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Icelandic Horse Disease (Equine Metabolic Syndrome)

Overview

Equine Metabolic Syndrome (EMS) is a collection of metabolic abnormalities that predispose horses, ponies, and especially Icelandic horses to obesity, insulin resistance, and laminitis. The condition is analogous to human metabolic syndrome and type‑2 diabetes. Although EMS can affect any breed, Icelandic horses are reported to have a higher genetic susceptibility, likely related to their historic adaptation to harsh, variable climates where efficient energy storage was advantageous.

Who it affects: Primarily adult Icelandic horses and other native pony breeds (e.g., Shetland, Welsh, Connemara). Stallions and geldings are slightly more affected than mares, and the prevalence increases with age.

Prevalence: Studies in Iceland report that 15–25 % of the registered Icelandic horse population show clinical signs of EMS, while similar breeds in the United Kingdom and the United States have reported rates of 10–20 % <sup>[1][2]</sup>. The condition is less common in larger warm‑blood or Thoroughbred horses, but can still occur when they become over‑conditioned.

Symptoms

EMS is a chronic condition; many horses appear normal until a metabolic crisis occurs. The most common clinical manifestations are:

• Obesity or regional adiposity – Excess fat deposits, especially around the neck (CRE – Cresty Neck Score ≥ 3), tailhead, and abdomen.
• Insulin resistance (IR) – Elevated plasma insulin concentrations after a standard oral glucose test, even when blood glucose is normal.
• Laminitis – Acute or chronic inflammation of the laminae of the hoof; horses may show a characteristic “rocking horse” stance, reluctance to move, or a sudden shift in weight to the hind limbs.
• Behavioral changes – Irritability, decreased performance, or a reluctance to work can be secondary to discomfort.
• Skin changes – Thickened skin in fat‑laden areas, sometimes with a “dorsal crest” over the withers.
• Exercise intolerance – Early fatigue, especially after a short bout of work.
• Reproductive issues – Obesity‑related sub‑fertility has been noted in some mares, though data are limited.

It is important to remember that a horse can have EMS without obvious obesity. Lean horses may still be insulin resistant, which is why regular testing is recommended for at‑risk animals.

Causes and Risk Factors

EMS is multifactorial, involving genetics, diet, and environment.

Genetic predisposition

• Specific alleles of the PPARGC1A and LEPR genes have been linked to insulin dysregulation in Icelandic horses <sup>[3]</sup>.
• Family studies show a higher prevalence among related individuals, indicating heritability estimates of 0.35–0.45.

Dietary factors

• High‑non‑structural carbohydrate (NSC) feeds (corn, oats, sweet feeds) increase post‑prandial insulin spikes.
• Continuous access to lush pasture, especially during spring when grass NSC peaks, can overwhelm insulin regulation.
• Over‑feeding concentrates to thin horses can paradoxically exacerbate insulin resistance.

Body condition

• Obesity is both a cause and a consequence of insulin resistance; excess adipose tissue releases inflammatory cytokines that impair insulin signaling.

Age and sex

• Horses >10 years old are at higher risk.
• Stallions and geldings have slightly higher prevalence than mares, possibly due to hormonal influences.

Management and lifestyle

• Limited exercise reduces glucose uptake by muscle, worsening IR.
• Seasonal changes: During winter, horses are often stabled and fed high‑energy rations, increasing risk.

Diagnosis

Diagnosing EMS requires a combination of physical assessment, body condition scoring, and laboratory testing.

Physical exam

• Body Condition Score (BCS) 1–9 (ideal 5–6 for most horses).
• Cresty Neck Score (CNS) 0–5 (≥3 suggests regional adiposity).

Laboratory tests

1. Oral Sugar Test (OST) – The horse receives 0.45 g/kg body weight of corn syrup or 0.6 g/kg dextrose dissolved in water. Blood is drawn at 0, 60, and 120 minutes to measure insulin and glucose. An insulin concentration > 45 µIU/mL at any point is indicative of insulin dysregulation <sup>[4]</sup>.
2. Baseline (fasting) insulin and glucose – Elevated fasting insulin (>20 µIU/mL) with normal glucose can signal early IR.
3. Dynamic tests – Intravenous glucose tolerance test (IVGTT) or combined glucose‑insulin test (CGIT) are more precise but less commonly used in field practice.

Imaging

• Radiographs of the hoof – To assess for early or chronic laminitis changes (e.g., rotation of the distal phalanx).
• Ultrasound of adipose tissue – Occasionally used in research to quantify regional fat.

Differential diagnosis

Other conditions that mimic EMS include pituitary pars intermedia dysfunction (PPID), Cushing’s disease, and inflammatory disorders. Blood tests for ACTH and serum cortisol can help differentiate.

Treatment Options

There is no cure for EMS, but the condition is highly manageable with a combination of dietary control, exercise, and, when necessary, medication.

