Peritonitis - Cats and Dogs

Introduction

Peritonitis is defined as inflammation of the peritoneum. The inflammatory response involves vasodilation, exudation of protein-rich fluid, cellular infiltration, pain and, chronically, formation of fibrous adhesion. The disease can be classified into primary and secondary cases.

Primary peritonitis occurs spontaneously without any pre-existing pathological process in the abdomen. In cats, feline infectious peritonitis is the most common cause of primary peritonitis.

Secondary peritonitis occurs as the result of a pre-existing pathological process within the abdomen. It can be further classified into septic or non-septic peritonitis, where septic peritonitis results from direct bacterial infection of the peritoneal cavity. Septic peritonitis is the most common form in the dog and its causes include:

• Perforation of the gastro-intestinal tract due to foreign bodies, intussuscepta, invasive neoplasia, deep ulceration or dehiscence of surgical wounds or biopsy sites. Peritonitis as a result of wound dehiscence is most likely to occur 3-5 days post-operatively.
• Penetration of the abdomen by a stick, gunshot or other foreign body.
• Rupture of an infected uterus (pyometra), biliary tract or urinary tract.

The bacteria causing septic peritonitis or their products may spread systemically causing sepsis or endotoxaemia.

Non-septic peritonitis may occur due to the leakage of bile, urine or pancreatic enzymes (chemical peritonitis) or due to the presence of foreign substances such as barium or glove powder (physical peritonitis). In some cases of urinary tract or biliary tract rupture however, septic peritonitis may occur if the tracts were previously infected.

Clinical Signs

The clinical signs are related to the presence of severe inflammation within the body cavity, with or without systemic infection.

• Abdominal pain, manifesting as a reluctance to move due to inflammation of the parietal peritoneum.
• Depression, anorexia and lethargy and non-specific signs of infection or systemic disease.
• Vomiting and diarrhoea may occur due to alterations in intestinal motility and functional ileus.
• Hypotension and (septic) shock due to effusion of fluid into the peritoneum and systemic vasodilation.
• Hypothermia or hyperthermia.

Laboratory Tests

Haematology

As with any severe inflammatory process, leucocytosis will occur. Initially, this is caused by neutrophilia which may have a left shift or, if very severe, a degenerative right shift. Severe localised inflammation may stimulate a leukaemoid response with massive mobilisation of neutrophils from the bone marrow pools.

Haemoconcentration (causing a raised packed cell volume (PCV) and total protein concentration) may occur due to loss of extracellular fluid.

Biochemistry

Hypoproteinaemia may occur due to loss of plasma proteins into the inflammatory exudate.

Hypoglycaemia may occur in cases of septic peritonitis.

Dehydration (which is also responsible for the haemococentration) may also result in pre-renal azotaemia, increased tissue lactate production and metabolic acidosis.

Hypokalaemia may occur as a result of chronic vomiting and it may contribute to the intestinal ileus which often develops in cases of peritonitis.

Diagnostic Imaging

Radiography

Plain radiographs of the abdomen may reveal the presence of free gas in the abdomen (pneumoperitoneum) due to intestinal perforation or bacterial production. The normal serosal detail may be effaced due to the presence of an abdominal effusion and, if a horizontal beam decubitus radiograph is made, a fluid line may be apparent.

In cases where neoplasia is thought to be the cause of the inflammatory process, thoracic radiograph should be assessed for signs of metastatic disease.

Ultrasonography

This modality has a high sensitivity for the detection of free fluid in the abdomen and it may be used to identify some specific causes of peritonitis, including abscesses of organs or rupture of the biliary tract.

Ultrasound scans can also be used to guide abdominocentesis.

Other Tests

Free abdominal fluid can be collected under ultrasound guidance and submitted for cytological analysis. If fluid cannot be obtained on aspiration, diagnostic peritoneal lavage can be performed by instilling a small volume (~20 ml/kg) of warmed isotonic crystalloid into the abdomen, agitating the abdomen and then re-aspirating this fluid. Grossly, the fluid may contain vegetable fibres if the gastro-intestinal tract has ruptured or it may be evidently green (indicating the presence of bile) or haemorrhagic. On cytological examination, the sample should be assessed for the presence of neutrophils (and other leucocytes) with intracellular bacteria. If motile organisms are observed when examining your sample microscopically, this is a good indication that you have accidentally taken a sample of intestinal contents.

