Thoroughbred foal in field

Equine Familial Isolated Hypoparathyroidism (EFIH)


  • Equine familial isolated hypoparathyroidism (EFIH) is a fatal autosomal recessive inherited disease in Thoroughbred foals.
  • There is no long-term treatment or cure for EFIH.
  • A genetic variant has been identified that causes EFIH. A DNA test is available to help breeders avoid producing affected foals.

What is equine familial isolated hypoparathyroidism?

Equine familial isolated hypoparathyroidism (EFIH), previously termed idiopathic hypocalcemia, is a fatal condition in affected Thoroughbred foals up to 35 days of age. It is inherited as an autosomal recessive disease, meaning that horses with two copies of the causative genetic variant are likely to suffer from EFIH. To date, the disease has only been identified in Thoroughbreds.

What are the clinical signs of equine familial isolated hypoparathyroidism?

Foals affected with EFIH suffer from low blood calcium concentrations, resulting in involuntary muscle contractions, muscle stiffness that leads to a stiff gait and can progress to an inability to stand, seizures, fevers, and an abnormally fast pulse. Parathyroid hormone is typically produced to increase calcium levels in the body, but in EFIH-affected foals, concentrations are low or “inappropriately” normal (i.e. they should be high due to the low calcium).

How is equine familial isolated hypoparathyroidism diagnosed?

Equine familial isolated hypoparathyroidism is diagnosed based on blood work (low calcium and low or normal parathyroid hormone levels), clinical signs, and genetic testing. Necropsy results usually reveal underdeveloped or absent parathyroid glands.

How is equine familial isolated hypoparathyroidism treated?

Calcium supplementation may prevent seizures in affected foals, but there is no cure for EFIH.

What is the prognosis for equine familial isolated hypoparathyroidism?

Disease onset and progression are likely determined by the amount of calcium in the diet early in life. This can vary based on dam milk calcium concentration and the amount of milk ingested. Affected foals die or are euthanized due to poor prognosis.

How can equine familial isolated hypoparathyroidism be prevented?

Breeders and owners can utilize the DNA test for EFIH to avoid producing offspring with two copies of the causative genetic variant. Hair samples may be submitted for testing to the UC Davis Veterinary Genetics Laboratory.

For more information:

Rivas, V.N., Magdesian, K.G., Fagan, S., Slovis, N.M., Luethy, D., Javsicas, L.H., Caserto, B.G., Miller, A.D., Dahlgren, A.R., Peterson, J., Hales, E.N., Peng, S., Watson, K.D., Khokha, M.K., Finno, C.J. 2020. A nonsense variant in Rap Guanine Nucleotide Exchange Factor 5 (RAPGEF5) is associated with equine familial isolated hypoparathyroidism in Thoroughbred foals. PLoS Genetics 16(9): e1009028.

UC Davis Researchers Identify Genetic Variant for Equine Familial Isolated Hypoparathyroidism in Thoroughbreds - Testing Now Available

UC Davis Veterinary Genetics Laboratory EFIH genetic testing