Peritonitis means inflammation of the peritoneal or abdominal cavity, the normally sterile space that surrounds the abdominal organs.

Inflammation of this cavity is most commonly caused by a bacterial infection. Bacteria most often come from the intestinal tract, and usually escapes the intestine through some sort of intestinal injury. This intestinal injury could be a penetrating wire or foreign body, or intestine damaged by twist or loss of blood supply.

Once bacteria enter the abdominal cavity they multiply, populating the large space surrounding the organs. The bacteria cause an inflammatory response. Infection fighting cells already in the peritoneal space become active, and more cells are recruited from the blood. Fluid seeps into the space from the circulation. In advanced peritonitis, gallons of pus, composed of white blood cells and their inflammatory products and bacteria, bathes the organs.

A poorly understood infection of the abdomen with the bacteria Actinobacillus can occur repeatedly in the same horse. This is reportedly common in Australia, but not in the United States or elsewhere.

Diagnosis of peritonitis has been made easier by ultrasound, which allows the examiner to detect excessive amounts of fluid in the abdomen. Fluid sampling through abdominocentesis gives information about the inflammatory process and allows culture of the bacteria populating the space. Ultrasound and abdominocentesis are helpful in monitoring response to treatment.

Treatment usually requires more than just antibiotics. Importantly, the cause of the peritonitis must be identified and treated. In some cases, this treatment could mean abdominal surgery. Often the pus-filled abdomen must be aggressively drained and flushed.