Dwarfism & Hydrocephalus

As a result of inbreeding, dwarfism and hydrocephalus, are prevalent in the Friesian breed and can affect both purebreds and crossbreds. At this time, the KFPS allows horses to be carriers. Eliminating carriers from breeding would make the available gene pool even smaller, possibly creating further inbreeding issues.  


The carrier status of KFPS Approved stallions, from 2014 on, is published by the KFPS.  However, it is important to be aware that the status of KFPS stallions approved before 2014, non-approved stallions and mares is frequently unknown. Therefore, it is important to test, to prevent the occurrence of these disorders.

Dwarfism and hydrocephalus are two separate genetic issues! You could (in theory, but not recommended) breed a hydro carrier to a dwarf carrier, with no risk of the abnormalities actually occurring.

Although rare in other breeds, hydrocephalus has been reported in Standardbreds, Orlov Trotters, American Miniatures, Hanoverians and Thoroughbreds.  Thoroughbreds and American Minitures are also known to occasionally carry dwarfism.  

During the reestablishment of the Friesian breed, dwarf mares were sometimes used for breeding. This practice was occurring as late as the 1990's. Since genetic testing was not yet invented, and most of the foals were healthy and appeared perfectly normal, breeders of these foals probably did not know that the mare they were breeding was passing on dwarfism to 100% of her foals! Even with the breeding practice of using dwarf mares, and in-breeding, hydrocephalus seems more prevalent than dwarfism.

As occurrences increased with the breeding of related horses, a hypothesis was probably formed that in-breeding was not a good idea. This probably lead of the creation of the in-breeding co-efficients. While certainly helpful, in-breeding co-efficients are NOT good predictors of health issues.


Being  a carrier of dwarfism or hydrocephalus itself does not affect the health of the horse.

It is only if you breed this horse, that carrier status matters. 


If your Friesian or part-bred Friesian is a carrier, they can still be used for breeding, but to avoid the risk, the status of all mates with Friesian lineage must be known, before breeding.

If you breed to any horse with Friesian lineage, you will want to have DNA test PROOF that the partner horse is NOT a carrier also.  

If you breed two carrier parents, the foal has a 1 in 4 (25%) chance of having dwarfism or hydrocephalus.

Hydrocephalus often causes the mare to abort the foal early. If the foal is carried to a point that is closer to full term, this causes a great risk to the mare, as abnormal deliveries are common and this can result in death of the mare.


Dwarfism foals are physically affected and many have problems that can result in having to be humanely euthanized.

Keep in mind that any carrier parent will pass on the genetic mutation to approximately 50% their foals, even if the other parent is clear or regardless of the breed.


So, if you plan to breed offspring from carriers back to Friesian lines, they should tested first.

Lineage verification and application DNA tests dwarfism and hydrocephalus KFPS 11/08/2017

At the meeting of the Member Council the following was decided with regard to lineage verification and the DNA tests for dwarfism and hydrocephalus

1. All approved stallions that are used for breeding have to undergo testing for dwarfism and hydrocephalus. 

2. In addition to lineage verification, all mares that are declared Star and all mares with a foal accepted for registration also need to undergo DNA testing for hydrocephalus, dwarfism and the chestnut factor.

What is involved for the mare keeper:

Testing of the mares will begin following the Star declaration/registration of the foal. For these tests hair samples will be used that were taken at the inspection or which the KFPS has received after chipping of the registered foal.


The incurred expenses will be invoiced together with inspection fees or registration fees of the foal. Costs may vary subject to the mare´s previous history regarding lineage verification and/or carrier status for hydrocephalus, dwarfism and chestnut factor. Administration fees will not be invoiced because the KFPS highly values the implementation of these tests.

• The mare has neither been tested for lineage verification nor for carrier status of hydrocephalus, dwarfism and chestnut factor; In this case the combined DNA gene tests including lineage verification will be carried out. 

• The mare has been tested for lineage verification but nor for carrier status of hydrocephalus, dwarfism and chestnut factor; In this case only the DNA gene tests for hydrocephalus, dwarfism and chestnut factor will be carried out. 

• The mare has not been tested for lineage verification but has been tested for carrier status of hydrocephalus, dwarfism and chestnut factor; In this case only the DNA test for lineage verification will be carried out. 

Lineage verification comprises testing of the sire and in case a DNA profile of the dam is available of both dam and sire.


The registered owner will receive the results by post after a period of around five weeks.


Results of the mare´s gene tests will not be published but are available on demand for owners on MY KFPS on the website.



Pioneer Equine Hospital - Unfortunately a 15 year old thoroughbred mare aborted her 6 month old foal, note the large head. The foal has hydrocephalus a developmental disorder involving an accumulation of cerebrospinal fluid within the skull, often resulting in premature birth and or stillbirth.