Endotoxemia Explained

Endotoxin release–which happens when the bowel’s checks and balances are altered–causes serious issues.

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When the bowel’s natural system of checks and balances is altered, the resulting endotoxin release can cause serious health issues

Although we’d like to think our horses are “sterile” on the inside, they are, in fact, harboring vats of micro-organisms within their intestinal tracts. In the normal, healthy bowel resident microbes limit the overgrowth of pathogenic bacteria. One type of microbial inhabitant, Gram-negative bacteria, which helps break down fibrous feed, releases a portion of its cell wall when it multiplies and then dies. What’s released is called endotoxin.

Normally, bacteria or their components are restricted from traveling beyond the bowel’s interior by a complex intestinal barrier; the bowel lining’s mucosa (mucus-secreting membrane lining all body passages that communicate with the air) is made of epithelial cells that shield against endotoxin. Also, enzyme and antibody secretions block endotoxin passage. And when small amounts of endotoxin do make it through the mucosal barrier into the liver’s circulation, specialized immune cells envelop and eliminate the toxin.

However, when the bowel’s natural system of checks and balances is altered and microbial overgrowth leads to a die-off of Gram-negative bacteria, the resulting endotoxin release can cause serious health issues. “Problems develop when the intestinal barrier is injured, such as occurs with gastrointestinal disease or surgery, from reduced intestinal blood flow related to intestinal displacements,” explains Anthony Blikslager, DVM, PhD, Dipl. ACVS, professor of equine surgery at North Carolina State University’s College of Veterinary Medicine. “In addition, nonintestinal events such as septicemia (bacteria in the bloodstream) or post-foaling retained fetal membranes can cause large amounts of endotoxin to be absorbed into the central circulation.” Bacterial, viral, or parasitic invasion might also alter gut flora and intestinal motility. And antibiotic therapy can change the gastrointestinal population, allowing overgrowth of some pathogenic bacteria strains.

What Does Endotoxin Do?

When large amounts of endotoxin overwhelm the body’s normal protective mechanisms, they are absorbed through injured intestinal mucosa and enter the general circulation. “There,” Blikslager says, “lipopolysaccharide (LPS) binding protein facilitates transfer of endotoxin to sensitized cell surface receptors.” This initiates an intricate inflammatory cascade. The subsequent release of potent inflammatory mediators and prostaglandins (fatty acid-derived compounds) causes the horse to respond with clinical signs such as fever, malaise and/or anxiety, dehydration, dark mucous membranes, delayed capillary refill time, sweating, rapid heart and respiratory rates, intestinal shutdown, and pain. Potentially, the horse can lapse into life-threatening endotoxic shock

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Written by:

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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