Equine Laminitis: Causes, Signs, and Management

Laminitis is a devastating, complex disease affecting the equine foot. It involves inflammation and failure of the lamellar attachment between the distal phalanx (P3, or coffin bone) and the inner hoof wall. This structural failure can lead to severe pain, rotation or sinking of P3, and often necessitates euthanasia. Laminitis remains one of the most common causes of euthanasia in aged horses, accounting for 5 out of 20 musculoskeletal deaths in a retirement facility study [14]. Understanding its causes, recognising early signs, and implementing prompt management are critical for improving outcomes.

Quick Q&A

Question: What is the most common early sign of laminitis in horses? Answer: The most common early sign is a characteristic shifting of weight between forelimbs and a "sawhorse" stance where the horse leans back to relieve pressure on the painful toes. Affected horses may also show reluctance to turn, increased digital pulses, and heat in the hooves.

Understanding Equine Laminitis

Laminitis is not a single disease but a clinical syndrome with multiple underlying mechanisms. The final common pathway is lamellar dysfunction, involving enzymatic degradation and separation of the basement membrane. Recent research has illuminated the critical roles of inflammation, vascular compromise, and metabolic dysregulation. For instance, cyclooxygenase-2 (COX-2) is markedly elevated in laminitic lamellar tissue, confirming the inflammatory component [24]. Additionally, vascular perfusion deficits, as visualised by 18F-FDG PET under continuous weight-bearing, suggest that ischaemia plays a key role, particularly in supporting-limb laminitis [33].

Laminitis can affect any horse or pony, but certain populations are at higher risk. A recent stakeholder survey identified laminitis as one of the top health concerns in the U.S. equine industry, alongside colic and metabolic disorders [12].

Causes of Laminitis

Endocrinopathic Laminitis (EMS and PPID)

The most common form of laminitis today is endocrinopathic, linked to either Equine Metabolic Syndrome (EMS) or Pituitary Pars Intermedia Dysfunction (PPID, equine Cushing's disease).

Equine Metabolic Syndrome is characterised by insulin dysregulation (ID), regional adiposity (cresty neck, tailhead), and a predisposition to laminitis. Hyperinsulinemia, not hyperglycaemia, is the primary driver. Insulin directly disrupts lamellar health, promoting lamellar cell stress and impairing keratinocyte function [7]. Adiponectin, an adipose-derived hormone, is protective; low adiponectin levels strongly predict hyperinsulinemia and laminitis risk [2]. In fact, leptin-to-adiponectin ratio >1 is a powerful predictor of corticosteroid-induced hyperinsulinemia [2]. Studies in Criollo horses have shown a direct correlation between body fat and radiographic signs of distal phalanx descent [38].

Seasonal changes in pasture non-structural carbohydrates (NSC) also trigger laminitis in susceptible horses. A study in Hucul mares found that glucose levels were significantly higher in laminitis-prone horses during May (high water-soluble carbohydrates) and October (high starch) [21]. The incretin hormones GLP-1 and GIP are also implicated, with GIP potentially involved in lipid clearance and obesity exacerbation [15].

PPID is common in older horses. It leads to excessive ACTH production, which induces hyperinsulinemia and increases laminitis risk. A large retrospective study confirmed that hyperinsulinemia-associated laminitis is independently associated with PPID [27]. Importantly, PPID does not shorten life expectancy, but affected horses experience more medical events, including laminitis [27]. The TRH stimulation test remains the standard for diagnosis; the post-/pre-TRH ACTH ratio does not improve diagnostic performance over post-TRH ACTH alone [25].

Carbohydrate Overload (Grain)

Ingestion of large amounts of high-starch concentrates or lush pasture can disrupt the hindgut microbiome, leading to acidosis and the release of endotoxins and vasoactive amines. This triggers systemic inflammation and lamellar damage. Although less common than endocrinopathic laminitis, it remains a risk in cases of accidental grain access or improper feeding.

