Colored Cowhorse Ranch

     There are a number of genetic issues that anyone breeding Paint and Quarter Horses needs to be aware of (or those that are breeding animals that could be or are descended from Paint or QH bloodlines).  All of these issues have the potential to cause foal losses or to cause health issues in a horse that could result in extensive veterinary bills or the death of the animal. 

OLWS....overo lethal white syndrome. 

This is a condition in which the intestines of the foal do not completely develope during gestation.  It either does not form a complete intestinal tract from mouth to anus or a segment of the intestine will lack the nerves necessary to move digested food/waste products through to be expelled.  It is a fatal condition.  Some foals with this are lost during pregnancy while others develop to full term but cannot process food and either die within a day or two or are humanely destroyed during that time period. 

The gene that causes this appears to be the same gene that makes the frame overo color pattern, a popular pattern in which the white markings are on the side of the horse with dark coloring down the centerline of the back, up the center of the bottom of the neck and dark legs.  The colored parts of the horse "frame" or outline the white markings.  Not all horses that have this gene show the frame overo color pattern however.  Solid horses as well as horses with other patterns can also have this gene in their makeup.  This includes QH's and TB's as well as Paints/Pintos of other patterns.

There is a test for this gene and probably most Paints should be tested even if they are tobiano, tovero or solids.  If one is considering breeding an OLWS positive horse the mate should be tested.  Breeding two positive horses will give a one in four chance of a lethal white.  And any  pair of horses that when mated produced a lethal can be assumed to both be positives.   

In general lethal white foals are born solid white and have blue eyes although there have been a few foals that have been lethals that do have small patches of colored skin.  Also...not all all-white foals are lethal whites.  Foals that are combination patterns, especially if there is a maximum expression of the sabino gene involved, will sometimes be born all white.  Only if a foal has been unable to pass the meconium (the tarry near black first stool) and is showing colic signs within 12-24 hours and comes from parents that are tested positive or are potentially OLWS positives should humane destruction be considered.  An all white or nearly all white foal that has meconium and is not showing colic signs should not be destroyed as it is unlikely to be a lethal white. 

HERDA.....Hereditary Equine Regional Dermal Asthenia (also called HC or Hyperelastosis Cutis).  This is a recessive genetic conditon tracing back to the foundation QH stallion Poco Bueno where it is thought that a mutation occured.  There are similar mutations in several other species of animals including humans in which there are a number of varieties in the way this is expressed.  In this condition the layers of connective tissue in the skin (and possibly in other structures as well) are defective and don't stay "glued" together as they should.  The skin, under stress, will split, blister, tear and shear.  It also does not heal well and produces more scar tissue than usual when it does heal. 

Since this is a recessive gene it takes two copies in a horse to make that horse show signs of this condition.  This means that the foal had to inherit one gene for this from each parent so each parent has to be a carrier.  Carriers are thought not to show symptoms so are not easily recognized.  Any horse tracing to Poco Bueno is a potential carrier unless the parent(s) that go back to Poco Bueno have already been tested and are negative.  When two carriers are bred together there is a one in four chance that the foal will not inherit this gene from either parent and will be negative for this.  There would be a one in two chance that the foal would inherit this gene from one or the other but not both parents and would be a carrier just as the parents are.  And there would be a one in four chance that the foal would inherit this gene from both parents and be an afflicted foal.  These tend to have "loose" skin, sometimes with wrinkles similar to those of a Shar Pei dog.  They may not heal well if they get minor cuts or scrapes as foals often do.  When they do heal they may have more scars that form than usual.  They may show symptoms of what looks like chronic fungal infections.  It isn't clear if these foals also have lax tendons or are more inclined to have tendon injuries where the tendons stretch more than usual.  Most often the first symptoms occur when the horse is saddled and ridden.  At that time the shearing effect of weight on the skin can cause blistering, tearing and sometimes skin peeling from the sides of the horse.  Many of these horses are destroyed before the age of 4 years.  In the past, before a clear diagnosis was possible, these horses were sometimes diagnosed as dog or cougar attacks, as fence accidents and as nutritional problems.  On occasion a mare with this condition was put into the broodmare band where her foals would all have been carriers but not shown symptoms (unless she were bred to a carrier) so no one, at that time, recognized it as a genetic condition. 

There is a test for the gene for this condition.  It is a simple hair test and any horse that traces to a known carrier or to Poco Bueno should probably be tested.  Breeding positives together has a one in four chance of an afflicted foal which, in general cannot be ridden and would probably be destroyed within a short life time.  Stallion owners can eliminate a lot of the potential problem by having any stallion tracing to Poco Bueno tested.  Mares of unknown background should probably be tested and certainly those going to Poco Bueno should be.  It is up to the mare owner to decide whether or not they wish to take the risk of a positive foal or an afflicted foal.  A stallion owner with a positive horse should consider whether or not to breed at all and whether or not to breed to positive mares.  Certainly mare and stallion owners need to discuss this. 

Scars on back of a mare with HERDA

 GBED.... glycogen branching enzyme deficiency.

PSSM...polysacharide storage myopathy

HYPP....hyperkalemic periodic paresis