The Horse

OCT 2018

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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Page 31 of 51

32 October 2018 The Horse | as interleukins (IL) and tumor necrosis factor (TNFα). These substances are important for initiating a physiological response to infections or injury and see- ing to a successful recovery. While inflammation might start in one tissue, excessive inflammation releases substances into the circulation that act systemically on other tissues throughout the body. Let's look at some of the nega- tive effects this process can have. Obesity, Insulin, and Inflammation Energy sources such as glucose— which horses obtain from consuming carbohydrates—can negatively affect the horse's endocrine system if not used or stored in the body's tissues properly. This can lead to bodywide inflammation. "When an animal is sick and off feed, there is continued need to preserve glu- cose for tissues that require it, such as the brain," says Jessica Suagee-Bedore, PhD, assistant professor of equine science at Sam Houston State University, in Hunts- ville, Texas, who has a special interest in obesity and insulin resistance in horses. "TNFα is important for initiating insulin resistance that is beneficial in the face of illness. However, the body doesn't dif- ferentiate between cytokines released to initiate the sickness response or cytokines released due to diet or obesity. Therefore, insulin resistance develops when an animal consumes a diet that promotes inflammation, such as high-starch and -sugar feeds, or in an animal that already experiences high levels of inflammation due to obesity." The association between inflammation and obesity depends on factors such as breed, exercise, diet, and aging—often re- ferred to as inflammaging. We now know that older horses naturally have higher circulating concentrations of cytokines, namely IL-6 and TNFα. "One chief concern of an animal's body is the prevention of hyperglycemia (elevated blood glucose levels) or hypogly- cemia (low blood sugar levels), as either of these can lead to a host of physiologi- cal problems," says Suagee-Bedore. For example, hyperglycemia results in inflam- matory free radical production that dam- ages tissue. Normally, the body stores any extra glucose in the appropriate tissues after eating. Skeletal muscle and the liver take up most of the glucose, converting it to glycogen the body can use for energy later on. When this system goes awry, cells become less able to respond to the insulin hormone transporting glucose from the bloodstream into storage tissues. Suagee- Bedore compares insulin resistance development to falling dominoes, with insulin being the signal that tips off the first domino. "For a healthy horse, the dominoes fall in a line, but in an insulin-resistant horse, one domino is slightly braced and doesn't completely fall over," she says. "More in- sulin has to keep hitting it to get it to tip." Without weight loss and exercise, af- fected horses remain insulin-resistant for prolonged periods. "Ultimately, it seems there is a positive feedback cycle, with insulin resistance resulting from inflam- mation and also promoting it bodywide," she says. She explains it like this: "Adipocytes (fat cells) secrete all sorts of cytokines, which have the same roles in initiating insulin resistance and disrupting metabo- lism as they would if they were secreted by the immune system during a bout of illness. In a normal healthy animal, these cytokines are synthesized and released ROBIN PETERSON, DVM ILLUSTRATION With leaky gut syndrome, bacteria and toxins leak into the bloodstream through damaged enterocytes. In insulin-resistant horses, if insulin (purple) is not attached to cell wall receptors, glu- cose (blue) cannot enter and circulates in abnormally high amounts in the bloodstream. Behavioral changes Obesity Laminitis Muscle soreness IR Normal Red blood cells and in ammatory cytokines breach the blood-brain barrier, in-ltrating central nervous system cells. IT'S ALL CONNECTED Bodywide inflammatory responses, such as those initiated by the immune system and the gut, can cause or contribute to a number of conditions, from obesity to behavioral anxiety to insulin resistance. Bodywide Inflammation

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