Winter laminitis is a circulatory condition that causes sore hooves in horses exposed to cold temperatures.

Horses affected by winter laminitis have impaired circulation within their hooves, leading to pain and inflammation in the laminae.

Signs of cold-induced laminitis include difficulty moving due to pain and, in rare cases, changes in hoof structure. Like other forms of laminitis, this condition typically only affects the front feet, or is most obvious in the front feet, since horses bear 60% of their weight on the front feet.

Winter laminitis is seen in horses with existing metabolic conditions, damaged circulatory mechanisms or horses that have suffered previous attacks of laminitis.

Diagnosis of winter laminitis should be made by veterinary examination after consideration of past medical history. This condition is best managed by addressing any underlying metabolic disease and providing appropriate feeding and management strategies.

What is Winter Laminitis?

Winter laminitis can occur in horses following a sudden drop in temperature. It results in pain and inflammation in the hooves of horses, ponies, and donkeys.

The horse’s body naturally reduces blood flow to the extremities in cold weather. Blood vessels constrict, and structures called arteriovenous shunts open, decreasing the delivery of oxygen to the hooves.

Arteriovenous shunts direct blood away from the capillaries that provide blood to the tissues and send it directly to the veins.

In normal horses, protective mechanisms ensure that blood oxygen levels do not get too low by opening up blood vessels and closing shunts as needed.

However, in horses with metabolic or circulatory issues, blood flow to the hooves may be insufficient, putting them at risk of winter laminitis.

Clinical Features

In horses that become laminitic in cold weather, higher levels of insulin result in greater vasoconstriction of blood vessels, causing pain.

Elevated insulin levels increase endothelin-1, a potent vasoconstrictor. [9][22][23] The combination of increased endothelin-1 and cold induced vasoconstriction may be what tips some horses over the edge into compromised circulation. [24]

This condition affects the soft structures of the hooves known as the laminae, which anchor the coffin bone to the hoof wall.

Unlike other forms of laminitis, this condition rarely progresses to founder which involves rotation and sinking of the coffin bone within the hoof capsule.

 
Healthy vs Laminitic Horse Hoof
 

Prevalence

Horses with metabolic disorders including Equine Metabolic Syndrome and pituitary pars intermedia dysfunction (PPID) are more likely to develop winter laminitis. These metabolic conditions impair normal circulatory mechanisms within the hooves.

Prior attacks of endocrine laminitis can also cause damage to the circulatory mechanisms of hooves and increase the risk of winter laminitis.

It is estimated that one in ten horses is affected by laminitis each year. [1] The proportion of horses specifically affected by winter laminitis is unknown.

Signs of Winter Laminitis

Winter laminitis often has a sudden onset despite no changes in diet or management. The condition may affect some horses at the same time each year and without intervention can continue until temperatures warm up in the Spring.

Horses with laminitis may show signs ranging from mild discomfort to sudden and severe lameness. [2] Affected horses will typically be more comfortable standing and moving on soft surfaces compared to those that are hard, uneven, and frozen.

Some of the possible indications of winter laminitis include:

Physical and Behavioral Signs

  • Sawhorse stance: Affected horses stand with their front legs positioned out in front of their body to reduce the weight-bearing load on them.

  • Reluctance to move: The severe pain caused by winter laminitis discourages horses from moving about normally.

  • Abnormal gaits: Horses with laminitis may exhibit a shortened stride and foot lameness.

  • Foot lifting: Horses with foot pain may lift their feet to reduce pain by shifting weight from one leg to another.

  • Increased digital pulse: Some horses may have a stronger digital pulse due to restricted blood flow and inflammation in the leg and hoof tissues. However, this is rare and most horses with winter laminitis also have cool/cold rather than hot feet.

Changes in the Hoof Structure

  • Distorted hoof wall: An unhealthy laminar attachment between the coffin bone and the hoof wall can cause a dished or flared appearance of the hoof.

  • Hoof rings: Weakening in the laminae and reduced blood supply to the area can promote the development of hoof rings.

  • Stretched white line: The white line which is present around the inner edge of the hoof may become widened when the laminar connection begins to fail.

Metabolic Abnormalities

  • Increased insulin levels: This is the greatest risk factor for winter laminitis.

  • Obesity: Obesity and excess body condition can be a marker of metabolic syndrome with increased insulin levels.

  • Regional fat deposits: Accumulations of fat deposits in areas including the neck, withers, rump, and genitalia are present in one or more of those locations in horses with metabolic syndrome.

What Causes Winter Laminitis?

Exposure to cold temperatures triggers the development of winter laminitis, although this is not the sole cause of the condition.

A sudden decrease in environmental temperature initiates multiple hormonal changes in horses to help their bodies cope with the cold. [11]

While most healthy horses can cope with winter temperatures, some with underlying diseases may develop winter laminitis due to the following factors:

Increased Cortisol Production

When temperatures drop quickly, the adrenal glands increase the production of the hormone cortisol.

