Laminitis is an important equine disease, with evidence of its significance going back to the beginning of recorded history. Today, laminitis is said to be the most common reason for a horse to be presented for veterinary treatment, with an overall incidence of about 2 percent in the United States.1
Changes to the foot are theorized to be due to a disturbance of blood flow to the region as a result of an inflammatory mechanism from toxic, metabolic or enzymatic effects or from traumatic or mechanical factors. The final common pathway is thought to be one of mediation of extreme inflammation within the hoof capsule that eventually damages the laminar tissue severely, leading to detachment of the coffin bone. Possible contributors to the development of laminitis include endotoxemia, glucocorticoid administration, endocrine disturbances and excessive concussion on the structures of the foot. Additionally, various nutritional factors have been linked to the initiation and recurrence of the disease process.
Nutrition and initiation of the disease
According to the National Research Council (NRC), several factors have been implicated in the etiology of acute laminitis.2 Nutrition-related factors include excessive ingestion of rapidly fermentable carbohydrates (i.e., starch, sugars or fructans) as a result of overfeeding cereal grains, excessive intake of lush pasture or bolus feeding of a commercial fructan (inulin). Intake of black walnut shavings, insulin resistance and obesity have also been linked to the development of laminitis.
"There are several nutritional laminitic models, but what they are actually doing to the horse is unknown — whether it's directly related to nutrition or an indirect effect is the question," says David Hood, DVM, PhD, Foot Diagnostic and Rehabilitation Clinic, Bryan, Texas.
Although several factors have been associated with the onset of laminitis, the exact mechanism by which these factors trigger laminitis is still unknown.
Influence of lush pasture
Although the role of pasture carbohydrates in the pathogenesis of laminitis (disturbance to hindgut function) has not been clearly demonstrated, some suggest that most laminitis cases occur because of excessive lush pasture intake, especially by ponies. For horses and ponies with a history of pasture-associated laminitis, there is some rationale for restricting their access to pasture to limit their intake of high levels of rapidly fermentable material, high levels of water-soluble carbohydrates (simple sugars, glucose, fructose, sucrose and more complex carbohydrates such as fructan) and starch.
Pasture content of water-soluble carbohydrates is highest in the spring, lowest in mid-summer and intermediate in autumn. Content also fluctuates at various times during the day. Sugar content coincides with patterns of energy storage (photosynthetic activity) and utilization. Pasture water-soluble carbohydrates tend to rise during the morning, peak in the afternoon and decline overnight. According to the NRC, a horse grazing in the afternoon, when compared with nighttime or morning feeding, may ingest between two and four times as much water-soluble carbohydrates.
There is limited information on the quantities of pasture fructan or other storage carbohydrates required to cause significant changes in hindgut function that may increase the risk of laminitis.
If pasture exposure is a concern, then it may be beneficial to restrict access to pasture and/or feed lower quality forage alternatives (those that are low in rapidly fermentable material), especially to those animals at risk of developing the disease.
Laminitis could be largely avoided in predisposed animals by allowing grazing when the levels of starch, sugar and fructans are low. However, this strategy is difficult to employ because these levels may vary season to season, location to location and throughout a particular day. Also, plant species, field topography and grazing patterns of individual animals can influence success with this strategy.
If some grazing is unavoidable, it has been advised to turn animals out very late at night or very early in the morning and remove them from pasture by mid-morning, since fructan levels are likely to be at their lowest level at night through early morning.3
And since mature stemmy grasses can contain more fructan, it may be beneficial to avoid pasture that has not been managed properly by regular grazing or cutting and to not allow animals to graze on recently cut stubble.3
Another consideration is that at the change from vegetative to reproductive development, water-soluble carbohydrate and fructan levels are likely to be high, so it is advisable to avoid turning horses out to pasture during the spring when water-soluble carbohydrate and fructan levels are rising. Furthermore, cold temperatures will reduce grass growth, resulting in the accumulation of fructan. Horses at risk should not be turned out to pasture that has been exposed to low temperatures and bright sunlight. These conditions are more typically encountered just after the autumn flush of growth or on bright, cool winter days.
In the United Kingdom, allowing animals to graze on pastures that contain timothy and cocksfoot is suggested because these grass species tend to produce lower levels of water-soluble carbohydrates and fructans and are thought to be less likely to cause hindgut acidosis than ryegrass.
