Pasture-associated laminitis (PAL) is a form of insulin-induced laminitis that can occur after horses have grazed on grasses and legumes that are high in hydrolyzable carbohydrates (HC).

This condition causes pain and stretching and weakening of the laminae of the hooves. If it is not treated promptly, it can progress to founder – a life-threatening condition that involves displacement of the coffin bone in the hoof capsule.

In a survey of American horse farms, over 50% of reported laminitis cases were thought to be due to grazing on lush pasture and grain overload. [4]

Lush grass contains higher levels of NSC – a measure of the sugar, starch, and fructan content of forage. Transitioning horses onto nutrient-rich grasses in the springtime can trigger a laminitic attack caused by the insulin response to high sugar and/or starch intake. [1][2]

Horses that have metabolic conditions involving insulin resistance, like equine metabolic syndrome are at risk for pasture-associated laminitis. [3]

Pasture Associated Laminitis

A common cause of lameness, pasture laminitis can occur in any breed or age of horses. It can affect any of the four feet, but most often occurs in the front hooves.

Pasture-associated laminitis is most likely to occur in the spring and early summer when new growths of grass have low fibre and high sugar content. It can also occur in the fall when many pastures experience a regrowth and cooler nighttime temperatures cause plants to retain higher levels of NSCs.

All pasture plants contain non-structural carbohydrates, but concentrations are higher in periods of rapid growth, with exposure to direct sunlight, and after times of stress such as drought or frost.

Pasture laminitis involves impaired blood flow to the laminae of the hooves – soft structures that anchor the coffin bone to the hoof wall. The condition can cause separation of the laminae and lead to a loss of support to the coffin bone. [15]

When the coffin bone is no longer supported by the laminae, it changes orientation within the hoof capsule and may rotate downwards causing pressure on or perforation through the sole. If laminitis progresses to this state, the condition is referred to as founder.

 

Healthy vs. Laminitic Horse Hoof | Mad Barn USAIllustration:

NSC Content of Grasses

Pasture growth contains fluctuating levels of simple sugars, fructans, and starch.

There is no single type of pasture grass that consistently has low NSC values because these levels are affected by a number of dynamic factors, including:

  • Type of plant
  • Soil composition
  • Use of fertilizer
  • Duration and intensity of sunlight
  • Time of day
  • Season and cutting
  • Exposure to environmental stressors
  • Height of grass

Cool season grass species including ryegrass, timothy, fescue, and orchard are known to be high in fructans and sugars.

Warm season grass species including Bermuda, Bahai, and crabgrass do not produce fructans but may be higher in sugars and starch. [13][14]

NSC concentration in grass typically increases during the morning, reaches its highest levels in the afternoon, and then declines overnight. Horses consume less NSCs when they graze in the nighttime or morning versus the afternoon.

Stress conditions that restrict growth within plants also increase the concentration of NSCs. When growth is restricted, energy produced by photosynthesis is stored as a reserve rather than being used for growth.

Stressors to plants include low temperatures, killing frosts, applications of herbicides, overgrazing and poor soil composition.

Causes of Pasture Laminitis

While the clinical signs of pasture laminitis are observed in the hooves, this condition begins in the digestive tract after carbohydrate-rich plants are consumed.

Pasture-associated laminitis is referred to as a form of endocrinopathic laminitis because it involves elevated levels of insulin – an endocrine hormone.

The exact mechanisms that contribute to this disease are still under investigation, but it appears to be mediated by elevated insulin levels (hyperinsulinemia) in the horse’s body. [15][17]

Although alterations in hindgut function and fructan overload have previously been implicated in PAL, new research demonstrates that elevated insulin is the main causal factor. [18]

Insulin Resistance

Pastures comprised of nutrient-rich grasses and new growth will supply high levels of easily digestible carbohydrates, including sugar and starch.

Sugar and starch are rapidly digested energy sources that increase blood sugar (glucose) levels and contribute to more insulin being secreted by the pancreas.

Horses that have metabolic syndrome or pituitary pars intermedia dysfunction (PPID) are at a greater risk for high insulin levels. [3] These horses are often also overweight, which increases the strain on the laminae.

These horses will have some degree of glucose intolerance/insulin resistance, meaning they require more insulin to regulate higher than normal blood glucose levels. [15]

Research demonstrates that high insulin levels are directly linked to the development of laminitis in horses and ponies. [7] Insulin influences the production of biological agents including Endothelin-1 which promotes the constriction of blood vessels in the hooves. [8][9][12]

A high insulin level can trigger separation of the laminae, although the exact mechanism through which this occurs is still being researched.

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Signs of Pasture Laminitis

Before laminitis develops, you may notice rapid weight gain and development or worsening of abnormal fat deposits. Fatty neck crests become very hard to the touch.

