Intra-abdominal infections generally occur after entry of enteric organisms into the peritoneal cavity. An abscess is the body's way of attempting to contain an infection. Intraperitoneal and retroperitoneal abscesses can develop as a result of appendicitis, diverticulitis, necrotizing enterocolitis, pancreatitis, pelvic inflammatory disease, tubo-ovarian infection, surgery, or trauma. Given the vast number of microbes in our alimentary tract, any penetration of the wall of the gastrointestinal (GI) tract as a result of a vascular, traumatic, or iatrogenic event introduces these microbes into the abdomen. The concentration of microorganisms increases with distal progression down the GI tract. The morbidity of an intra-abdominal infection is 40%. The mortality is 20% in immunocompetent patients, and can be as high as 70% in the immunocompromised. This chapter explains types of peritonitis, locations of abscesses, diagnosis, treatment, common organisms associated with community-acquired and healthcare-associated infections, and suggested use of antimicrobials.

Abdominal abscess often follows or complicates peritonitis (see Chapter 57, Peritonitis). Primary peritonitis is an infection of the peritoneal cavity without an underlying violation of the intestinal wall; the most common cause of primary peritonitis is spontaneous bacterial peritonitis (SBP). The etiology of SBP is thought to be translocation of bacteria through the intestinal wall and into the abdomen. Clinical features of SBP may be subtle or absent, but usually SBP causes abdominal pain from infected ascites. The mainstay of treatment of primary peritonitis is antibiotics. Secondary peritonitis results from perforation of hollow viscera with spillage of intestinal contents, often from appendicitis, diverticulitis, or ulceration. The patient may initially present with severe abdominal pain, tenderness, rigid abdomen, or shock. The peritonitis can be focal or diffuse. If the spillage is small, the patient may not initially seek medical attention. Over the course of a few days the body will attempt to contain it, and an abscess may develop. If the abscess is less than 3 cm, the patient may only need antibiotics. An abscess 3 cm or greater usually needs drainage, and the percutaneous approach is preferred. Some abdominal abscesses progress to severe sepsis and shock, particularly when left untreated. Immunocompromised patients may have perforation with gross contamination of their abdomen, yet be relatively asymptomatic, making diagnosis more challenging. Tertiary peritonitis is a persistent or recurrent infection following treatment of primary or secondary peritonitis and is often found in patients with pre-existing comorbidities or who are immunocompromised.