"We're doing some work related to how structural carbohydrates might impact the hindgut," says Paul Siciliano, PhD, a nutritionist at North Carolina State University. "Although a threshold for induction of laminitis using a nasogastric bolus of fructans has been defined, the threshold for consumption of pasture nonstructural carbohydrates (of which fructan is often a major component) through grazing is not clear. Grazing generally results in a clearly different intake pattern than a nasogastric bolus."
Siciliano is also working with endophyte-friendly tall fescue and its concentration of total nonstructural carbohydrates. "We're seeing those concentrations at 13 to 14 percent, with about a 3 percent variation between morning and night," Siciliano says. "Nonstructural carbohydrates increase in the fall and in the spring, when people tend to report pasture laminitis. If you have a horse that is truly at risk, you might want to select hay with the lowest sugar content, but you want to make sure that you're not suddenly feeding poor-quality forage and underfeeding the horse. In 1977, Butler and Hintz showed that energy intake influences hoof growth, so you don't want to impede that. Good management and good feeding practices are what you're after."
Siciliano and his team also examined fecal pH as an indicator of hindgut pH. They compared horses on pasture that were consuming higher nonstructural carbohydrates with those fed hay that had about half the amount. "Fecal pH tended to be a little bit lower in the horses on pasture, but it was still above 7, so it didn't appear to be anything that upset them," says Siciliano.
Siciliano's team then did some in vitro fermentations using the feces from each horse. They took the feces, made a batch culture for each horse, and then measured the volatile fatty acids that were produced. "We did see some differences in fermentation. We haven't looked at lactate — that's the one we are really interested in, but I don't know how that will come out."
The possible role of glucose, insulin
Researchers have found that "glucose is important in maintaining lamellar integrity and was shown to be essential for the viability of hoof explants in culture (in vitro)."3 Hoof culture without glucose or inhibition of glycolysis causes basement membrane zone separation when under tension. This finding suggests that high-circulating corticosteroid concentrations or inhibition of insulin activity could result in such an effect in vivo. It is possible that in an insulin-resistant state, glucose transporters are downgraded in the lamellar tissues, so glucose entry into the epithelial cells may be impaired, as has been demonstrated in chronic laminitis.
Infusion of large amounts of insulin has been reported to induce laminitis, but the amounts infused lead to circulating insulin concentrations well above physiological concentrations. Although diet influences the insulin concentration in the blood, the relationship among diet, blood insulin concentrations and laminitis remains unclear.
Insulin sensitivity has also been found to be affected by diet. Large fluctuations in glucose and insulin after meals high in sugar may supply inappropriate signals of energy availability to the glucose regulatory system, altering insulin sensitivity of the tissues.
In Thoroughbred weanlings adapted to a sugar-and-starch diet, insulin sensitivity was lower compared with weanlings adapted to a feed rich in fat and fiber. Mature Thoroughbred geldings with normal body condition scores (BCSs) tended to have decreased insulin sensitivity when adapted to a sugar- and starch-based diet. There may be a progression of insulin resistance in laminitis-prone ponies, from compensated insulin resistance to decompensated insulin resistance later in the course of the disease.
Specific breeds suggested to have an increased risk of developing insulin resistance include pony breeds, Morgans, domesticated Spanish mustangs, European warmbloods and American saddlebreds. But there is insufficient epidemiological information to confirm whether there is an increased susceptibility to laminitis in these breeds.
Diet modification to reduce the risk of developing laminitis due to insulin resistance in predisposed breeds may be indicated. Replacing starch- and sugar-based diets with properly formulated fat- and fiber-based feeds will produce a low glycemic index and an insulinemic response and avoid the insulin insensitivity that develops during chronic adaptation to sweet feed.
Other nutrition-related factors
The NRC notes that "specific, objective information regarding nutritional management of horses with chronic laminitis is not available."2 However, it might be best when feeding such horses to minimize energy intake and prevent excess body weight in order to minimize mechanical stress on the foot. To provide nutrients necessary for hoof growth and repair, rations should be of proper nutritional plane, have high-protein quality and include a complete balance of required micronutrients such as biotin.
Out of concern for the development of obesity and laminitis, it is prudent to encourage owners to provide regular exercise, monitor body weight (BCS) and maintain animals at an optimal weight (BCS 4 to 6 on a 9-point scale).
