1. Equine Genetic Disorders
By Elli Hayes

2. The Horse’s Karyotype
Notice the
absence of a Y-
chromosome.
It is a female
horse.

3. Intro:
• The horse has 32 pairs of chromosomes as compared
to our 23
• Some of the chromosomes correspond to ours or have
similar functions (ex: the horse’s 27th chromosome is
equivalent to our 21st)
• Some disorders are fatal due to complications with
internal organs and/or their functions, while others
don’t have much effect of the horse’s usefulness
• Since the horse’s genome hasn’t been extensively
researched and explored, many generic disorders
haven’t been pinned to a specific chromosome yet.

4. Dominant Trait Example

5. Lethal Trait Example:

6. Recessive Trait Example:

7. Table of Contents:
Lethal Non-lethal
Overo Lethal White Syndrome Equine Juvenile Epilepsy
Guttural Pouch Tympany
Glycogen Branching Enzyme Deficiency
Hairless Foal Syndrome
Lavender Foal Syndrome
Equine Dwarfism
Severe Combined Immunodeficiency
Disorder Trisomy 27
Hereditary Equine Regional Dermal Asthenia
Junctional Epidermolysis Bullosa
Hyperkalemic Periodic Paralysis
Degenerative Suspensory Ligament Desmitis
Works Cited

8. Overo Lethal White Syndrome
• Lethal white overo (LWO), Overo lethal white syndrome (OLWS)
• Caused by an autosomal recessive gene, this disorder is fatal
100% of the time. It causes a defective large intestine, which is
narrow in one part causing a blockage. There is no treatment.
OLWS foals are either euthanized or die of colic in the first few
days of life, they cannot pass stool.
• All breeds with white and patterned coats are at risk including
thoroughbreds, quarter horses, and, predominantly, the paint
horse breed. The disorder can only be prevented through careful
and responsible breeding.
• All horses in breeding programs that are suspected of being
carriers should be tested for the gene. Carrier horses usually
have a certain coat pattern called the frame, which is a sought
after color.
more info back to table of contents

