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HCP (Friesian)

Hydrocephalus causes an excessive accumulation of fluid in the brain of foals. This usually causes the foals to be stillborn.

Full name


Breeds known to be affected

This particular mutation is found in Friesian horses.

Inheritance pattern

Autosomal recessive


Hydrocephalus is the distension of the ventricular system of the brain due to the accumulation of cerebrospinal fluid. Cerebrospinal fluid is produced in the ventricles of the brain and is then meant to flow over the surface of the brain and down the length of the spinal cord. Friesian horses affected with this form of HCP have a narrowing of the pathway that the cerebrospinal fluid normally travels through.  This causes the fluid to accumulate within and distend the skull.

HCP in Friesians is caused by a mutation in the B3GALNT2 gene and is autosomal recessive, so affected horses will have inherited the defective allele from each parent (HCP/HCP). If a horse is a carrier (n/HCP), it will not show any clinical signs of hydrocephalus. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.

There appear to be a number of different forms of hydrocephalus and this test will only identify this specific mutation, which has been documented as causing hydrocephalus in Friesian horses. The test will not identify carriers of other forms of hydrocephalus.

Interpretation of results

n/n: Horse does not carry the mutation associated with HCP.

n/HCP: Horse has one copy of the mutation associated with HCP and will pass this on to approximately half its offspring.

HCP/HCP: Horse has two copies of the mutation associated with HCP and will likely be affected.


Ducro et al. A nonsense mutation in B3GALNT2 is concordant with hydrocephalus in Friesian horses. BMC Genomics (2015) 16:761 doi: 10.1186/s12864-015-1936-z

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