If your horse is affected by pasture laminitis, they may display a wide range of symptoms including the following:

  • Reluctance to move so they can avoid bearing weight on painful hooves
  • Standing with the front legs positioned in front of the body to avoid bearing weight on them
  • Lameness and shortened stride
  • Softening of the coronet band; In severe cases, exudate may seep through the coronet band
  • Heat in the hooves
  • Increased digital pulse
  • Dish-shaped appearance of affected hooves due to begging of failure of laminar attachments
  • Rings on the hoof due to damage to laminae
  • Stretched white line. The inner edge of the hoof may become elongated when the laminar connection begins to fail
  • Frequent abscesses in hooves

If you notice any of these signs after your horse has been turned out in a lush grass pasture, call your veterinarian to obtain a diagnosis.

Risk Factors for Pasture Laminitis

Some horses are at a greater risk of developing this condition than others. Risk factors for pasture laminitis include:

Genetics

Horses and ponies of certain breeds including Welsh, Shetland, Icelandic, Norwegian fjords, and Morgan have a higher risk of pasture-associated laminitis. [10] Any breeds prone to metabolic syndrome also have an increased risk for the condition.

These animals are commonly referred to as easy keepers. They are often overweight, obese, or have fatty deposits in certain areas of their bodies. Obesity increases the strain on laminae and can exacerbate the damage of PAL.

They are genetically predisposed to store more fat to use as energy reserves when food is scarce. While these genes may have aided the ancestors of these horses living in harsh environments, they are detrimental to modern-day horses that have calorie-dense foods readily available.

There is mounting evidence that metabolic syndrome is genetic , which explains why it is absent in some breeds. [19][20][21][22]

Insulin Resistance

The primary role of the hormone insulin is to stimulate tissues to remove glucose from the bloodstream. When insulin signalling is impaired, tissues fail to uptake glucose, triggering the production of more insulin to clear glucose from the blood.

Elevated insulin is a known cause of laminitis in horses and ponies.[7] High insulin may interfere with circulation in the feet by increasing endothelin-1. [9][25] High insulin may be responsible for the proliferation and lengthening of laminae observed early in endocrinopathic laminitis. Cell death can also occur which weakens the hoof-to-bone connection and increases the risk for laminitis.

Horses with metabolic syndrome have cells that fail to respond to insulin properly, which results in above-normal insulin levels, but blood sugar levels typically remain normal.

Insulin typically has vasodilatory effects, but with insulin resistance, vasoconstrictive substances can prevail. For example, higher levels of endothelin-1 are seen in insulin resistance. [8] This is a protein produced by the cells lining the interior of blood vessels in the connective tissues of the equine hoof. Increased Endothelin-1 promotes vasoconstriction (the constriction of blood vessels) which increases the chance of laminitis developing.

Insulin resistance also decreases the production of nitric oxide, a key signalling molecule that regulates the flow of blood and oxygen to tissues. Reduced levels of nitric oxide could promote laminitis by interfering with normal circulation.

PPID

Pituitary Pars Intermedia Dysfunction (Equine Cushing’s disease) is an endocrine-related disease that commonly affects older horses of all breeds.

Research indicates that more than 50% of horses or ponies that have PPID develop laminitis. [6][11]

These horses are predisposed to laminitis due to an overproduction of pituitary hormones that alter metabolic responses. Horses with PPID have high levels of circulating adrenocorticotrophic hormone (ACTH). This hormone stimulates the adrenal glands to increase the production of the stress hormone, cortisol.

Increased cortisol indirectly contributes to vasoconstriction in the hoof by increasing the sensitivity of blood vessels to vasoconstricting agents, including catecholamines.

Catecholamines are hormones such as epinephrine/adrenaline released by the adrenal glands in response to pain. These hormones cause peripheral vasoconstriction, which decreases blood flow to the hooves resulting in inflammation and tissue destruction.

Increased cortisol levels in horses with PPID not only impact peripheral blood flow but also promote insulin resistance by interfering with insulin signalling, thus increasing the risk of laminitis.

Age

As horses get older, the risk of developing pasture laminitis increases because insulin resistance is correlated with age. Insulin resistance rarely develops in growing animals but may appear as early as 3 to 5 years of life.

Insulin concentrations in the blood in response to oral sugars are higher in aged horses than in younger adult animals. This suggests they are more insulin resistant. [16]

Diagnosis of Pasture Laminitis

Veterinary assessment is necessary to accurately diagnose laminitis and the extent of damage to the laminae. In some cases, X-rays will be used to determine if coffin bone displacement has occurred and provide a baseline for therapeutic farriery.

