Laminitis is the inflammation and subsequent separation of the laminae of the hoof. Causes in horses include carbohydrate overload, excess weight bearing, and endotoxemia. The hallmark clinical sign is severe lameness with bounding digital pulses. P3 rotation or sinking can result from laminar separation in the most severe cases, and it is associated with a guarded to poor prognosis. Treatment includes addressing the primary systemic disease present, pain management, restricted exercise, supportive footing, corrective farriery, and sometimes debridement after the acute phase of disease has passed.
Laminitis is one the most important and catastrophic diseases in horses. Manifestation of this disease can result in a vast spectrum of consequences from temporary loss of use to career-ending or life-threatening circumstances.
The bony column of the limb of the horse is suspended from the inside of the hoof capsule by interlocking dermal and epidermal laminae. In laminitis, a series of pathophysiological events causes injury to dermal (sensitive) and epidermal (insensitive) laminae, weakening this attachment. In more severe cases, this weakening can result in separation of the hoof capsule from the underlying tissues.
Laminitis can be divided into stages, or phases, that are somewhat subjective:
Developmental phase. The period of time between the inciting cause and the first evidence of foot pain.
Acute phase. A period of ≤ 72 hours after the onset of clinical signs during which the coffin bone remains undisplaced in relation to the hoof capsule
Subacute phase. The period of time beyond 72 hours after the onset of clinical signs during which the coffin bone remains undisplaced in relation to the hoof capsule.
Chronic phase. An undefined period of time during with the coffin bone is displaced in relation to the hoof capsule.
Etiology and Pathophysiology of Laminitis in Horses
Inflammation of the dermis and degeneration of the laminae in the hoof can be initiated by various mechanisms, resulting from a single initiating cause or several contributing factors.
Diet-related laminitis in horses is thought to result from excessive ingestion of nonstructural carbohydrates. With grain overload, excessive concentrate consumption leads to the passing of appreciable amounts of undigested starch to the hindgut, where the starch is rapidly fermented and lactic acid is produced. The resulting lactic acidosis stresses and kills a proportion of hindgut bacteria, causing them to release toxins that are absorbed through the intestinal walls. The absorption of these toxins leads to endotoxemia and the associated systemic inflammatory response syndrome.
With pasture-associated laminitis in horses, the consumption of large quantities of nonstructural carbohydrates in a pasture has a result similar to that of excessive concentrate consumption. Horses with obesity and insulin resistance are predisposed to developing pasture-associated laminitis.
Inflammatory, infectious, or toxin-related laminitis can arise in horses from endotoxemic processes such as metritis, pleuritis, enteritis, or colitis.
Endocrine dysfunction is probably the most common underlying cause of laminitis in horses today. More specifically, hyperinsulinemia is thought to be the predisposing condition. Equine metabolic syndrome and pituitary pars intermedia dysfunction (PPID) are the main disease processes in this category. PPID alone may not be an inciting cause; however, many horses with PPID that develop laminitis have concurrent insulin resistance.
Mechanical or support limb laminitis in horses results from a single limb being forced to bear excessive weight because of an injury or disease process in the contralateral limb. The lack of cyclic loading and unloading in the weight-bearing limb is theorized to cause a lack of perfusion to the internal structures of the hoof. The timing of onset varies.
Other causes of laminitis in horses include the administration of corticosteroids and plant poisoning.
Anecdotal evidence suggests that systemic or intra-articular corticosteroid administration can be a cause of laminitis. This evidence is not supported by controlled data in the literature but should be considered, especially in a horse with a predisposing condition such as PPID or equine metabolic syndrome.
Ingestion or contact with a poisonous plant may also induce laminitis; black walnut (Juglans nigra) shavings are the classic example. (See the table Poisonous Range Plants of Temperate North America.)
Chronic laminitis in horses can be characterized by how the coffin bone displaces relative to the hoof capsule. The manner of this displacement depends on the severity of the tissue necrosis within the laminae, as well as the stresses of weight bearing and movement.
With chronic rotational laminitis, the tension of the deep digital flexor tendon causes rotation of the coffin bone relative to the hoof capsule. This rotation causes compression of the solar dermis beneath the descending distal margin of the coffin bone, which in turn causes compression or shearing of blood vessels and nerves under the distal margin of the coffin bone and in the dorsal coronary region.
