Diarrhea can range from mild to severe.

Treatment with antibiotics is appropriate if one of these bacteria's isolated from the faeces in large numbers from an animal with signs of diarrhea. There is some evidence to suggest however that treatment with antibiotics in the case of Salmonella may lead to the development of a carrier state with prolonged shedding of the bacteria in the faeces.

More recently a syndrome called secondary bacterial overgrowth has been recognized. In this condition the bacteria causing diarrhea are those which grow normally in the gut and play an important role in its function. These bacteria are kept in balance by a number of factors including the flushing action of food moving through the intestine, the bodies immune system, and the protective lining of the gut. Should any of these fail the normal bacteria can significantly increase in number overgrowing the intestine.

Secondary bacterial overgrowth should be considered as a potential problem following intestinal surgery for obstructions or tumors, inflammatory bowel disease, immune disorders and secondary to pancreatic insufficiency.

In most cases this condition is diagnosed on the basis of history or failure to respond to treatment for other specific gut conditions. Specific diagnosis can only be made by assessing the number of bacteria in samples taken directly from the intestine.

Treatment with a broad-spectrum antibiotic is usually effective but should be continued for 10-14 days. Improvement within the first 7 days of therapy also supports the diagnosis of bacterial overgrowth. Relapses are common and may require repeated or prolonged courses of the antibiotics.