bacteria are selected (amoxicillin, metronidazole, tetracycline, and tylosin are good choices). If there is a rapid response to therapy, an attempt to dis­continue antibiotic administration can be made after 2 to 3 weeks.

If long-term antibiotic administration seems to be necessary, my preference is usually to use either metronidazole or tylosin powder. The recom­mended dose of tylosin is 5 to 10 mg/lb orally every 12 hours mixed with food. Tylosin powder has a bitter taste, and some dogs will accept it bet­ter if it is mixed in the food initially in very small amounts.

It is emphasized that concurrent problems may be present with SIBO, and even if the bacterial overgrowth is adequately treated, an underlying disorder not yet diagnosed and managed may itself cause persistence of clinical signs.