Small Intestinal Bacterial Overgrowth (SIBO)


The upper part of the small intestine is responsible for the continued digestion of food as it exits the stomach and for the commencement of the process in which nutrients are absorbed. Bacteria normally aids in the process of breaking down the partially digested foodstuff within the intestines. In dogs, the overgrowth of this bacteria within the small intestine results in a common process descriptively labeled, “small intestinal bacterial overgrowth” or “SIBO.”

It’s a big problem for some dogs, and it occurs for a variety of reasons:

1. Ileus. When normal peristaltic waves moving the partially digested foodstuff forward aren’t present, bacterial populations increase. Neurologic problems, gastrointestinal obstruction, pancreatitis, peritonitis, parasitism and other processes can all cause the lack of forward intestinal movement we call “ileus.”

2. Too much foodstuff. This can happen when digestion and absorption is impaired, as with exocrine pancreatic insufficiency and other disorders that lead to poor digestion and absorption of nutrients.

3. Malnutrition. When the body’s overall condition is adversely affected, all the normal defenses can break down, leading to uncontrolled bacterial growth.

The above tend to be labeled “secondary” causes of SIBO. But SIBO can also be “idiopathic,” which means we don’t understand the exact mechanism by which it happens. In dogs, this problem is believed to be hereditary.

Symptoms and Identification

Diarrhea and flatulence are the most common signs of SIBO. Chronic, intermittent diarrhea is most typical of the idiopathic form, with many dogs also suffering weight loss, stunted growth and/or generalized unthriftiness. Some dogs may appear inordinately hungry and may even engage in pica or stool consumption.

SIBO is easily identified by noting small bowel diarrhea (characterized by limited straining and its large volumes) and finding large numbers of bacteria in the fecal material. Diagnosis of the idiopathic form is aimed primarily at ruling out all potential causes of secondary SIBO. Because these are numerous, the process usually involves X-rays, serial fecal examination (not cultures, which are notoriously unreliable), and sometimes endoscopy to test the upper part of the small intestine for high bacterial counts.

Blood tests for the vitamins known as folate and cobalamine are also indicative of the process. High folate levels and decreased cobalamine typically result. That’s because folate is synthesized by the bacteria, and cobalamine is bound by them.

Affected Breeds

German Shepherd Dogs are overrepresented among those who suffer the idiopathic, hereditary form of this disease. Apart from GSDs, other young, large breed dogs are most frequently affected.

Secondary SIBO can occur in any dog of any breed.


Treating the underlying process is the approach best undertaken for secondary SIBO. For idiopathic SIBO, it’s clear that antibiotics are highly effective in helping owners manage their dogs’ clinical signs. That’s why this version of the disease is often referred to as “antibiotic-responsive SIBO.”

Veterinary Cost

Though antibiotics are not very expensive in and of themselves, diagnosing idiopathic SIBO by ruling out all other causes can be time consuming and expensive, especially if endoscopy is elected (this can easily cost between $500 to $1,000). Therefore, it’s not unusual for a careful diagnostic plan undertaken by a specialist to run well over $1,000.

For secondary SIBO, the expense of the disease depends on similar factors: How hard is it to identify the root cause? If the cause is simple and easily identified (parasites, for example), then the process can be eliminated quickly and inexpensively. But if an intestinal obstruction is the cause, surgery to repair it can run into the thousands of dollars.


Though we don’t understand the mode of inheritance, dogs identified as having idiopathic SIBO should be removed from all breeding programs to eliminate the risk of passing on this hereditary trait.


Textbook of Veterinary Internal Medicine: Disease of the dog and cat, 5th edition, eds. Ettinger and Feldman, pp 1223-1228.

Small Animal Medicine, eds. Nelson and Couto, pp 453.

Strombeck's Small Animal Gastroenterology, third edition, eds. Guilford, Center, Strombeck, Williams, and Meyer, pp 370-373.