Although obesity and mechanical trauma due to increased weight load may be related to the development of laminitis, it is possible that the risk in obese horses or ponies (BCS > 7) is more appropriately attributed to the development of insulin resistance or equine metabolic syndrome.
"From my personal perspective, there is a lot of misinformation out there that I think is detrimental to the horse," Hood says. "The concept that you have to take weight off horses because their front feet hurt, and if you take weight off, they hurt less, is not quite valid because these horses need to grow their feet and need lots of protein and lipids to do so. If you cut back their diets, you're potentially going to have a detrimental effect. It's obvious if you're overfeeding, a horse can get into a laminitis problem that really impacts the disease. But there is very little data as to exactly how specific nutrients or supplements can affect the horse. If you overfeed on corn or energy, you put the horse at risk, but the mechanism by which that works is unknown."
"As for chronic laminitis, you're really just trying to prevent another episode," says Siciliano. "You're trying to keep affected horses from being overconditioned and overfed. Know what their digestible energy requirements are and provide them. Just practice good feeding management, making sure what you're feeding them is balanced, with adequate micronutrients to help stimulate hoof growth. Probably equally important — maybe more important — is the farrier and the environment that owners keep their horses in. Ensure that the horses are in a good environment for their feet and have proper trimming, shoeing and hoof care. That's going to have equal or even more impact on their condition than what they're being fed."
For a horse with chronic laminitis or one prone to laminitis, "we don't really stress the energy portion as much as we do the sugars and the starches in the diet," says Carey Williams, PhD, Equine Science Center, Rutgers University. "We really stress keeping them on a low-starch diet by using more poor-quality hay. If they need extra calories, which most of them don't, then it needs to be a low-starch, low-carb feed. If the horse feels good enough to be exercising and needs more energy to maintain body weight, then we recommend using fat to increase the energy content of the diet. Overfeeding is an issue, but if you have a horse that needs the energy, we recommend not to add the calories from carbohydrates or sugars; it needs to come from a different source."
"As far as nutrition goes, one thing we've got going on is looking at the basic nutritional requirements of the equine hoof — its physiologic aspects, what it's made up of, what it theoretically needs and how we can provide it," says Hood.
Hood is working with Josie Coverdale, PhD, a nutritionist at Texas A&M University, to determine how best to feed a chronically foundered horse. "Although there is much information out there on how to feed a chronically foundered horse, there is very little data," Hood says. "That will be the purpose of what we're trying to do — to determine what's wrong with the horse, medically, and then how that affects the nutritional needs of the horse," he says. "It's a bit complex because you have horses that are essentially normal and then those that have chronic laminitis, metabolic syndrome or pituitary pars intermedia dysfunction. That makes each case a distinct individual — the highly stressed horse needs one nutritional protocol, while the metabolic syndrome patient might need an entirely different one."
"I think we keep looking at triggers, not at how to maintain or even basically treat laminitic horses nutritionally," says Coverdale. "We're convinced there are some protein aspects at play — not just crude protein-related, but possibly an amino acid imbalance or a greater requirement. Whether it's sulfur-containing amino acids or branched-chain amino acids, I'm not sure. I can't imagine that laminitis-affected horses have the same amino acid requirements as a healthy horse at maintenance."
"We don't even honestly know the limiting amino acids for maintenance or for growth, so for laminitis we're starting from scratch," Coverdale says. Most cereal grains are either limiting or marginal in methionine. "I've done some methionine work for growth in weanlings, and we've not been able to limit growth with normal cereal grains available to us or to make a methionine-deficient diet. Although it is difficult to produce such a diet, whether there is a benefit of super-supplementation of amino acid-containing products is not known."
Research into this common debilitating equine disease is ongoing. Veterinary practitioners should consider an individualized and, in many cases, multifactorial approach to prevention and management of laminitis.
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. He is based in Seattle.
1. USDA. Lameness & Laminitis in U.S. Horses. USDA: APHIS:VS, CEAH, National Animal Health Monitoring System, April 2000.
2. National Research Council. Nutrient requirements of horses. 6th ed. Washington, D.C.: National Academies Press, 2007.
3. Harris P, Bailey SR, Elliott J, et al. Countermeasures for pasture-associated laminitis in ponies and horses. J Nutr 2008;136:2114S-2121S.