Chronic distal displacement (sinking) of the coffin bone may be symmetrical or asymmetrical.
With symmetrical distal displacement, the entire bony column moves distally within the hoof capsule.
With asymmetrical distal displacement, one side of the bony column moves distally within the hoof capsule. Because of differences in weight bearing, this asymmetrical distal displacement usually occurs medially in the forelimbs and laterally in the hindlimbs.
Clinical Findings of Laminitis in Horses
Clinical signs of laminitis in horses vary with the stage and severity of the disease. Usually, both forefeet are affected; however, any combination of affected limbs is possible.
Horses with bilateral forelimb involvement commonly move with an extremely stiff, short-strided gait in which the forelimbs are placed in front of their normal position and the hindlimbs are placed under the horse, also in front of their normal position (see laminitis posture image).
Courtesy of Dr. Thomas Lane.
When laminitis affects all four limbs, the horse is less likely to move with the hindlimbs in the forward position. Severely affected horses are usually unwilling to move or pick up a foot that is contralateral to an affected limb.
During the acute phase of laminitis in horses, the hoof may be warm to the touch (this sign can be difficult to detect, depending on the ambient temperature). In many cases, a bounding, exaggerated digital pulse is palpable in the digital arteries in the region of the fetlock. Hoof tester examination often yields a response over the sole or apex of the frog; however, many horses refuse to pick up a foot during this acute stage of laminitis.
In the chronic phase of laminitis in horses, the clinical signs are more variable, ranging from mild lameness to recumbency.
Generally, chronic laminitis in horses is associated with some recognizable external characteristics of the hoof capsule. The altered position of the coffin bone in the hoof capsule causes changes in hoof wall production. The pressure of the displaced coffin bone on dermal tissue can result in a decrease of horn growth in the sole and toe.
Courtesy of Dr. Thomas Lane.
The severity and duration of the disease influence the extent of hoof capsule distortion. Palpation of the coronary band may reveal a dip or ledge in the area where the coffin bone has descended in the hoof capsule. The skin in this coronary region can separate at the hair line, and serum may ooze from the separation. The sole of the foot will often take on a flattened or convex shape as a result of the rotated or distally displaced coffin bone. In severe cases, the coffin bone and attached dermis may prolapse through the bottom of the sole.
When the coffin bone has stabilized and the hoof is growing out, a notable widening of the white line is often evident. The hoof wall generally develops a convergent pattern of growth rings in the toe region because of delayed growth of the dorsal wall and an increased growth rate in the heels (see laminitis lesions image).
Complications associated with laminitis in horses include sloughing of the hoof capsule, prolapse of the coffin bone through the sole, recurrent hoof abscesses, and pedal osteitis.
Diagnosis of Laminitis in Horses
The diagnosis of acute laminitis in horses is often suggested from the history and clinical signs. If only the hindlimbs are affected, acute laminitis may be more difficult to identify, because it can mimic neurological signs.
Initial radiography during the acute phase indicates only slight thickening of the dorsal hoof due to laminar edema.
With chronic laminitis, the characteristic hoof capsule changes are often enough to make a diagnosis.
Radiography is useful in chronic laminitis to help determine the severity of the disease. Lateromedial and horizontal dorsopalmar or dorsoplantar views are adequate to evaluate the position of the coffin bone in relation to the hoof capsule (see laminitis radiographic image).
Courtesy of Dr. Brian Beasley.
The lateral view enables evaluation of the angle of the dorsal surface of the coffin bone relative to the dorsal surface of the hoof capsule, the thickness of the sole under the tip of the coffin bone, and the relative distance between the coronary band and the extensor process of the coffin bone.
The horizontal dorsopalmar or dorsoplantar view enables the identification of asymmetrical distal displacement, in which one side of the coffin bone is appreciably closer to the ground and the ipsilateral side of the coffin joint space appears widened.
With increased chronicity, radiographic findings may include remodeling at the tip of the coffin bone and evidence of periosteal new bone formation on the middorsal surface of the coffin bone.
Management and Treatment of Laminitis in Horses
No currently available drug or agent stops the onset of laminitis, and there is no universally accepted standard approach for the treatment of acute laminitis in horses.