Inflammatory/Sepsis-related Laminitis

Systemic inflammatory conditions can precipitate laminitis. These include:

• Potomac Horse Fever (PHF) caused by Neorickettsia risticii, where laminitis is a well-known complication [16]. The bacterium can be detected in faecal samples and has been recovered from aborted equine fetuses, confirming transplacental transmission [39].
• Retained fetal membranes (RFM) in mares, especially after dystocia, can lead to metritis, endotoxemia, and secondary laminitis [3].
• Generalised steatitis, an uncommon inflammatory adipose tissue disease, has been reported in a miniature horse filly presenting with laminitis and pyrexia [22].

Supporting Limb Laminitis

When a horse is non-weight-bearing on one limb for extended periods (e.g., due to fracture or septic arthritis), the contralateral weight-bearing limb is at high risk. PET studies have shown that prolonged standing weight-bearing creates perfusion deficits in the medial sole and lamellae, which may predispose to tissue ischaemia and laminitis [33]. These deficits resolve with ambulation, underscoring the role of movement in maintaining foot health.

Corticosteroid-induced Laminitis

Intra-articular (IA) corticosteroids can cause transient hyperinsulinemia in some horses, which may increase laminitis risk. Intra-articular triamcinolone acetonide consistently induces hyperinsulinemia, while IA methylprednisolone acetate did not produce significant metabolic changes in metabolically normal horses [30]. However, caution is warranted in horses with pre-existing insulin dysregulation [30].

Recognising the Signs

Early and Acute Signs

Classic signs include:

• Bounding digital pulses
• Heat in the hoof wall and sole
• Reluctance to move, especially on hard ground
• Shifting weight or "camped out" stance (fore feet placed forward)
• Painful response to hoof testers over the toe

Gait evaluation often reveals a short, stilted stride. The Obel grading system (grades 1-4) is used clinically but can be subjective.

Diagnostic Imaging

Radiography is essential for assessment. Standard views (lateromedial, dorsopoplantar) allow measurement of:

• Sinker distance: degree of distal phalanx descent
• Rotation angle: angle between P3 and dorsal hoof wall
• Palmar angle (target 3-10°)

A recent study using nonuniform rational B-spline (NURBS) ROC smoothing has refined diagnostic thresholds for these measurements, improving diagnostic accuracy over conventional empirical thresholds [11]. In Miniature Horses and Ponies, hoof wall thickness ratios (dorsal, lateral, medial) provide repeatable metrics for screening and monitoring [32].

Venography is a valuable technique to assess vascular perfusion of the digit. Intravenous contrast (e.g., iodinated agent) is injected into the palmar digital vein under tourniquet control, and serial radiographs show reduced contrast filling in laminitic feet [20]. Decreased vascular filling can guide shoeing choices.

18F-FDG PET has emerged as a research tool to detect perfusion deficits, especially in the medial lamellae and sole under weight-bearing conditions [33].

Chronic Signs

Chronic laminitis presents with:

• Divergent growth rings (wider at heels)
• Bruised soles, abscess formation
• Dropped sole, "elf slipper" foot shape
• Ongoing lameness (Obel grades 1-2)

Chronic laminitis is frequently associated with endocrine disease. A 5-year-old mare with recurrent endocrinopathic laminitis refractory to treatment highlighted the severe progression possible with EMS [7].

Emergency Management

Immediate action is crucial when laminitis is suspected.

1. Remove the inciting cause: If grain overload, administer mineral oil via nasogastric tube; if due to lush pasture, move to a dry lot or stall.
2. Supportive care: Provide deep bedding (sand, shavings) to encourage recumbency. Perform a thorough lameness exam and obtain radiographs.
3. Analgesia: NSAIDs (phenylbutazone or flunixin meglumine) are first-line. Avoid corticosteroids. Opioids (morphine, butorphanol) may be needed for severe pain.
4. Farriery: Stabilise the hoof with a heart-bar shoe or other support. Deep digital flexor tenotomy (DDFT) can relieve tension on the lamellae and reduce rotation. In a large retrospective analysis, DDFT produced a 20-fold improvement in comfort 3-6 months post-surgery [10]. A high-metacarpal DDFT combined with Steward clog shoeing achieved 100% 6-month survival in horses (88% in ponies) with chronic refractory laminitis [29].
5. Vascular support: Botulinum toxin injection into the deep digital flexor muscle has been shown to transiently reduce peak ground reaction force at the toe and shift the centre of pressure palmarly, potentially reducing lamellar stress [17]. This is a promising adjunctive treatment.

