Preventing Laminitis - Dr Jennifer Stewart

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Preventing laminitis in horses 97 Table 1 Known or Presumed Factors that Can Cause or Contribute to the Development of Laminitis in Horses, Ponies, and Other Domestic Equids Causes Contributing Factors* unrestricted access to improved pastures during periods of high fructan content (eg, spring, fall, cold stress) grain overload (ingestion of large quantities of dietary starch) fiber carbohydrates (NFC), as distinct from the fibrous structural carbohydrates (cellulose, lignin, etc.) which give plants their structural integrity and support (ie, the plant’s skeleton). Nonstructural carbohydrates are produced by the plant during daylight hours (via the process of photosynthesis) and are used by the plant as an energy source for growth and reproduction. Any excess NSCs are stored in the leaves and stems for later use, and, once the plant flowers, in the seeds to history of laminitis (even if previous episode was mild) high-grain diet (>5 lb of grain-based feed for the average sized horse) severe intestinal disease (eg, “surgical” colic, enterocolitis) consumption of endophyte-infested tall fescue other serious systemic illness (eg, pleuropneumonia, overweight (BCS of 6 or greater); the greater the BCS, the Potomac horse fever) greater the laminitis risk endo/metritis secondary to retained placenta pony or Morgan untreated equine Cushing’s disease (older horses) inactivity severe hypovolemia (eg, severe dehydration, severe blood occasional strenuous exercise (“weekend warriors”) loss, toxemia) exercise at speed on a hard surface (“road founder”) sustained, nonweight-bearing lameness in one limb (ie, glucocorticoid administration (the higher the total dose, contralateral limb laminitis) the greater the risk) exposure to black walnut heartwood (usually as wood stress (eg, high-stress occupation or environment, shavings used for bedding) transportation, hospitalization?) small feet for the size or weight of the horse (usually linked to BCS >6) flat or thin soles, or excessive trimming of the wall or sole (ie, less-than-optimal protection for palmar/plantar digital vasculature) BCS body condition score (see Table 2). *These factors, while they may not cause laminitis on their own, appear to increase the risk for laminitis and may cause laminitis when two or more are present concurrently. lush pasture 46% grain overload 7% unknown 15% colic or diarrhea 3% other known 27% retained placenta 2% Figure 1 Causes of laminitis identified in a recent nationwide survey (from Lameness and Laminitis in U.S. Horses. USDA: APHIS: VS, CEAH, National Animal Health Monitoring System. USDA, 2000). “Other known” causes included feed problems and complications of injury, obesity, or pregnancy. Each comprised less than 2% of all cases, so these diverse factors were combined into one group for this chart. fuel germination and sustain the seedling until it can produce sufficient NSCs of its own. Grains such as oats, corn, barley, wheat, and millet— which actually are the seeds of grasses—are high in starch and simple sugars. Pasture grasses also are high in sugars at certain stages of their growth cycle and during times of environmental stress (eg, cold weather). These two feed sources, grain and grass, must therefore be fed with care in horses. The circumstances in which excess carbohydrate intake can lead to laminitis can be divided into two categories: 1. carbohydrate overload—ie, a large meal that is high in starches or sugars (whether grain or grass) 2. chronic overfeeding (usually coupled with relative inactivity), resulting in a horse that is overweight. Either circumstance increases a horse’s laminitis risk. When both are present, the horse’s risk for developing laminitis greatly increases, particularly when other triggers (eg, illness, injury, stress) occur. Following are some dietary strategies that, when consistently followed, can substantially decrease a horse’s laminitis risk. Pasture Management Access to lush pasture is the single most important risk factor for laminitis on farms where pasture is available for grazing. While pasture grasses contain a wide variety of structural and nonstructural carbohydrates, the ones that appear to be most important, with respect to laminitis risk, in many of the pasture grasses in the U.S. are called fructans. Fructans are NSCs that cannot be broken down by enzymes in the horse’s small intestine; they bypass digestion and absorption in the small intestine and reach the large intestine, where they are rapidly fermented by the resident microbes.