The extent and severity of the initial laminar pathological change seem to influence the outcome more than does the treatment plan.
Considering each case individually when developing a treatment plan offers the greatest possibility of success. The following general goals should be considered in the treatment of laminitis:
identifying and urgently treating any disease processes occurring in other body systems
identifying and treating or eliminating other factors that can cause a predisposition to or triggering of laminitis in horses
preventing further damage to the laminae, and maintaining the blood supply to the dermis of the hoof
Additional goals include using mechanical aids to prevent displacement of the coffin bone, and decreasing pain and inflammation.
A variety of measures can help minimize the effects of acute laminitis in horses:
Cryotherapy during the developmental and acute stages is currently the most effective treatment to decrease the severity of laminar pathological change.
Drugs administered to help maintain or improve the blood supply to the foot include acepromazine, pentoxifylline, isoxsuprine, aspirin, and heparin. NSAIDs are most commonly used to manage pain and inflammation.
Removal of existing horseshoes must be a careful consideration because in the acute phase of laminitis, the act of removing the shoe could result in further laminar damage.
Supporting the frog or sole with materials such as expanded polystyrene, dental impression material, or hoof boots with a soft deformable insert during the acute phase helps to decrease the load borne by the laminae and redistribute it to other parts of the hoof.
Appreciable elevation of the heels can decrease the tension of the deep digital flexor tendon. Commercially available wedge shoes can be secured to the hoof without the use of nails.
Horses in the acute phase of laminitis should be restricted to a stall with deep bedding.
The treatment of chronic laminitis also requires an approach customized to the individual condition of the horse. The main goal for the treatment of stabilized chronic laminitis is to promote healing of the tissues and regrowth of a healthy hoof wall and sole. The likelihood of achieving these goals is influenced by the extent of rotation or sinking, the extent of deformation of the hoof capsule, and the state of the blood supply within the hoof capsule.
Supportive care in the form of therapeutic shoeing is the mainstay of treatment for chronic laminitis in horses. The fundamental areas of focus in therapeutic shoeing or trimming in such cases include the tension of the deep digital flexor tendon, breakover, the thickness of the sole under the tip of the coffin bone, and the alignment of the dorsal surface of the coffin bone in relation to the dorsal hoof wall. Radiographic examination is useful in making therapeutic shoeing decisions about the alteration of breakover and the amount of heel elevation.
Surgical intervention consisting of deep digital flexor tenotomy is sometimes necessary in cases of laminitis in horses to decrease the tension of the tendon when therapeutic shoeing is not adequate. This surgery is indicated primarily in cases with progressive rotation of the coffin bone and persistent pain despite all other attempts to control it. This procedure may improve the blood supply to the dorsal and subsolar dermis of the coffin bone by decreasing the forces of compression and tension.
Horses with distal displacement of the coffin bone are more problematic to treat because of the loss of laminar support around the circumference of the foot. Standing the horse on a soft, deformable surface may improve comfort. A foot cast that has a rounded bottom may also provide comfort and ease of movement.
The prognosis for horses with laminitis varies and depends on many factors, including the underlying cause, extent of lameness, type and extent of displacement, body weight, relative stability of the coffin bone within the hoof capsule at the time of diagnosis, and response to treatment. Generally, if laminitis is diagnosed and treated early enough in the disease process, the prognosis is better for horses with only rotational displacement and worse for horses with symmetrical distal displacement. The prognosis is guarded to poor for horses with severe acute laminitis, regardless of the type of displacement.
Key Points
Laminitis is a serious condition in horses. Severe cases are marked by debilitating pain due to permanent structural deformity of the support structures of the foot.
Severe acute laminitis is usually associated with severe systemic disease, such as colitis, pleuropneumonia, or metritis.
Mild recurrent laminitic episodes are common in horses with insulin dysregulation or pars pituitary intermedia dysfunction (Cushing syndrome).
For More Information
Grenager NS. Endocrinopathic laminitis. Vet Clin North Am Equine Pract. 2021;37(3):619-639. doi:10.1016/j.cveq.2021.08.001
van Eps A, Engiles J, Galantino-Homer H. Supporting limb laminitis. Vet Clin North Am Equine Pract. 2021;37(3):657-668. doi:10.1016/j.cveq.2021.08.002
Also see pet health content regarding laminitis in horses.