9. More Info
• Although a horse may
have the frame
pattern, he may not carry
the OLWS gene
• Not all white foals born
are OLWS foals, some just
have a white coat
• Many carrier horse are
A horse with the frame pattern. He is a
bred either intentionally
carrier of the lethal white gene. or accidentally because
the frame coat is a very
popular coat
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10. Glycogen Branching Enzyme Deficiency
• Glycogen Branching Enzyme Deficiency (GBED)
• This disorder is caused by a recessive, mutated gene. Foals
affected with the disorder are missing the enzyme that
turns glucose into glycogen, the fuel for the muscles, heart
and brain. Therefore it is fatal; no fuel, no foal. Many of the
affected foals are stillborn or miscarried. Some are born
alive, but none have survived past 3 months. There is no
treatment.
• It is present mostly in quarter horse and paint horse
breeds. The gene is present in approximately 10% of all
quarter horses. The only prevention is carefully planned
and responsible breeding. Have all horses that are being
bred tested for the gene.
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11. Lavender Foal Syndrome
• Coat Color Dilution Lethal (CCDL), Lavender Foal Syndrome (LFS)
• This disorder is most commonly found in Egyptian Arabians. It is
present in approximately 10% of all Egyptian Arabians, although
it has been reported in other breeds. The foals generally have a
complicated birth, and cannot stand. Their coat is usually pale,
and can look pink, silver, or lavender (hence the name). It is
thought to be caused by a brain lesion.
• Currently, the inheritance pattern has not been established, but
research points towards the condition being autosomal
recessive. There is no treatment, all foals die within a few days.
Prevention consists of careful breeding and testing for the gene
and carrier horses.
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12. Severe Combined Immunodeficiency
Disorder
• Severe Combined Immunodeficiency Disorder (SCID)
• This disorder causes foals to be born without an
immune system and without the means to build up
one. Affected foals will die within 4 to 6 months
without any intervention. Few cases have been
corrected with bone marrow transplants, however this
is expensive and not always effective. For most
affected foals, the condition is fatal.
• It is an autosomal recessive disorder found mostly in
Arabian horses and related breeds. Between 15 and
28% of Arabian horses are carriers.
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13. Junctional Epidermolysis Bullosa
• Junctional Epidermolysis Bullosa (JEB)
• Caused by a recessive mutation, this disorder results in
the absence of a protein that keeps the hoof wall and
skin structure intact.
• It usually occurs in Belgian horses and other draft breeds.
Foals will die within 7-14 days because of infection in
sores on the skin from pressure points or because of the
disconnection of the hoof wall.
• Unfortunately, there is no cure. Prevention includes:
having draft horses in the breeding stock tested, and not
breeding two carrier horses together.
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14. Equine Juvenile Epilepsy
• Equine Juvenile Epilepsy (EJE), Juvenile Epilepsy Syndrome
(JES), Juvenile Idiopathic Epilepsy (JIE)
• This disorder causes seizures in young horses and is usually
outgrown by 18 months of age. It is unclear what causes it, and
how it is inherited. EJE is common in Arabian horses and related
breeds.
• Affected foals must be kept in safe enclosures to prevent harm if
the foal were to have a seizure and fall to the ground. They are
also given anti-seizure medication. It is usually not lethal unless
the foal is seriously injured during a seizure.
• There are no tests for the disorder, so therefore no means of
prevention. EJE horses are perfectly normal once they mature.
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15. Guttural Pouch Tympany
• Guttural Pouch Tympany (GPT)
• Thought to be in a polygenetic gene, this disorder causes the
Eustachian tube (entrance to the windpipe) to act as a one way
passage. As a result, air builds up behind the jawbone making it
difficult for the horse to breathe. This can be fixed temporarily by
the insertion of a catheter to allow air to escape. If this does not
fix the problem, surgery is required, however the outlook is good.
• GPT usually shows up in horses from six months to a year and a
half, but it can come up at any point in life. Females are about four
times as likely to inherit the disorder than males. Many breeds are
affected including (but not limited to) Arabians, warmbloods, and
Quarter Horses. Since the cause of the disorder and specific genes
affected are unknown, there isn’t much prevention other than not
breeding affected horses.
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16. Hairless Foal Syndrome
• Hairless Foal Syndrome (HFS), Naked Foal Syndrome (NFS)
• HFS causes minimal keratin production resulting in a foal born
with no hair. It is passed recessively. It is a rare occurrence for all
breeds, but shows up more often in Ahal-Teke horses. Affected
foals are born without any or little hair. Hoof and digestive
problems are common in these horses. In Ahal-Teke horses, it is
lethal because of the severity of the inhibited keratin production.
• Affected horses of this breed and other light breeds usually die in
the first two years of life because of laminitis (non hereditary
condition of the hooves) or digestive problems. Draft horses with
the disorder usually can make it to their late teens because they
tend to have stronger hooves and are more hardy. These horses
must be properly cared for: blanketed, turned out only at
night, skin moisturizer, balanced diet, etc.
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17. Equine Dwarfism
• This disorder causes foals to be born extremely small in stature and
generally out of proportion. There are four known types or variations of
the gene
– Type 1: Diastrophia: these foals have twisted legs and other extremities. It is
the least severe form of equine dwarfism
– Type 2: Achondroplasia: this form produces foals with short legs, necks, etc in
