HYPP in Performance Horses

Written By: Lindsay Riddell, Alamance County Livestock Advisory Committee Member

Pictured above left is the famous halter stallion Impressive, pictured right is a cute paint doing dressage.  My cute paint!  As someone who specialized in English riding disciplines at an early age, I never thought I would need to know much about a World Champion halter stallion, but then an APHA registered yearling called Peppy came into my life.  I am still learning about Impressive to this day.  Although they look nothing alike they are related through Peppy’s sire, Ima Classy Solis, who is a grandson to Impressive.  Why does this lineage matter?  Impressive carried a gene for a disease known as HYPP that he passed to all his many descendants, including my horse. 

      

Sounds like devastating news on a performance horse doesn’t it?  It’s been a journey that I would like to share with you in case you also have a horse of this lineage or know someone who does.

1.   THE DISEASE:

What exactly are we talking about?  When I bought my horse the breeders said he was tested for HYPP and the results were heterozygous (h/n).  They assured me he had never had an attack in the 18 months he’d been alive and that he probably never would.  At that time I had no idea what that meant or how it could effect his life as a performance horse.  I don’t think the breeders did either.  

This disease is an inherited defect of the muscles in affected offspring.  While it may seem unlikely that a horse suffering muscular disorders could ever be a high performance equine, it can be done.  Peppy does it every day!  

1. THE 

A.     Hyperkalemic Periodic Paralysis (HYPP) – hyper means high or in excess, kalemic refers to the amount of potassium present in the bloodstream.  Periodic paralysis is self explanatory.
                     i.            Impressive (#0767246) – AQHA Appendix stud born in 1968.  Impressive was a proven champion in the early 1970’s winning all 31 of his classes. He achieved World Champion Open Aged Halter status with 48 halter points in 1974.  With Championship status came many breeding contracts, unfortunately it wasn’t until the 1990’s that the disease and genetic link to Impressive came to light and even longer before research study results and testing were in place.  Testing was not required by AQHA until  Jan. 1^st of 2007 at which time they also enacted a rule refusing registration to homozygous horses testing h/h. The Appaloosa and Palomino horse registries put similar rules in place that year, as well.
                   ii.            The Genetics – HYPP is an autosomal dominant gene mutation, which means it is not gender linked, all offspring are affected and will pass the gene on to their offspring.  Horses of the Impressive line that do not show symptoms still carry the gene and will pass it on.  Basically, while not all offspring suffer visible illness, all offspring are “carriers”.  It is not a result of “inbreeding,” it is a mutation.  Unfortunately the progenitor of this mutation (Impressive) was not identified until after Championship status created a large generation of affected offspring.  It is estimated that Impressive has more than 100,000 descendants and the number continues to grow.  Testing is available from UC Davis Veterinary Genetics Laboratory.  These tests look for a specific gene pair that can be homozygous, heterozygous, or normal.  Results are express h/h, h/n, and n/n respectively.  Horses testing homozygous h/h are most likely to experience the severe symptoms and are at highest risk of death early in life. The heterozygous horses testing h/n are more moderate and may not suffer visible attacks at all.  Horses testing normal n/n are at no risk.  As far as breeding goes the inheritance possibilities look like this.

Breeding Outcomes
N/H x N/H have 25% chance of producing normal offspring, 50% chance of producing heterozygous carriers and 25% chance of producing homozygote offspring			N/H x N/N have 50% chance of producing normal (NN) offspring and 50% chance of producing carriers (NH).
	N	H		N	H
N	25% NN	25% NH	N	50% NN	50% NH
H	25% NH	25% HH

Diagram from www.vgl.ucdavis.edu

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