Sepsis is actually a continuum of a disease process that begins with SIRS (Systemic Inflammatory Response Syndrome).

SIRS is the systemic inflammatory response that can occur due to a variety of severe clinical insults. The response is manifested by the following conditions:

SIRS in Dogs¹ - presence of 2 or more criteria

• Temperature <100.4°F or >104°F
• Heart rate >120 beats/min
• Respiratory rate > 20 breaths/min or PaCO2 <30 mmHg
• WBC >16,000 cells/mm3, <6,000 cells/mm3

SIRS in Cats² - presence of 3 or more criteria

• Temperature <100°F or >103.5°F
• Heart rate <140 beats/min or >225 beats/min
• Respiratory rate > 40 breaths/min
• WBC >19,500 cells/mm3 or , <5,000 cells/mm3 (or 5% bands)

Sepsis is the systemic response to an infection. This systemic response is manifested by two or more of the above-mentioned SIRS criteria as a result of proven or suspected infection. Severe sepsis is sepsis associated with organ dysfunction, hypoperfusion or hypotension. Hypoperfusion and perfusion abnormalities may include, but are not limited to, lactic acidosis, oliguria, or an acute alteration in mental status. Septic shock is severe sepsis with hypotension, despite adequate fluid resuscitation, along with the presence of perfusion abnormalities that may include, but are not limited to, lactic acidosis, oliguria, or an acute alteration in mental status. Patients who are on inotropic or vasopressor agents may not be hypotensive at the time that perfusion abnormalities are measured.

An infection may or may not occur in the presence of SIRS. The following diagram depicts the relationships between SIRS and infection. Only when SIRS occurs with an infection does sepsis or septic shock occur.

While sepsis is heavily studied in human diagnostics and there is a wealth of tools for the physician, the veterinarian is left primarily with the clinical impression on how to handle the compromised animal. The best prognosis for an animal with an infection is to identify it early before it evolves into sepsis.

"The most important determinant of mortality is not the pathogen but rather the degree and nature of the host response" - William C Aird

Cases of severe sepsis are expected to rise in the future due to the increased awareness and sensitivity for the diagnosis, number of immuno-compromised patients, use of invasive procedures, number of resistant microorganisms, and the growth in the senior canine and feline population. Currently, severe sepsis has a 50% mortality rate in companion animals.

Sepsis develops when the immune system becomes over activated in response to an existing infection, setting in motion a cascade of dangerous inflammatory and coagulation responses throughout the body. The diagnosis is made by an index of suspicion, physical examination findings, blood cultures, and complete blood count and chemistry abnormalities. Unfortunately many patients are undiagnosed or misdiagnosed due to the lack of an objective test to help the clinician confirm the diagnosis.