INTRODUCTION

Peritonitis is defined as inflammation of the peritoneal cavity and may be classified according to the underlying etiology (primary or secondary), extent (localized or generalized), or the presence of infectious agents (septic or nonseptic). Primary peritonitis refers to a spontaneous inflammatory condition in the absence of underlying intraabdominal pathology. Secondary peritonitis occurs more commonly and is the consequence of a preexisting aseptic or septic pathologic, intraabdominal condition. Secondary septic peritonitis is the more common form in the dog and cat, most commonly resulting from leakage of gastrointestinal (GI) contents from a compromised GI tract. Because of the multitude of conditions that may lead to peritonitis, the types of clinical signs and their severity are varied.

Hematogenous dissemination of infectious agents has been postulated as the mechanism of development of primary peritonitis and is likely facilitated by impaired host immune defenses. The most common form of primary peritonitis is the effusive form of feline infectious peritonitis, caused by feline coronavirus, which should be included on any differential diagnosis list for cats with peritoneal effusion. Other infectious agents reported to have caused primary peritonitis in dogs and cats include Salmonella typhimurium, Chlamydia psittaci, Clostridium limosum, Mesocestoides spp, Blastomyces spp, and Candida spp.

Inflammation of the abdominal cavity in the absence of infectious pathogens (aseptic peritonitis) most commonly occurs in response to exposure of the peritoneum to sterile fluids (i.e., gastric, biliary, or urine), pancreatic enzymes, or foreign material. Aseptic bile and urine cause minimal peritoneal inflammation, and gastric fluid and pancreatic enzyme leakage lead to a more intense peritoneal reaction. Both microscopic and macroscopic foreign material, including surgical glove powder, surgical materials (suture, cotton swabs, surgical sponges), hair, and impaled objects (sticks, plant material, metal) may elicit a granulomatous response. To minimize iatrogenic causes of aseptic peritonitis, it is recommended that the surgeons rinse or wipe surgical gloves with sterile saline or use powder-free gloves, perform a surgical sponge count before opening and closing a celiotomy, and use surgical sponges with radiopaque markers.

More commonly, secondary peritonitis can be identified as a septic process, with the most frequent source of infection being the GI tract. Leakage of GI contents may occur through stomach and intestinal walls that have been compromised by ulceration, foreign body obstruction, neoplasia, trauma, ischemic damage, or dehiscence of a previous surgical incision. Spontaneous gastroduodenal perforation may be associated with nonsteroidal antiinflammatory drug administration but may also be seen with corticosteroid administration, neoplastic and nonneoplastic GI infiltrative disease, gastrinoma, and hepatic disease.^1,2 GI linear foreign bodies in dogs have been reported to lead to the development of peritonitis in 41% of cases, higher than that previously reported for cats.³ Dehiscence occurs in 7% to 16% of postoperative patients requiring intestinal enterotomy or anastomosis, with mortality rates of 75% to 85% in this population. One study identified dogs as being at high risk for leakage following intestinal anastomosis if they had two or more of the following conditions: preoperative peritonitis, intestinal foreign body, and a serum albumin concentration of 2.5 g/dl or less.⁴ Other causes of septic peritonitis can be found in Box 133-1.

CLINICAL SIGNS

Historical information may provide clues regarding the underlying cause of peritonitis. Previous and current maladies and surgical procedures (including neutering), current medications (particularly those which may predispose to GI ulceration), and duration of current clinical signs should be investigated. Owners should be questioned specifically regarding potential for trauma exposure and foreign body ingestion.

Clinical signs of dogs and cats with peritonitis vary in both type and intensity and may reflect the underlying disease process. Peritoneal effusion is a consistent finding but may be difficult to appreciate on physical examination if a small volume of fluid is present, and may even be difficult to detect sonographically in animals exhibiting dehydration. Abdominal pain may be appreciated on palpation, with a small number of dogs exhibiting the “prayer position” in an attempt to relieve abdominal discomfort. In a retrospective study focusing on cats with septic peritonitis, only 62% exhibited pain on palpation of the abdomen.⁵ Most animals with septic peritonitis are systemically ill and exhibit nonspecific clinical signs such as anorexia, vomiting, mental depression, and lethargy. It should be noted that animals with uroperitoneum may continue to urinate with a concurrent leakage into the peritoneal cavity. These patients may arrive in progressive states of hypovolemic and cardiovascular shock, with either injected or pale mucous membranes, prolonged capillary refill time, tachycardia with weak pulses, and with either hyperthermia or hypothermia reflecting poor peripheral perfusion. A significant number of cats (16%) with septic peritonitis exhibited bradycardia⁵ (see Chapter 106, Sepsis).

DIAGNOSTIC TESTS

Patients with suspected or confirmed peritonitis should have routine hematologic, biochemical, and coagulation analyses. A marked neutrophilia with a left shift is the predominant hematologic finding, although a normal or low neutrophil count may be present. It is anticipated that animals recovering without incident from GI surgery may also have a transient inflammatory leukogram; however, the overall peripheral white blood cell counts typically fall within normal limits.⁶ An increasingly left-shifted neutrophilia (or neutropenia) paired with clinical signs of peritonitis may raise the clinician’s index of suspicion for postoperative intestinal dehiscence (which typically occurs 3 to 5 days postoperatively).

