Page 161 - September 2018

Hereditary equine regional dermal asthenia carrier (HERDA), an inherited autosomal recessive connective tissue disorder which weakens collagen fibers that allow the skin to stay connected to the rest of the horse, was traced to the stallion Poco Bueno.
didn’t have the computer capacity we have today. We asked the IT people to get us the data on how many living Quarter Horses traced back to those sires. We discovered that almost every living Quarter Horse at that time traced back to King, and 2/3 of them traced back at least once to Poco Bueno. Thus, you can’t just look at a pedigree today and think
you are safe. GBED is common, and we’re not sure what bloodlines it will be found in. You just have
to test for it. HERDA is a little different because we know how it is transmitted, and generally a minimal problem for speed horses. In my opinion MH and PSSM are the crucial ones, and maybe GBED, for speed horses,” Winand says.
WHY WE SHOULD TEST
Dr. Stephanie J. Valberg, Michigan State University, says it is important to do genetic test- ing for many reasons, including making breeding decisions, checking the status of an animal you are purchasing for breeding, and making an accurate diagnosis in a horse with signs of muscle disease. “When contemplating breeding certain animals, we want to know whether or not they are likely to pass on a genetic trait,” she says. “With AQHA now having a 5-panel test, which tests for HYPP, GBED, HERDA, LWFS and PSSM, it’s possible to get a relatively economical overview
of what your horse’s genetic status is on those diseases.” Owners can then either select mates to breed away from a trait or decide it is not worth the risk of passing on the genetic condition.
It is important to understand dominant and recessive traits, because this will influence breeding decisions. “If your horse has a recessive trait, you’d want to breed to a horse that does not carry that trait to avoid any chance of having an affected foal. If your horse has a dominant trait, however, and is heterozygous (one normal gene, along with the gene for the genetic disease), no matter what horse you breed to there is a 50% chance that the offspring will be affected,” she says.
This could lead to the heartbreak of producing an affected foal. “If you have an affected foal and are trying to sell it later, this will affect its value if genetic testing is done as part of a pre-purchase exam. If you are purchasing a horse and wonder if it might have an underlying genetic disorder, it’s wise to find out, since this may affect the horse’s ability to perform. If you have done genetic testing on animals you are selling, you have a duty to disclose the results of those tests,” says Valberg.
The 5-panel test makes testing more affordable. “When you do AQHA’s 5-panel test, that informa- tion becomes available with the horse’s registra- tion. If you use a private lab for genetic testing, results are confidential to you and not linked to the horse’s registration,” she says. This could be an issue later if the horse is sold and the owner does not disclose test results to the new owner.
Breeding horses is a huge financial investment. “You want to minimize chances for financial loss,”
Two measures are commonly used in deter- mining frequency and characteristics of a genetic disease. Allele frequency is the broad category that covers both dominant and recessive diseases and is therefore an estimate of the percentage of disease- causing alleles of a gene among all the alleles in the horse population. Carrier frequency describes the percentage of the population having a single copy of a disease-causing allele that does not cause a disease phenotype (recessive diseases such as LWFS, GBED, and HERDA). These carriers are important because they have the potential to create diseased animals when bred to other carriers, even though they do not show the disease themselves.
Breeding two carriers will result in affected offspring 25% of the time and carrier offspring 50% of the time, with only 25% chance of a non-carrier). For dominant diseases like HYPP and PSSM, a single allele is enough to cause the disease. Breeding an affected animal to a non-affected animal will result in a 50% chance of creating a normal animal and 50% chance of an affected animal.
In the past, many Quarter Horse and Paint breeders did not use DNA testing unless there was a known carrier related to the horses used for breeding, yet nearly all sub-groups carry these dis- ease alleles. Use of AI, embryo transfer and super- ovulation and cloning can speed up appearance of genetic diseases. Only a few years are needed to pass many copies of a gene to thousands of horses from a popular sire or mare. The need for genetic testing has increased dramatically.
“This research paper serves as a reference for occurrence of various genetic diseases in different performance disciplines,” says Winand. “Even though racing and barrel racing horses seem to
have the lowest incidence, there are still problems. Owners of these horses might think there is low incidence, but it is significant in terms of malig- nant hyperthermia (MH), a serious genetic disease which is not included in the panel test now offered by the AQHA and PSSM—especially the type of PSSM that we still don’t know the cause of. There is a test for type 1 PSSM, but there are at least two other types of PSSM in Quarter Horses, Paints, and other breeds.”
“Speed horses like racing and barrel horses often have issues with muscle diseases, tying up, etc., and owners need to be aware of this,” she says. “We
may not have all the necessary tests yet, but if I were breeding for speed horses I would definitely test for PSSM, MH, and GBED since these genetic diseases can occur throughout the population. I wouldn’t worry so much about HYPP and HERDA because these are associated with specific bloodlines and you could know what to test for and what not to worry about if you know your bloodlines.”
“You have to look back in the pedigree a long way to be sure, but we understand where these problems came from. By contrast, PSSM and GBED are much more widespread, and go back a very long way. Originally it was thought that GBED came from a mutation passed on by King P234, but essentially all modern Quarter Horses trace back to King at least once, so it could have come from farther back,” she explains.
In some of her own research, she worked with AQHA to look at the impact a specific sire could have on the breed. “We pulled sire records from Little Peppy, King, Poco Bueno, Doc Bar, and other horses perceived to have had big influences on the breed. It wasn’t easy at the time (2004) because we
SPEEDHORSE, September 2018 159
EQUINE HEALTH