Endotoxemia remains the leading cause of death in horses, being intimately involved in the pathogenesis of
gastrointestinal disorders that cause
colic and neonatal foal
septicemia.
Endotoxins, normally present within the bowel, gain access to the blood across damaged intestinal mucosa, or
endotoxemia occurs when gram negative organisms proliferate in tissues.
Endotoxins are removed from the circulation by the mononuclear phagocyte system, and the response of mononuclear phagocytes to these
lipopolysaccharides (LPS) play an important role in determining the severity of clinical disease. Macrophages become highly activated for enhanced secretory, phagocytic and cidal functions by LPS. Macrophage-derived
cytokines are responsible for many of the pathophysiologic consequences of
endotoxemia. The
arachidonic acid metabolites,
prostacyclin and
thromboxane A2 likely mediate early hemodynamic dysfunction and the
leukotrienes may potentiate tissue
ischemia during
endotoxemia.
Interleukin 1 (IL-1) induces
fever and is responsible for the inflammatory cascade, which constitutes the
acute phase response.
Tumor necrosis factor (TNF), an important proximal mediator of the effects of LPS, acts to initiate events and formation of other molecules that affect
shock and tissue injury. Systemic administration of TNF produces most of the physiologic derangements that are associated with
endotoxemia and
antibodies that are directed against TNF significantly reduce LPS-induced mortality in experimental animals. In response to
endotoxins, mononuclear phagocytes express
thromboplastin-like procoagulant activity (PCA), which initiates microvascular
thrombosis. Both
IL-1 and TNF induce PCA expression, creating a positive feedback loop for LPS-induced coagulopathy. A macrophage-derived
platelet activating factor contributes to coagulation dysfunction and further stimulates
arachidonic acid metabolism. The ultimate consequences of
endotoxemia are multiple system organ failure and death. The numerous feedback loops and intertwining cascades of mediators during
endotoxemia defy simplistic methods of treatment. The optimal
therapy likely involves methods to alter the generation of inflammatory mediators by mononuclear phagocytes.