Congenital Conditions

last updated 11/6/00

Let's start with a definition.  Websters defines congenital as: Existing at or dating from birth.  That's a little broad for our purposes here, so we will narrow it to include only those conditions that are life threatening, or severely handicapping, and are genetic in nature.  Horses have several conditions which fall under this category, fortunately, most of them are rare.  We will discuss HYPP, CID (sometimes calles SCID), , Hemophilia, Dominant White, Roan, Overo White, and may touch on several others that are either less common or less serious.  Some horses affected with these conditions die soon after birth, some are never born, and some require special care or treatment to live.

HYPP

This is the one that is on everyone's mind lately, if you own a Quarter Horse, Paint , or Appaloosa.  Hyperkalemic Periodic Paralysis is a flaw in a specific protein neccessary for proper muscle function.  It is called the voltage-gated sodium channel.   This causes muscular twitching or even complete muscular failure.   Symptoms include periodic attacks of muscular twitching, trembling, weakness and collapse.  The condition can be fatal if the muscles attacked are related to air flow or the heart muscle.  Hypp may be mis-diagnosed as Tying-Up Syndrome, Colic, or a seizure disorder.  Attacks tent to occur when the horse is stressed on some way, therefore, many horses have no attacks until later in life.  There is a definitave blood test to diagnose HYPP. The genetic angle comes into play with the fact that all HYPP sufferers are direct decendants of one horse, the Quarter Horse stallion, Impressive.  The HYPP gene appeared in him by a fluke of nature, called a mutation.  Genetic mutations happen frerquently, but most are either unimportant or instantly lethal to the unfortunate recipiant, preventing them from being passed on into the gene pool.  This one is not always fatal, and does not always strike before the animal is full grown, therefore it was passed along to the gene pool.  To make matters worse (from a breeders perspective) the horse with the original mutation was one of the most sought aafter and successful show horses and sire of show horses, ever to come along in the stock horse world.  In fact, in 1992, of the top 15 Halter horses, 13 were decendants of Impressive.  Thanks to the desirability of this horse to breeders, and the fact that HYPP was not identified immediately, and it took even longer to be sure that it was genetic,and did in fact trace to Impressive, this gene is now widespread within the stock horse industry. HYPP is a dominant genetic trait, therefore, if it is present it will be expressed.  This makes eradication easier but the condition more widespread.  Impressive himself has only one copy of the defective gene, therefore, about half of his offspring are normal and unable to pass along HYPP.  Unfortunately, the gene seems to be linked to genes that cause the large muscles so desired in stock horses, therefore, his most successful offspring are also most likely to have HYPP.  With the blood test on the market, breeders are now advertising their impressive bred stock as HYPP N/N.  These horses are genetically normal and cannot pass on HYPP.  Those whose papers say H//N carry one copy, and about half of their foals will suffer from HYPP.  Those whose papers are marked H/H have two copies of the HYPP gene and are certain to pass in on to any offspring.  These horses also tend to suffer the condition most severely.  This gene can be erradicated totaly from the gene pool by testing all decendants of Impressive, and only breeding those who are N/N.  Whether or not this will happen depends on the integrity and persistance of those who breed stock type horses.