Horses that have experienced laminitis should be tested for any underlying metabolic conditions that may have contributed to their condition.

Treatment for Pasture Laminitis

Horses with pasture laminitis should be removed from pasture immediately and assessed by a veterinarian. The horse should be placed in a deeply bedded stall or small area with soft footing to restrict movement.

Depending on the severity of lameness, horses with laminitis may require extended stall rest with or without occasional hand-walking.

Horses that have suffered acute laminitis may need to be confined to a stall even after their lameness resolves until their laminae have healed. However, in most cases it is best to allow horses to move at will, as long as they are not on pain-blocking medication. Preferably this should be done in a small enclosure attached to their stall. This improves circulation and promotes recovery.

Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly prescribed but often are not very effective because endocrinopathic laminitis is not inflammatory. If pain is severe, Tramadol hydrochloride is an option. [23] NSAIDs complications are related to inhibition of nitric oxide, which makes them a poor choice for endocrinopathic laminitis.

Icing the hooves (cryotherapy) may provide temporary pain relief but is contraindicated in endocrinopathic laminitis because it worsens vasoconstriction. Icing does help with other causes of laminitis that involve inflammation.

It is also essential to treat any underlying endocrine conditions and to implement diet and management strategies to prevent future episodes of this condition. Getting insulin under control is the most important factor in controlling pain and stopping laminitis. With the proper diet, pain can abate within 2 or 3 days.

Horses with laminitis may benefit from therapeutic farriery. Heart-bar shoes and other types of corrective shoeing may aid in the rehabilitation of horses that have suffered laminitis. Some horses may recuperate from the condition better without shoes (barefoot trimming) and can be kept in boots and pads for comfort. This approach allows for more frequent trimming, which is often required during laminitis recovery.

Strategies to Prevent Pasture Laminitis

You can take the following precautions to help your horse avoid developing pasture laminitis:

Avoid pasture turnout: Horses and ponies with active laminitis should not be given access to pasture until there has been a complete resolution of the condition. If your veterinarian allows it, you may be able to turn your horse out in a dry lot with no grass growing.

Restrict grass intake: Using a grazing muzzle can reduce grass intake for horses that can safely graze on limited amounts of pasture. Some horses will require turnout on dry or sparse lots to prevent overconsumption.

Manage pasture: Ensure pastures are not stressed by being overgrazed to reduce the level of sugar ingested by your horse. Grasses that have grown to full height and released their seed are the safest to graze but will still vary in sugar and starch levels over the day.

Pasture analysis: Grasses can be submitted for analysis to determine approximate levels of non-structural carbohydrates at different times of the year. However, because these levels fluctuate widely a low NSC value on a grass sample does not necessarily mean that the grass is safe for laminitis-prone horses to graze on. Grasses must be flash frozen immediately in liquid nitrogen for accurate analysis.

Hay selection: If turning out your horse on a dry lot, you should provide appropriately-selected hay for your horse to forage on. Choose hay that contains less than 10% ESC + starch. If you cannot submit a sample of your hay, it should be soaked to reduce the sugar content.

Maintain appropriate body weight: Overweight horses put more strain on their feet. Regularly monitor your horse’s body composition and make adjustments to their diet as needed. Although research has shown that gaining weight does not necessarily lead to insulin resistance, losing weight does improve it. [24]

Introduce dietary changes slowly: Don’t turn a horse out to pasture to graze all hours of the day without first allowing a transition period from a hay-based diet. Monitor your horse for signs of laminitis as the transitions from a hay-based diet to pasture.

Choose turnout times carefully: When turning out laminitis-prone horses to graze, turn them out in the early morning and evening when the NSC levels are lowest.

Monitor horses for signs of laminitis: Horses at risk for laminitis should be monitored daily. Early signs of the condition include warmth in the hooves, increased digital pulse, and reluctance to move.

Provide regular hoof care: Have your horse’s hooves trimmed regularly. A farrier can assist with checking your horse’s hooves for signs of laminitis and other hoof problems that could cause damage to the laminae.

Consult an equine nutritionist: Speak with an equine nutritionist to formulate the ideal feeding plan for your horse. Your horse’s diet should be nutritionally balanced to support healthy hooves, metabolic function and overall well-being.

You can submit your horse’s information online for a free consultation with our equine nutritionists. Our nutritionists can help you design a turnout plan for your horse that minimizes the risk of pasture-associated laminitis.

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References

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  3. Grenager, N.S. Endocrinopathic Laminitis. Vet Clin North Am Equine Pract. 2021. View Summary
  4. Kane, A.J. et al. A cross-sectional survey of lameness and laminitis in U.S. horses. Proc 9th Intern Symp Vet Epid Econ. 2000.
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