Pharmacological Advances

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) have revolutionised management of hyperinsulinemia. Ertugliflozin (0.05-0.1 mg/kg, once daily) is well-tolerated and effectively reduces resting and dynamic insulin levels [4]. In metabolically normal horses, ertugliflozin significantly blunts the insulin spike after intra-articular triamcinolone [6][19]. Velagliflozin (0.3 mg/kg, q24h) also decreases insulin concentrations and body condition score over 20 weeks [35]. SGLT2 transporters are expressed in equine kidney, liver, and pancreas, suggesting extra-renal effects [26].

Alpha-glucosidase inhibitors such as 1-deoxynojirimycin (DNJ, from Morus alba) reduce postprandial glucose and insulin spikes by delaying carbohydrate absorption. A three-phase study in 27 horses found DNJ (5000 mg q12h) significantly reduced peak insulin and insulin AUC during oral sugar tests [8].

Adiponectin receptor agonists (e.g., AdipoRon, full-length adiponectin) reduce lamellar endoplasmic reticulum stress ex vivo, offering a potential future therapeutic avenue [5]. Similarly, sex hormone-binding globulin (SHBG) improved mitochondrial function and reduced inflammation in equine adipose-derived stem cells from EMS-affected horses [34].

Long-term Management and Prevention

Management focuses on addressing the underlying metabolic disorder.

Dietary Control

• Forage restriction: Limit NSCs; use hay tested for low water-soluble carbohydrates (<12%). Soaking hay removes some water-soluble carbohydrates.
• Pasture management: Implement grazing muzzles during high-risk periods (spring, fall). Rotational grazing and managing grass-fungal interactions can lower NSC levels [23]. A study in Romania confirmed that horses consuming carbohydrate-rich forages had higher insulin, leptin, and BCS [31].
• Balanced concentrates: Avoid high-starch feeds; use a ration balancer or low-NSC complete feed.

Exercise

Controlled exercise (hand-walking, turn-out on a dry lot) improves insulin sensitivity and promotes weight loss, but must be tailored to comfort level. Over-ambitious exercise during acute laminitis can worsen damage.

Medical Management of EMS and PPID

• PPID: Pergolide mesylate is the treatment of choice. Regular monitoring of ACTH (ideally using TRH stimulation test) is essential. Although the post-/pre-TRH ACTH ratio does not improve diagnostic performance, the test itself is recommended [25].
• EMS: SGLT2 inhibitors (ertugliflozin, velagliflozin) are first-line when diet and exercise insufficient. Ensure adequate water intake to prevent dehydration and monitor for ketosis.
• Corticosteroid caution: Use IA corticosteroids judiciously in ID-prone horses. If required, co-administer ertugliflozin for 4 days before and 4 days after injection to mitigate hyperinsulinemia [6].

Monitoring and Prognosis

Regular hoof care (every 4-6 weeks) and radiographic monitoring every 3-6 months are recommended. A venogram can identify compromised perfusion and guide shoeing adjustments [20]. Prognosis depends on degree of rotation/sinking, chronicity, and control of underlying factors. A large survey found that horses with a prior history of laminitis were 10.9 times more likely to develop strangulating lipoma colic, highlighting the systemic impact of metabolic disease [36][37].

For severe refractory cases, salvage procedures like DDFT may allow pasture soundness, though outcomes are variable [10][29]. Euthanasia remains a compassionate option when pain cannot be controlled.

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