This hormone has a vasoconstrictive effect on blood vessels in extremities. [18]

Cortisol also worsens insulin resistance. However, the cortisol response to acute weather changes is short-lived and it is unclear what role it may play in winter laminitis.

Changes in Insulin Sensitivity

Horses tend to exercise less in winter which may affect insulin sensitivity in tissues.

High insulin increases the production of Endothelin-1 in the connective tissues of the equine hoof. [9] This promotes vasoconstriction in the hooves which could make it more likely for winter laminitis to develop.

Risk Factors

Several factors are believed to contribute to the onset of winter laminitis, including metabolic health, circulatory function and pituitary hormone imbalance.

Horses are more likely to have problems with hoof circulation and an increased risk for laminitis if they are affected by any endocrine disorders. [3][4][5]

Equine Metabolic Syndrome (EMS)

Equine metabolic syndrome is a disorder associated with difficulty regulating glucose (sugar) levels in the blood.

Insulin is a hormone that stimulates muscle, liver, and fat to remove glucose from the bloodstream.

Horses with EMS are insulin resistant, meaning their cells are less responsive to insulin. When insulin signalling is impaired, the tissues fail to uptake glucose normally, thus triggering the production of more insulin to clear glucose from the blood.

An overproduction of insulin can lead to chronically high concentrations of this hormone in the blood, leading to health issues including laminitis. [6]

Insulin-Induced Laminitis

In experiments, elevated levels of the hormone insulin are known to promote laminitis in horses and ponies. [7]

It is not well understood how high insulin contributes to laminar damage in the horse. However, it is known that insulin influences the production of biological agents that promote the constriction of blood vessels.

In particular, insulin is known to affect nitric oxide and endothelin-1. Endothelin-1 is a potent constrictor of blood vessels while nitric oxide dilates them.

Vasoconstriction

Vasoconstriction increases resistance to blood flow in the hooves. In cold weather, this can lead to laminar changes, and increased pain.

Chronically high levels of insulin are associated with decreased production of nitric oxide – a key signalling molecule that regulates blood flow and tissue oxygenation. Lower levels of nitric oxide lead to vasoconstriction. [8]

High blood insulin concentration is also known to increase the production of Endothelin-1, a protein produced by the cells lining the interior of blood vessels.

Endothelin-1 is a potent vasoconstrictor associated with insulin dysregulation and present in the hooves of horses with insulin-related laminitis. [8][9][10]

Pituitary Pars Intermedia Dysfunction (PPID)

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

The condition results in an overproduction of pituitary hormones, causing metabolic dysfunction.

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.

Cortisol

As cortisol production increases, blood vessels narrow and circulation to and inside the hoof decreases. Cortisol increases the sensitivity of blood vessels to vasoconstricting agents, but doesn’t cause blood vessel constriction itself.

Cortisol enhances the effects of hormones called catecholamines, which are released into the bloodstream by the adrenal glands in response to pain.

Catecholamines cause peripheral vasoconstriction, which decreases blood flow to and in the hooves, resulting in inflammation and destruction of tissues.

Pain and the associated stress response lead to an even greater increase in the production of cortisol, resulting in a vicious cycle. Breaking the cycle of pain is an important part of successful treatment for laminitis.

The increase in cortisol production in horses with PPID contributes to insulin resistance. Just as horses with EMS-related insulin resistance are at risk for laminitis due to the vasoconstrictive effects of Endothelin-1, those with PPID are as well.

Damaged Circulatory Mechanisms

Laminitis can cause permanent damage to the circulatory mechanisms of hooves. AVA (arteriovenous anastomosis) shunts located in the sensitive laminae may no longer open and close efficiently to maintain sufficient blood circulation. [19]

In healthy horses with no prior damage to the circulatory mechanisms of their hooves, blood vessels constrict and shunts open in response to cold. But blood supply at the tissue level is normally restored when oxygen tension drops too low.

If the vascular network of the hoof is damaged, or if constriction is higher than normal due to hormonal issues, AVA shunt functioning may not occur properly.

Diagnosis of Winter Laminitis

Examination by a veterinarian is important to correctly diagnose winter laminitis.

If your horse is showing signs of hoof pain in cold weather, it is important to distinguish whether they are experiencing cold-induced laminitis or general foot pain such as sore frogs and soles caused by navigating hard, uneven, and frozen ground conditions.

Your veterinarian will review your horse’s full medical history and assess your horse for clinical signs. Horses suspected of having winter laminitis should have their hooves, body condition, and movement examined.

In rare cases, X-rays may be taken to assess any potential damage to the hoof. Imaging tests are useful to determine whether hoof rotation and sinking have occurred.

Treatment of Winter Laminitis

Non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone are often administered to horses with laminitis caused by metabolic disease and grass or grain overload.

However, horses with winter laminitis typically don’t respond well to NSAIDs because this pain is not related to inflammation. [12] This condition is better managed by helping affected horses stay warm, supporting circulatory health and stimulating healing. <