SCID

CID is a condition that exists in the Arabian breed, half Arabs, and several of the breeds which have been created from the Arabian in modern times such as the National Show Horse and the Pintabian.  Combined Immunodeficiency (or Severe Combined Immunodeficiency)  has been around for a number of years, first identified in 1973,  and has spread throughout the gene pool.  This condition is caused by the lack of functional T- and B- lymphocytes, types of white blood cells, which combat infection.  Foals affected seem normal at birth, and may live several months, until the passive immunity, afforded by colostrum, wears off.  Any time after that, an infection can set in.  Antibiotics sometimes work for a while, but quickly lose their effectiveness.  The foal can literally die of the sniffles!  Vaccination is useless for CID foals as they have no ability to create antibodies in response to the vaccine.  Foals are succeptable to disease anyway, and have fewer reserves than a mature horse, so not every foal that succumbs to infection is a CID foal.  This is part of what made the condition difficult to identify. Genetically, CID is a recessive trait.  This means that there are three possible conditions for a horse.  N/N is the normal condition.  These horses have no ability to pass on the defective gene.  N/C is the carrier stage.  These are the horses who pass on the condition, however, since it is recessive, affected foals only appear when a N/C horse is bred to another N/C horse.  In that case, 25% of the foals will be normal, and unable to pass on the gene.  50% will be carriers, like the parents, not suffering from CID, but able to pass it on to their offspring.  The final 25% are the foals which are C/C.  These foals are the affected ones, who do not reproduce as they do not live long enough.  About 3% of the Arabian foals born in a given year are thought to die of CID.  Recently, a test has been developed to identify the presence or absence of the CID gene in the innapparent carriers.   Previously, the only way to identify a carrier was after it had produced a CID foal.  Since the horse may have produced some, or even many foals carrying the CID gene prior to the affected foal's birth, the gene continuse to be spread.  it is even possible that CID exists, at a very low level, in other, older, breeds based on the Arabian.  The test is still fairly expensive, and it remains to be seen how many Arabian breeders wil go to the expense of testing their horses, and how many will stop breeding carriers as opposed to beeding their carriers to known non-affected horses.  This latter course, while effective in the short term (no CID foals will be born) allows the gene to remain active in the breed, allowig for the possibility of CID foals in the future when these foals are bred.  Again, the solution is testing, and the integrity of the breeders in removing known carriers from reproduction.

Dominant White (W) & Roan

The Dominant White genetic lethal was first recognized in Denmark at the Royal Fredricksborg Stud.  Iw was originally called the Fredricksborg lethal.  It is caused by a dominant gene.  When a horse inherits one copy of the gene, it is White in color with dark, or possibly blue or hazel eyes.  They may also be a light cream color.  If the horse inherits no white genes, it is colored normally.  If it inherits both white genes, it is aborted early in pregnancy.  These foals are never born.  The presence of the lethal was inferred by conception rates as well as ratios of white to colored foals born. Roan is also a dominant genetic lethal.  As it is considered a desirable trait by Quarter Horse breeders, the management of this gene is important.  The mechanism is the same as the White gene.  Foals recieving two copies of the roan gene are aborted early in pregnancy, and  are never seen by the breeder.  These lethals show up in reduced production.  The best way to safely produce roans is to  breed roans to solids, which will produce approximately 50% roan and 50% solid colored foal, but removes the issue of the early aborted  roan lethal foals.

Hemophilia

This disease exhibits itself by preventing the blood from clotting normally.   It is not instantly lethal, but eventually the horse will suffer an injury severe enough that it will bleed to death.  It is extremely rare in horses.  Hemophilia is a sex linked recessive trait, making it interesting from a genetic perspective.  The unique thing aboaut sex linked recessives is their position on the chromosome.   These genes are found only on the X chromosome, on the leg that has no analagous position on the Y chromosome.  Therefore, for a mare to exhibit the trait, she must inherit two copies of the gene,  wheras a stallion woud only need to inherit one copy, which he would get from his dam, in order to exhibit the trait.  Since very few Hemophiliac horses survive to breeding age, it is almost impossible to find a hemophiliac mare.  A mare who carries the abnormal gene will pass it on to half of her offspring.  The fillies who inherit the disease will become carriers, while the colts will suffer the disease itself.

Overo White

The Overo White Lethal is as yet poorly understood from a genetic perspective.  We know that it occurs in Frame Overos, but are uncertain if it appears in other forms of overo.   It has also shown up very rarely in other colors of horses.   In it's common form, Letha White appears when two overos are bred together.  A foal will be born, apparently normal, but all white with blue eyes.  These foals have incomplete digestive tracts, and therefore will soon colic and die within a day or two.  It is assumed to be a recessive, but may be a more complex relationship between two or more genes.  Some suspect it to be a dominant like the roan, but rates of occurrance do not seem to bear this out.

Information for this page came primarily from  The Horse by Evans,  but much of the information on HYPP was researched on the internet.  Updates will be added if new information presents itself.  If you have any data, or photos, or if you have any suggestions for future topics, you can reach me by clicking on the mailbird.