Page 160 - September 2018

IMPORTANCE OF GENETIC TESTING
5-Panel Test
is Useful for Quarter Horses
by Heather Smith Thomas
1 polysaccharide storage myopathy (PSSM). This muscle condition is dominantly inherited and occurs most fre- quently in heavily muscled horses such as Quarter Horses, draft horses, some warmbloods and breeds with Quarter Horse or draft horse bloodlines.
Dr. Nena Winand, retired veterinary molecular geneti- cist of Cornell University, researched the HERDA mutation and has worked on muscle disorders in horses and other species. She owns horses herself and has a personal interest in Quarter Horse genetics.
HOW COMMON ARE GENETIC DISEASES?
A research study published in 2009 utilized a genetic survey of more than 650 elite performance Quarter Horses, along with 200 control Quarter Horses and 180 control Paint Horses, to try to determine the frequency of several genetic diseases found in Quarter Horses and related breeds, and to see if some disciplines/sub-types showed more or less frequency than the breed average.
They looked for HYPP, lethal white foal syndrome (LWFS), GBED, HERDA and type 1 PSSM in horses performing in seven different competitive disciplines, including barrel racing, cutting, halter, racing, reining, western pleasure and working cow horse. These horses were genotyped for these five different disease-causing alleles or variant forms of a given gene, and the age-matched control Quarter Horses and Paints were used to establish compara- tive across-the-breed estimates of allele frequencies.
The highest frequencies among the control Quarter Horses were for type 1 PSSM and GBED. Incidence of HERDA and HYPP were less prevalent. The group of control Paint horses harbored a high level of LWFS and hadahigherprevalenceofHYPP.Inthesub-groups,halter horses had significantly greater frequencies for HYPP and PSSM. Cutting horses had highest incidence for HERDA and western pleasure horses had a high incidence for GBED, though GBED, HERDA and PSSM were found broadly throughout all subgroups. Racing and barrel horse subgroups had the lowest frequencies of all 5 disease genes.
Only a few years are needed to pass many copies ofageneto thousands of horses from a popular sire or mare.”
During the past 3 decades, research has shed light on many genetic diseases in horses, including several defects that appear in Quarter Horses and breeds
that utilize Quarter Horse bloodlines. Some of these diseases originated from mutations in horses that were ancestors of today’s popular animals and the new trait was perpetuated in numerous offspring. Mutations are com- mon in humans and animals, but generally don’t cause problems because they are greatly diluted in a large gene pool with random matings. If the genetic change occurs in a horse that has hundreds or thousands of descendants, however, this may affect many horses.
Mutations that cause concern in Quarter Horses include hyperkalemic periodic paralysis (HYPP), a dominant trait first identified in 1985 and eventually traced back to the stallion Impressive. A serious recessive trait, originally termed hyperelastosis cutis and later renamed hereditary equine regional dermal asthenia (HERDA) was documented in
the late 1960’s and finally traced back through Poco Bueno. More recently, a recessive muscle disorder called glycogen branching enzyme deficiency (GBED) was recognized as causing late-term abortions, stillbirths or weak foals that don’t survive. This trait was traced back through King.
Some disorders are due to recent mutations like HYPP, while others, like muscle diseases that cause “tying up” episodes, have existed a long time, maybe since the Middle Ages. Muscle cramping associated with exercise, or with exercise after a period of inactivity, has been recognized for more than a century. Terms like azoturia, Monday morning disease, exertional rhabdomyolysis, etc, have been used to describe this abnormality, and in recent years’ researchers have found several forms of this syndrome,withdifferentcauses.
Any horse may experience a “tying up” episode if muscle stress is severe with no underlying abnormality in muscle tis- sue or function, but some horses continually have problems because of a defect in which muscles collect an abnormal amount of sugar. The predisposing factor for this muscle malfunction is altered carbohydrate metabolism due to type
Heather Smith Thomas
158 SPEEDHORSE, September 2018
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