comparison to their bodies
– Type 3: Brachiocephalia: The foals affected have very compact bodies with
short necks, large heads, and pot bellies.
– Type 4: Hypochondrogenesis: These foals are underdeveloped and not carried
to term. They are very small and have extreme deformities.
• All types are recessively inherited. The disorder is most commonly found
in miniature horses (proportional to normal horses), however type 1 has
been found in Fresians.
• Dwarfs foals with leg deformities can sometimes be casted to encourage
normal leg growth. Braces and sometimes even surgery can correct more
serious problems.
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18. Trisomy 27
• This disorder is caused by nondisjunction of the 27th
chromosome. Any horse can potentially be affected. Not
many cases have occurred on record. One of them, a
Standardbred colt. He was born with an abnormal
personality, and was slow in picking up on social cues from
other horses and seemed to violate the others’ “personal
space”.
• Many abnormalities in this colt were similar to those seen
in children with Down syndrome (Trisomy 21). Other
trisomies have occurred in horses including:
23, 26, 27, 28, 30, and 31. Although the colt was euthanized
at 24 months, he showed no physical signs that he could
not have led a somewhat normal life and matured into an
adult.
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19. Hereditary Equine Regional Dermal
Asthenia
• Hereditary Equine Regional Dermal Asthenia (HERDA),
Hyperelastosis Cutis (HC)
• This connective tissue disorder causes skin in certain areas to
be extremely thin and fragile making them prime candidates
for infection and slow healing. Cornell and Mississippi State
are working to find the affected gene and research how the
gene is expressed.
• Mild cases managed well on special diets and restricted
turnout can sometimes be ridden with the right equipment.
However this isn’t the case for most affected horses who are
either euthanized or donated for research by age 4. Many
carriers and affected horses are inbred and can be traced back
to one quarter horse sire, Poco Bueno. Paint and Appaloosa
horses are also affected by the disorder. The homozygous
recessive disorder can be directly tested for and carriers
should not be bred together.
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20. Hyperkalemic Periodic Paralysis
• Hyperkalemic Periodic Paralysis (HYPP)
• An autosomal dominant muscle disorder, HYPP causes sudden,
unexplained muscle twitches, collapse, and paralysis mainly in the Quarter
Horse breed. The disease can be traced back to Impressive, a halter horse
stud from the 70’s and 80’s. Heterozygous individuals are less likely to
have attacks than homozygous HYPP horses, but it is still possible. The
muscles of HYPP affected horses take less influence to contract, making
them prone to collapse and/or seize randomly. These horses can die from
an attack because the pharynx and larynx collapse or because the heart
develops arrhythmia that cannot be stopped. Any kind of stress or heavy
sedation can trigger an episode on affected horses. Blood or hair samples
can be tested for the gene. The American Quarter Horse Association
(AQHA) has taken active measures to prevent against the disorder. As of
January 1st, 2007, they require that all foals in the bloodline of Impressive
be tested for HYPP and any foal of any bloodline that tests homozygous
positive cannot be registered.
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21. Degenerative Suspensory Ligament
Desmitis
• Degenerative Suspensory Ligament Desmitis (DSLD), Equine
Systemic Proteoglycan Accumulation (ESPA)
• The condition causes the suspensory ligament to loose the
ability to support the horse’s weight. More advanced cases
show the pastern dropping towards the ground. In later
stages, it can be very painful for the horse. If caught early
on, the progression can be slowed with corrective shoeing
and diet adjustments varying case to case.
• It has appeared in the Quarter Horse, Saddlebred, Arabian,
Paso Fino, and Thoroughbred. There has not been extensive
research on the inheritance and prevention of the disorder,
therefore the mode of inheritance and preventative
measures are still widely unknown.
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22. back to table of contents
Works Cited:
"Advanced Topics." The Equine Tapestry. 3 Nov. 2011. Web. Feb. 2012. <http://equinetapestry.com/category/advanced-topics/>.
"DSLD." Horseshoes.com. Web. 27 Feb. 2012. <http://www.horseshoes.com/anatomy/esad/information/dsld/dsld.htm>.
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<http://www.thehorse.com/ViewArticle.aspx?ID=6169>.
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"Genetics and Genetic Disorders." Welcome to ArabianHorses.org. Arabian Horse Association. Web. 27 Feb. 2012.
<http://www.arabianhorses.org/education/genetic/default.asp>.
"Guttural Pouch TympanyBy: Dr. Melissa Mazan." Guttural Pouch Tympany. Web. 27 Feb. 2012.
<http://www.petplace.com/horses/guttural-pouch-tympany/page1.aspx>.
"Horse Forum." HORSE FORUM. Copyright 2008 - Horse Grooming Supplies, 2008. Web. 2011.
<http://www.horsegroomingsupplies.com/horse-forums/breeders-guide-to-equine-genetic-disorders- 367267.html>.
"Juvenille Epilesy." Genetic Disorders. Arabian Horse Association. Web. Feb. 2012.
<http://www.arabianhorses.org/education/genetic/docs/Juvenile_Epilepsy.pdf>.
"Lethal Chart." Lethal Chart. Photobucket. Web. Feb. 2012.
<http://i872.photobucket.com/albums/ab283/talkingmongo0se/Lethal-Breeding-Chart.png lethal>.
"Simple Dominant Breeding Chart." Simple Dominant Breeding Chart. Web. Feb. 2012.
<http://i872.photobucket.com/albums/ab283/talkingmongo0se/Simple-Dominant-Breeding-Chart.png>.
"Simple Recessive Breeding Chart." Simple Recessive Breeding Chart. Photobucket. Web. Feb. 2012.
<http://i872.photobucket.com/albums/ab283/talkingmongo0se/Simple-Recessive-Breeding-Chart.png recessive>.
Thomas, Heather S. "Genetic Diseases in Quarter Horses and Related Breeds." The Equine Chronicle. Heather Smith Thomas, July-Aug. 2009.
Web. Feb. 2012. <http://www.equinechronicle.com/health/genetic-diseases-in-quarter-horses-and-related-breeds.html>.