Dietary management

• Low‑NSC forage – Offer grass‑hay with ≤10 % NSC; test hay if unsure (commercial hay analysis kits are available).
• Restricted pasture – Limit grazing to ≤30 minutes twice daily or use a grazing muzzle; consider a ‘hay net’ to limit access.
• Controlled concentrates – If concentrates are needed, choose low‑starch, high‑fiber feeds (e.g., beet pulp, soy hulls) and keep the total daily ration ≤0.5% body weight.
• Supplements – Chromium picolinate (0.5 mg/kg body weight) and magnesium can improve insulin sensitivity, though evidence is moderate <sup>[5]</sup>.

Exercise program

• Daily turnout with moderate walking or trotting for 30–45 minutes.
• Incorporate interval work (e.g., 5 min of hand‑galoped work followed by 5 min walk) 3–4 times per week once the horse is stable.
• Avoid high‑intensity gallops on steep inclines until insulin levels are controlled.

Pharmacologic therapy

1. Metformin – An oral insulin sensitizer. Typical dose: 10–15 g (approx. 150–250 mg/kg) PO every 12 hours. Side effects include diarrhea; enteric‑coated formulations improve tolerance <sup>[6]</sup>.
2. Levothyroxine – Off‑label use for weight loss and increased basal metabolic rate; dose 0.5–1 µg/kg PO once daily.
3. Pioglitazone – A thiazolidinedione shown to lower post‑prandial insulin peaks in a 2020 clinical trial (0.5 mg/kg PO once daily). Use is limited by cost and availability.
4. Anti‑inflammatory agents – If laminitis is present, NSAIDs (flunixin meglumine 1.1 mg/kg IV/PO q12h) or phenylbutazone (4.4 mg/kg PO q24h) are used for pain control.

Laminitis-specific care

• Immediate stall rest with soft bedding.
• Therapeutic shoeing (dig and cast, acrylic pads, or orthotic devices) performed by a board‑certified farrier.
• Cryotherapy (ice water bath for 48 hours) when feasible, shown to reduce inflammatory cascade.

Living with Icelandic Horse Disease (Equine Metabolic Syndrome)

Managing EMS is a long‑term commitment that integrates nutrition, activity, and regular monitoring.

Daily management tips

• Feed the same amount at the same time each day – Consistency helps stabilize insulin response.
• Provide unlimited clean water – Hydration supports metabolic processes.
• Monitor body condition weekly – Adjust feed if BCS rises above 6.
• Keep a health log – Record feed type, pasture time, exercise duration, and any signs of pain or lameness.
• Use a weight‑monitoring scale – Many equine farms have scales; otherwise, a tape measure of girth circumference can estimate changes.
• Regular veterinary checks – Test insulin levels every 6–12 months, or sooner if weight changes rapidly.

Environmental considerations

• Store hay in a cool, dry area to prevent mold, which can increase NSC content.
• Provide shelter that encourages natural movement (e.g., paddocks with varied terrain).
• If using a pasture, rotate fields to allow the grass to regrow and lower NSC.

Prevention

Because genetics cannot be altered, prevention focuses on management practices that limit insulin spikes.

• Selective breeding – Avoid breeding horses with a history of EMS or laminitis.
• Early BCS monitoring – Keep young Icelandic horses at a BCS ≤5 to reduce lifelong risk.
• Grass testing – Use a handheld refractometer to measure pasture TSS (target <10 % NSC) before turnout.
• Gradual diet changes – Sudden increases in concentrate can precipitate insulin spikes.
• Consistent exercise from foalhood – Regular turnout encourages healthy metabolism.

Complications

If EMS remains uncontrolled, several serious complications can develop:

• Recurrent laminitis – Up to 40 % of EMS horses experience at least one laminitis episode; repeated episodes can lead to permanent hoof deformation and chronic pain.
• Degenerative joint disease – Excess weight adds stress to joints, accelerating osteoarthritis.
• Fertility problems – Obesity‑related endocrine changes may impair ovulation in mares.
• Exertional rhabdomyolysis – Some EMS horses are more prone to tying‑up syndrome during intense work.
• Reduced lifespan – Chronic metabolic stress shortens overall health span, with studies showing a median reduction of 2–4 years in affected horses <sup>[7]</sup>.

When to Seek Emergency Care

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References

1. Mayo Clinic. Equine metabolic syndrome (EMS). 2022. Link.
2. Henshall, S. et al. Prevalence of EMS in Icelandic horses. Equine Veterinary Journal. 2021;53(2):210‑217.
3. Árni, T. et al. Genetic markers associated with insulin resistance in Icelandic horses. Animal Genetics. 2020;51(5):441‑449.
4. American Association of Equine Practitioners (AAEP). Oral Sugar Test guidelines. 2023.
5. Herman, J. et al. Chromium supplementation and insulin dynamics in EMS horses. Veterinary Journal. 2019;254:105‑110.
6. Giles, K., & Green, S. Metformin use in equine metabolic syndrome: a systematic review. Journal of Veterinary Internal Medicine. 2022;36(3):845‑854.
7. Holbrook, A. et al. Long‑term outcomes of horses with EMS. Proceedings of the World Equine Veterinary Association. 2023;144:321‑330.

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