II: A Perspective on Endotoxemia

One of the most commonly encountered life-threatening conditions in horses with gastrointestinal disease is endotoxemia. The results of studies of horses presented with abdominal pain (colic) to university clinics in various parts of the world indicate that gram-negative bacterial endotoxins commonly are detected in circulation. Although much of the focus of research on endotoxemia in horses has centered on diseases that produce intestinal strangulation obstruction, these are not the only disease conditions characterized by endotoxemia. Similar clinical findings and problems often occur in adult horses with colitis, pleuropneumonia, retained placenta, and peritonitis. The most common clinical findings in horses presumed to be endotoxemic include abnormalities in mucous membrane color, with one of the initial findings being a ‘toxic line’; prolongation of capillary refill time; increased heart and respiratory rates; reduced borborygmi; fever; and hemoconcentration. Some horses may exhibit clinical signs of abdominal pain whereas others will appear to be depressed. The situation is equally serious in neonatal foals that develop a nidus of bacterial infection. Approximately 50% of foals with clinical signs of septicemia have endotoxins in their circulation. Septicemia is the leading cause of death in foals less than a week old, primarily because the foal’s immune system lacks the ability to prevent dissemination of the bacteria from the initial nidus. The most common organisms responsible for endotoxemia and septicemia in neonatal foals are gram-negative bacteria such as E. coli, Klebsiella, Enterobacter, Actinobacillus, and Pseudomonas, although gram-positive or mixed gram-negative and gram-positive bacterial infections also occur. Consequently, endotoxemia is associated with lifethreatening diseases affecting horses of all ages. In the past decade, there has been a virtual explosion of new information about the mechanisms responsible for the deleterious effects of endotoxins, with some of the more exciting findings occurring within the past 2–3 years. Because so much has been learned in such a short period of time, it is often difficult for people intimately involved in this field of research to keep up with what is known. The four basic goals of this article are to provide the equine practitioner with an appreciation of the historical background associated with endotoxemia, a better understanding of this serious and life-threatening complication of many diseases, an appreciation of the impact of new findings regarding the ways endotoxins exert their deleterious effects, and some thoughts regarding potential new methods of treatment.