Etiology and Epidemiology

Peritonitis may be focal or diffuse and can also be classified as primary or secondary peritonitis. Primary peritonitis is peritonitis that has no identifiable underlying cause. In secondary peritonitis, a reason for bacterial leakage into the abdomen can be identified.

In humans, primary peritonitis has also been referred to as spontaneous bacterial peritonitis and most often complicates development of ascites (e.g., secondary to cirrhosis, hepatitis, or congestive heart failure).7,8 In contrast, conditions that predispose to ascites are rarely present in the history of dogs and cats with primary peritonitis. Some primary peritonitis in dogs or cats may result from hematogenous spread of bacteria to the peritoneum or bacterial translocation from the gastrointestinal tract.⁷

Secondary peritonitis is the most common form of peritonitis in dogs and cats and occurs when bacteria are introduced into the peritoneal space as a result of gastrointestinal perforation, penetration of the abdominal wall, rupture of the genitourinary tract, rupture of intra-abdominal abscesses, gallbladder rupture, or ascending infection of the umbilicus in neonates (Table 88-2).^7,9–15 In dogs, low preoperative total protein or serum albumin concentrations are risk factors for development of bacterial peritonitis after gastrointestinal surgery.¹⁶

The bacterial species involved in peritonitis reflect the normal flora of the gastrointestinal tract. Mixed aerobic-anaerobic infections, with up to four different bacterial species, occur in more than 50% of affected dogs and cats.7,10 Escherichia coli is the most common isolate from both dogs and cats, followed by Enterococcus and Clostridium spp.^{7,10,11,14,15,17,18} Other isolates include staphylococci, streptococci, Pseudomonas aeruginosa or Acinetobacter spp., a variety of anaerobes, gram-negative Enterobacteriaceae (Proteus, Citrobacter, Serratia, Klebsiella, or Enterobacter), Actinomyces, or in cats, Pasteurella multocida. In one study, a greater proportion of dogs with primary peritonitis had gram-positive bacterial infections than dogs with secondary peritonitis.⁷ Uncommonly, Candida albicans can be involved, especially if there is a history of antibiotic treatment (see Chapter 67).

The mean age of dogs with peritonitis is around 5 to 7 years, and the mean age of cats is 7 to 10 years, although dogs and cats of any age can be affected.9,12,19 Age was not found to relate to development of primary versus secondary peritonitis in dogs or cats.⁷ There is no known breed or sex predisposition. In one study, three quarters of affected cats were indoor-only.¹⁰

Laboratory Abnormalities

Diagnostic Imaging

Microbiologic Tests

Antimicrobial Treatment

Parenteral antimicrobial drug treatment should be initiated as soon as possible after diagnosis of peritonitis, because delayed antimicrobial drug treatment in severe sepsis and septic shock may increase mortality (see Chapter 86). The initial choice of antimicrobial drugs in dogs and cats with bacterial peritonitis should include a broad-spectrum combination of antimicrobials with activity against both obligate anaerobes and facultative anaerobes (especially E. coli and Enterococcus) (Table 88-3). Subsequently, treatment should be adjusted on the basis of cytology and culture and susceptibility results. Because obligate anaerobes may not grow reliably in the laboratory and are commonly involved, continued use of an antibiotic that provides activity against anaerobes is recommended even when anaerobic cultures are negative.

Surgical Treatment

Careful surgical exploration for a site of bacterial leakage is indicated after initial stabilization with fluids, vasopressors, and antimicrobial drug treatment. The underlying cause must be surgically corrected (e.g., intestinal resection and anastomosis), and the peritoneum should be debrided and lavaged thoroughly with warm isotonic saline. The need for subsequent drainage is controversial. Drainage is generally selected when debridement and lavage cannot adequately reduce contamination. Methods of drainage include primary closure with closed suction drains, open peritoneal drainage, and, most recently, vacuum-assisted peritoneal drainage.10,12,17,20–22 In retrospective studies, no significant differences in survival have been identified among methods, but prospective studies are required. Complications of drainage are ascending nosocomial infection, obstruction of closed suction drains by omentum, and hypoproteinemia. Evisceration and bowel desiccation have the potential to occur with open peritoneal drainage, and fluid production cannot be quantified. Dogs and cats that undergo open drainage are also more likely to require plasma or blood transfusions and may have longer hospitalization times.¹⁷ Frequent sedation or anesthesia for bandage changes is also required.

Supportive Care

Supportive care for cats and dogs with bacterial peritonitis generally includes intensive crystalloid and colloid fluid therapy, administration of vasopressors, and enteral or parenteral nutritional support. Blood transfusions may also be required. Dogs that received early nutritional support had significantly shorter hospitalization times than those for which nutritional support was delayed.¹³

Prognosis

Survival rates of 32% to 67% have been reported in dogs with secondary peritonitis.7,9,11,12,16,21,22 Survival rates of 44% to 70% after surgery have been reported for cats.7,10,15 Typically hospitalization is required for 3 to 16 days (median around 5 to 6 days). When death occurs, it usually results from septic shock, DIC, or multiple organ dysfunction. Negative prognostic indicators identified in dogs with bacterial peritonitis include a diagnosis of primary as opposed to secondary peritonitis⁷; preoperative anemia, leukocytosis, or hypoproteinemia¹⁶; and postoperative administration of glucocorticoids.¹⁶ Low preoperative systolic blood pressure was a negative prognostic factor in a study that included both dogs and cats.¹⁴ Low preoperative median serum ALT activity (56 U/L for surviving cats compared with 179 U/L for non-survivors) was the only positive prognostic indicator in one study of cats with peritonitis.¹⁰ Age, preoperative heart rate, rectal temperature, the results of other laboratory parameters, or the presence of polymicrobial infection did not affect survival. In another study, a high plasma lactate concentration was a negative prognostic indicator in cats with bacterial peritonitis.¹⁵