Laminitis

What it is

Laminitis is a lesion of the laminae of the hoof, the tissue that joins the horny wall to the third phalanx (distal phalanx). The epidermal laminae on the inner face of the wall interdigitate with the dermal laminae covering the bone, and this attachment carries the horse's weight and the pull of the deep digital flexor tendon. When the laminae are damaged the attachment weakens and the distal phalanx can rotate (its tip moves away from the wall and down towards the sole) or sink inside the hoof. In severe cases the bone penetrates the sole.

Three forms are distinguished by cause:

• Endocrinopathic laminitis: the most common. It is associated with the insulin dysregulation of equine metabolic syndrome and with pituitary pars intermedia dysfunction (PPID, formerly equine "Cushing's disease"). Hyperinsulinaemia, often after access to sugar-rich pasture, triggers the laminar lesion.
• Sepsis- or systemic inflammatory response syndrome (SIRS)-associated laminitis: a complication of colitis, enteritis, strangulating colic, metritis from retained placenta, pleuropneumonia or grain overload.
• Supporting limb laminitis: develops in the sound limb when the horse does not bear weight on the opposite limb for weeks because of a fracture, joint infection or other severe lameness. It is attributed to the loss of the normal cycles of loading and unloading that maintain laminar blood flow.

Who it affects

Horses and ponies of any age, with higher risk in ponies, native breeds and obese animals or those with regional fat deposits (crest of the neck, tail head). The endocrinopathic form is more common in spring and summer with abundant pasture; PPID makes it more common in older horses. The SIRS-associated form affects horses hospitalised with severe disease, and the supporting limb form mainly heavy horses with prolonged unilateral lameness.

Anatomy involved

• Hooves (ungulae): the wall is attached to the distal phalanx by the laminae, about 600 primary epidermal laminae each carrying numerous secondary laminae, which multiply the attachment surface. The sole, frog and coronary band complete the hoof capsule. When the phalanx is displaced, the coronary band sinks, the wall grows divergently (rings wider apart at the heel than at the toe), and a widened white line and a flat or convex sole appear.
• Phalanges of the thoracic and pelvic limbs: the distal phalanx (third phalanx) is the bone that rotates or sinks; the deep digital flexor tendon, which inserts on its palmar surface, pulls it backwards and favours rotation. The thoracic limbs, which carry about 60% of the weight, are affected more often and more severely; supporting limb laminitis can affect any limb.

Clinical signs

Lameness of variable severity, almost always bilateral in the thoracic limbs. The horse adopts a typical stance: it shifts its weight back, places the thoracic limbs forward to unload the toes and is reluctant to walk or turn. At the walk it takes short, stiff steps, especially on turns and on hard ground. Digital pulses are increased, the hoof may be warm and hoof testers over the toe are painful. Severity is graded with the Obel scale (I to IV). In chronic laminitis there are divergent rings on the wall, a depression at the coronary band, a flat sole and, if the phalanx penetrates the sole, discharge of pus or exposure of the bone itself.

Diagnosis

• Clinical examination: stance, digital pulses, response to hoof testers and Obel grade.
• Radiographs, lateromedial and dorsopalmar views of both feet with markers on the wall: they measure rotation, sinking of the distal phalanx and sole thickness, and serve as a reference for shoeing and follow-up.
• Endocrine testing in every case without an obvious inflammatory cause: basal insulin or insulin after an oral sugar test for insulin dysregulation, and basal ACTH for PPID, interpreted according to the season.
• Digital venography shows the loss of perfusion of the coronary and sole regions in severe cases.
• Differential diagnosis: subsolar abscess, fracture of the distal phalanx, sole bruising and other causes of bilateral lameness.

Treatment and prevention

• Acute phase: rest on deep bedding, frog and sole support (pads, impression material, therapeutic shoes) to spread the load, analgesia with non-steroidal anti-inflammatory drugs and other analgesics, and treatment of the cause.
• Digital cryotherapy: keeping the feet and pasterns in ice water continuously in horses at risk of sepsis-associated laminitis, such as those with colitis; the evidence comes from experimental models and retrospective studies.
• Endocrinopathic laminitis: a diet low in non-structural carbohydrates (low-sugar hay, sometimes soaked), no cereals, restricted or no grazing, weight loss in obese animals and exercise once the horse is sound. PPID is treated with pergolide. The aim is to control insulin, because that is what predicts recurrence.
• Supporting limb laminitis: support of the sound foot from the start, pain control in the injured limb so the horse loads it again, and daily monitoring of the supporting limb.
• Chronic cases: corrective trimming and shoeing guided by radiographs, sometimes deep digital flexor tenotomy; euthanasia on welfare grounds is common when pain cannot be controlled.
• The horse is a food-producing animal in many countries and drug use is subject to national regulations and withdrawal periods. In the EU, the equine identification document (passport) states whether the horse is excluded from slaughter for human consumption, and this decides which medicines may be given.

Key studies

• Wylie et al. (2013a), prospective cohort study of veterinary-diagnosed laminitis in Great Britain, with 577 cases. The prevalence of active laminitis was 0.49% (95% CI 0.43–0.55%) of horses registered with practices and the incidence 0.5 cases per 100 horse-years. Both forelimbs were most often affected (53.5%), and the most common signs were increased digital pulses, difficulty turning and a short, stilted gait at the walk. The figures are much lower than earlier estimates, partly because of a different case definition.
• Wylie et al. (2013b), case-control study with 191 cases of active laminitis and 819 controls from the same population. Risk was increased by weight gain in the previous 3 months, new access to grass in the previous 4 weeks, box rest in the previous week, a history of laminitis, lameness after trimming or shoeing and existing endocrine disease (PPID or equine metabolic syndrome); greater height was associated with lower risk. It is observational and based on owner questionnaires.
• Knowles et al. (2023), prospective cohort of 374 non-laminitic ponies followed for up to 4 years (891 pony-years). Incidence was 4.8 cases per 100 pony-years. Basal insulin and insulin 60 minutes after an oral sugar test were the best predictors: with high basal insulin (>45.2 µIU/ml), 10% of the population, the estimated 4-year incidence was 69%, compared with 6% with low insulin. ACTH was not independently associated. The cut-offs depend on the insulin assay.
• de Laat et al. (2019), prospective cohort of 317 horses and ponies with acute laminitis, of which 276 endocrinopathic cases were followed for 2 years. Laminitis recurred in 34.1%. The risk of recurrence increased with basal fasted insulin, especially above about 45 µIU/ml, and with a previous diagnosis of laminitis; cases with a higher Obel grade recurred sooner. Data were provided by practising veterinarians without a common diagnostic protocol.
• ECEIM consensus statement on equine metabolic syndrome (Durham et al., 2019): defines the syndrome as a collection of risk factors for endocrinopathic laminitis, of which insulin dysregulation is the most important. It recommends basal or dynamic insulin testing and control based on diet and exercise, with drugs only as an aid in some cases. Several authors declare links with diagnostic laboratories or the pharmaceutical industry.
• Kullmann et al. (2014), retrospective study of 130 horses with colitis and a systemic inflammatory response at 2 university hospitals. Laminitis developed in 7 of 69 (10%) treated with digital cryotherapy and 20 of 61 (33%) not treated (OR 0.11; 95% CI 0.03–0.44). Only 48% of horses with laminitis survived, compared with 98% of those that did not develop it. It is not a randomised trial: cryotherapy depended on the hospital and the clinician, which may bias the result.