Further possible tests include:

• Measurement of amylase and lipase where the cause is suspected to be pancreatitis
• Bile where biliary tract rupture is suspected.
• Creatinine and potassium if the effusion is thought to be a uroabdomen; creatinine levels in a peritoneal sample that are higher than serum concentrations indicate uroperitoneum.
• Glucose and lactate should be measured; where their values are <2.8 mmol/l and >5.5 mmol/l respectively the inflammation is likely to be septic.

Lactate levels are indicative of a hypoxic crisis and levels are occasionally used as a prognostic indicator - rising levels in the face of vigorous treatment of peritonitis seem to be anecdotally more prognostically useful than absolute cut off values. NB. Measurement of lactate on some machines such as hand held blood gas analysers may require a heparin sample.

• Culture is indicated where a bacterial infection is suspected, and should include an anaerobic culture.

In cats with Feline Infectious Peritonitis, the effusion usually has a high protein content (>35g/l) with a high globulin: albumin ratio. There is a variably high cellularity mainly composed of lymphocytes.

Treatment

It is vital to identify severe cases promptly as these will require emergency surgical intervention. Any of the following criteria is a major indication for surgery:

• Presence of intracellular bacteria in leucocytes in the abdominal exudate
• Pneumoperitoneum
• Presence of penetrating injuries to the abdomen.

Medical Management

Fluid Therapy

Aggressive fluid therapy with crystalloid and colloid should be given on initial presentation to improve haemodynamic parameters and this should be maintained into the peri- and post-operative periods where appropriate and until the patient is normotensive. If the patient remains hypotensive, the use of a vasopressor such as dobutamine, dopamine or even vasopressin should be considered as animals in septic shock are likely to have systemic peripheral vasodilation.

Glucose and potassium should be supplemented where these parameters are found to be abnormal and, in cases of severe metabolic acidosis, the use of sodium bicarbonate may be considered. This product should be used with care as overdoses may result in overshoot metabolic alkalosis, tissue anoxia due to a left-shift of the haemoglobin-oxygen dissociation curve and paradoxical cerebral acidosis as carbon dioxide (not bicarbonate) crosses the blood brain barrier.

Septic peritonitis can cause disseminated intravascular coagulation (DIC) which represents a very large therapeutic challenge. Plasma may be administered to replace used clotting factors and some authors advocate the use of low doses of heparin to prevent further coagulation.

Antimicrobial Drugs

Broad spectrum bactericidal antibiotics should be administered and, where possible, the choice of product should then be guided by culture and sensitivity of samples of peritoneal fluid. Escherichia coli, Clostridium spp. and Enterococcus faecalis are the species most commonly isolated in cases of septic peritonitis.

Surgical Management

Surgical intervention is indicated if the cause of peritonitis is undetermined or if it has been caused by intestinal rupture, intestinal obstruction or mesenteric avulsion. Abdominal lavage is a controversial procedure as it carries a risk of disseminating infection throughout the body and it is therefore indicated in cases of generalised peritonitis but should be used with care in cases of localised peritonitis. A volume of around 200 ml/kg fluid should be used to lavage the abdomen and as much of this fluid as possible should be re-aspirated as its continued presence will hinder the immune system by diluting bactericidal factor and preventing leucocyte migration.

It is beneficial to maintain peritoneal drainage after lavage by either open or closed drainage. Open drainage involves leaving part of the abdominal wall loosely sutured so that peritoneal fluid can leak out under gravity. The wound must be dressed under sterile conditions and there is a high risk of ascending infection and of continued protein loss with this technique.

Closed drainage involves implanting a drain into the abdomen (often a Jackson Pratt drain) to which suction can be applied to aspirate fluid from the peritoneal cavity. The drain usually has multiple fenestrations along its length so that it does not become blocked by omentum or fat.

Prognosis

Guarded. Peritonitis is a multifactorial disease and the consequence is fatal in most cases. A rapid diagnosis and treatment may improve the prognosis but it is generally poor in cases of septic peritonitis.

Peritonitis - Cats and Dogs Learning Resources
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References

• Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2) Elsevier Saunders Company
• Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition) Mosby Elsevier
• Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition) Mosby Elsevier.
• Tilley, L. P. & Smith, F. W. K. (2007) Blackwell's Five-minute Veterinary Consult: Canine & Feline (Fourth Edition) Blackwell Publishing

For further information on peritonitis see the following In Practice article on SA Peritonitis: maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=haemoabdomen&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT

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