98 C. King and R.A. Mansmann Mechanism of Pasture-Associated Laminitis The exact mechanism by which pasture-associated laminitis occurs is still being researched. What is known is that intake of a large amount of fructans causes the pH in the large intestine to drop (ie, become more acidic), as lactic acid is produced by the fermentation of fructans. The now-acidic environment in the large intestine kills off the more acidsensitive microbes and allows the more acid-resistant microbes to multiply. Certain of these bacteria produce substances which can activate enzymes (matrix metalloproteinases, or MMPs) in the hoof wall that break down the cellular bond which anchors the hoof wall to the pedal bone. While this step remains to be fully investigated, it is assumed that these bacterial products reach the tissues of the hoof via the bloodstream, after being absorbed from the large intestine into the circulation. The net result is partial or complete separation of the hoof wall from the pedal bone, and possibly displacement of the pedal bone under the weight of the horse’s body and the pull of the deep flexor tendon on the pedal bone. Pain and distortion of the hoof are the two most obvious manifestations and hallmarks of laminitis. Fructan Content of Pasture Grasses The fructan content of pasture grasses varies greatly with plant type (species and variety), stage of growth, and growing conditions, including soil composition, rainfall, temperature, and hours and intensity of sunlight. Some generalizations can be made, but it is important to understand that they are by no means hard-and-fast rules. 1. In general, the fructan content is highest during periods of active plant growth, such as during the spring and early summer months. However, it can increase anytime soil moisture content, sunlight, and daytime temperatures are sufficient for fructan production. All that is required for the fructan content to increase in a pasture is for the rate of production (ie, photosynthesis) to exceed the rate of use (ie, growth or reproduction). In many parts of the country, pasture-associated laminitis is just as common in the fall as it is in the spring; in fact, it is perhaps more common in the fall, as many horse owners don’t consider their pasture to be high-risk once the spring flush has passed. A “green-up” after a drought is another potentially dangerous period any time of the year it occurs. The fructan content may also increase in response to plant stress, particularly cold stress (temperatures below 40°F). On a bright but chilly autumn day, for example, the amount of fructans produced by the plant may exceed the amount used for growth (which is inhibited by cold temperatures); as a result, fructans accumulate. Thus, pastures may continue to be a laminitis risk in the late-fall and even during the winter months in areas of the country where conditions are mild enough for horses to graze year-round. It is a dangerous misconception that only lush spring pasture is a problem. 2. In general, the fructan content is highest during the day (when the plant is actively photosynthesizing carbohydrates) and lowest at night and in the early morning hours (when the plant is using the carbohydrates for growth). However, on cool, overcast or rainy days this pattern may be off by half a day, or more. In a study in Great Britain, researchers evaluated the fructan content of a perennial ryegrass pasture on a warm, sunny day in May and on two cool, overcast or rainy days in July. On each day, they measured the fructan content at several different times over a 24-hour period. As might be expected, the fructan content on the warm, sunny day was highest between mid-morning and mid-afternoon, peaking around midday, and was lowest late at night and in the early morning hours. In contrast, on the cool, overcast or rainy days, the fructan content was highest in the afternoon and evening; it remained high overnight, and was lowest between dawn and midday. In that study, the fructan content of the pasture ranged from 0 to 30% dry matter (which can be enough to cause laminitis if grazing is unrestricted). On one overcast day it ranged from 0 to 22% within a 10-hour period! This study illustrates an important fact: even in the same pasture, the fructan content can vary greatly over the course of a day, and it cannot necessarily be predicted based on the time of day. 3. In general, grass that has already flowered and gone to seed has a low fructan content. Mature seed heads and dry, stemmy or “woody” stalks usually are a good indicator that the grass is low in fructans and other NSCs. However, flowering grass (ie, grass in the process of producing seed heads) can be quite high in fructans. 4. In general, the fructan content of pasture grasses is higher in the stems than in the leaves, particularly in the lower part of the stems. Overgrazing should therefore be avoided. Not only does overgrazing cause plant stress (potentially resulting in an increase in fructan storage), it means the horse is forced to eat more of the stem material than when the grass is taller (when more leaf than stem is eaten). What constitutes “overgrazing” depends on the grass species (ie, its hardiness and growth characteristics). With the more upright-growing pasture grasses, it may be best to suspend use of a pasture once the grass is grazed down to a height of 4 to 6 inches. 5. In general, native pastures and unimproved grass varieties are safer for grazing horses than are “improved” pasture varieties. Among other attributes, many improved pasture varieties have been selected and bred for a high NSC content because of the requirements of dairy farmers for high milk yields in their cows. Some varieties of improved pasture grasses are very high in NSCs, including fructans, making these pastures a considerable laminitis risk for horses at certain times of the year. In addition, these improved varieties are often sown as a single pasture crop (a monoculture), whereas native pastures typically consist of several different grass species and a variety of other plants. However, intensive irrigation and fertilization of native or unimproved pasture varieties can result in NSC levels that approach those of improved grass varieties, making these pastures a considerable laminitis risk, particularly for laminitis-prone horses. Pasture grasses adapted to warm climates
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Preventing Laminitis - Dr Jennifer Stewart

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