Furthermore, acid-base and electrolyte abnormalities may be noted. Hyperkalemia may indicate uroperitoneum, particularly if trauma or urinary tract dysfunction has been noted historically. Hypoproteinemia may be a result of the loss of protein within the peritoneal cavity. Patients with a concurrent septic process may be hypoglycemic. Hepatic enzymes, creatinine, and blood urea nitrogen may be elevated, indicating primary dysfunction of these organs or perhaps reflecting states of decreased perfusion or dehydration. The serum of patients with bile peritonitis may be icteric if the total bilirubin is elevated.

Patients with suspected peritonitis should be evaluated for peritoneal effusion. Little or no fluid may be detected initially if patients arrive early in the disease process or before fluid resuscitation if they are dehydrated. Large volumes of effusion may be obtained via blind abdominocentesis or, alternatively, via ultrasonographic guidance. Single paracentesis attempts are successful in only 20% of patients with low volumes of peritoneal effusion (3 ml/kg) and in only 80% with larger volumes (10 ml/kg). Ultrasonographic guidance will facilitate the retrieval of smaller volumes of peritoneal fluid. If single-site sampling is negative for fluid, four-quadrant sampling should be performed.

A diagnostic peritoneal lavage should be performed when peritonitis is suspected despite the absence of detectable effusion or when a minimal volume of effusion makes it difficult to obtain a sample. Diagnostic peritoneal lavage ideally is performed using a peritoneal dialysis catheter but can also be performed using an over-the-needle, large-bore (14 to 16 gauge) catheter. The technique is performed by infusion of 22 ml/kg of a warmed, sterile isotonic saline solution through the catheter inserted in an aseptically prepared site just caudal to the umbilicus and retrieval of a sample for analysis and culture and sensitivity. It is important to remember that the lavage solution will dilute the sample and therefore may alter the analysis. A repeated diagnostic peritoneal lavage may increase accuracy of the technique when results of the first procedure are equivocal (see Chapter 156, Diagnostic Peritoneal Lavage).

Leukocyte counts in peritoneal fluid are normally less than 500 cells/μl. White blood cell counts between 1000 and 2000 cells/μl indicate mild to moderate inflammation, and a higher peritoneal fluid leukocytosis suggests marked peritonitis.^6,7 However, cell counts in peritoneal lavage fluid obtained from postoperative patients undergoing intestinal resection and anastomosis may also show evidence of significant inflammation in the absence of surgical complications. In the patient that has undergone a celiotomy, 7000 to 9000 cells/μl suggests mild to moderate peritonitis. In these patients, intracellular bacteria or increasing inflammation (numbers of neutrophils or morphologic features of toxicity in these cells) observed in serial samples correlated with clinical findings may prove more useful than single leukocyte counts in abdominocentesis samples when deciding whether reoperation is indicated. It is also of note that dogs receiving antibiotics may have no observable bacteria in peritoneal fluid samples, despite having peritoneal contamination.

In addition to the presence of bacteria and a high nucleated cell count, the glucose concentration of abdominal effusion is a useful predictor of bacterial peritonitis in dogs. A concentration difference of more than 20 mg/dl between paired samples for blood and peritoneal fluid glucose is a reliable predictor of a bacterial peritonitis. Additionally, a blood-to-fluid lactate difference less than 2 mmol/L was predictive of septic peritonitis in dogs but has not been as useful in cats.^8,9 Intravenous administration of dextrose or the presence of a hemoperitoneum may decrease the accuracy of this test.

Samples for aerobic and anaerobic cultures should be obtained at the time of initial sampling so that additional samples are not required after confirming the presence of a septic process and initiating antibiotic therapy.

The diagnosis of uroperitoneum in dogs can be made if the peritoneal fluid creatinine or potassium concentration exceeds that of the serum creatinine (>2:1) or potassium concentration (>1.4:1).¹⁰ Similarly, biliary rupture will lead to a bilirubin concentration that is higher in the peritoneal fluid than in the serum. In addition, bile pigment or crystals may be visible on cytologic examination of the peritoneal effusion in animals with bile peritonitis (Color Plate 133-1). These changes may not be seen in patients with bile peritonitis secondary to a ruptured gallbladder mucocele because the gelatinous bile often fails to disperse throughout the abdomen.

Plain radiographs may reveal a focal or generalized loss of detail that is otherwise known as the ground glass appearance. A pneumoperitoneum (Figure 133-1) suggests perforation of a hollow viscous organ, penetrating trauma (including recent abdominal surgery) or, less commonly, the presence of gas-producing anaerobic bacteria. Intestinal tract obstruction or bowel plication should be ruled out. Prostatomegaly in male dogs and evidence of uterine distention in female dogs should be noted. Thoracic radiographs should be performed to rule out concurrent illness (infectious, neoplastic, or traumatic). The presence of bicavitary effusions increased the mortality rate of patients 3.3-fold compared with that of patients with peritoneal effusions alone.¹¹ Ultrasonography may be useful for defining the underlying etiology of peritonitis, in addition to its use in localizing and aiding retrieval of peritoneal effusion. In the case of a confirmed uroabdomen, preoperative contrast radiography (excretory urography or cystourethrography) is recommended to localize the site of urine leakage and aid in surgical planning. It should be noted that all patients should be hemodynamically and medically stabilized before diagnostic imaging is carried out.

Figure 133-1 Lateral abdominal radiograph showing free peritoneal gas and possibly ingesta free within the abdomen. Pneumoperitoneum without a history of recent surgery or open-needle abdominocentesis indicates the need for abdominal exploratory. This cat was diagnosed with a ruptured gastric mass during surgery.

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