AQHA Magazine AQHA July-Aug 2023 | Page 15

HYPP : Hyperkalemic periodic paralysis ( HYPP ) is characterized by sporadic attacks of muscle tremors ( shaking or trembling ), weakness , and / or collapse . Attacks can also be accompanied by loud breathing noises resulting from paralysis of the muscles of the upper airway . Occasionally , sudden death can occur following a severe paralytic attack , presumably from heart failure or respiratory muscle paralysis .

Manifestation of clinical signs of HYPP depends on many factors including stress , diet , and changes in exercise . Some horses may manifest severe signs of the disease while others exhibit little or no signs .

• Horses with N / N genotype will not have hyperkalemic periodic paralysis and cannot transmit this hyperkalemic periodic paralysis variant to their offspring .

• Horses with N / H genotype can display episodes of hyperkalemic periodic paralysis and are Carriers . They may transmit this hyperkalemic periodic paralysis variant to 50 % of their offspring .

• Horses with H / H genotype can display episodes of hyperkalemic periodic paralysis and typically are more severely affected . They will transmit this hyperkalemic periodic paralysis variant to all of their offspring .

• HYPP carriers are not registerable with the AQHA unless spayed or gelded .

PSSM1 : Horses with Type 1 Polysaccharide Storage Myopathy ( PSSM1 ) have a muscle disease characterized by accumulation of abnormal complex sugars ( glycogen ) in skeletal muscles . The accumulation of abnormal sugars can cause breakdown of muscle fibers ( rhabdomyolosis ) which leads to muscle pain , weakness , skin twitching , sweating , and reluctance to move .

• Horses with N / N genotype will not have type 1 Polysaccharide Storage Myopathy and cannot transmit the PSSM1 variant to their offspring .

• Horses with N / PSSM1 genotype will have the PSSM1 variant and may show signs of type 1 disease . Horses with this genotype may transmit the PSSM1 variant to 50 % of their offspring .

• Horses with PSSM1 / PSSM1 genotype are homozygous for the PSSM1 variant and may be more severely affected than N / PSSM1 horses . Horses with this genotype will transmit the PSSM1 variant to all of their offspring .

MH : Malignant hyperthermia ( MH ) is an inherited disease in which affected horses can be triggered by halogenated

anesthetics , succinylcholine , stress , or excitement , which can induce a hyper-metabolic state characterized by symptoms including muscle contracture , elevated temperature , and an irregular heart rhythm .

• Horses with N / N genotype will not have malignant hyperthermia and cannot transmit this malignant hyperthermia variant to their offspring .

• Horses with N / MH genotype will be affected by malignant hyperthermia . They can transmit this malignant hyperthermia variant to their offspring . Matings with N / N genotype will result in a 50 % chance of producing a malignant hyperthermia-affected foal .

• Horses with MH / MH genotype will have malignant hyperthermia .

GBED : Glycogen branching enzyme deficiency ( GBED ) is a fatal genetic disorder that results from the inability to correctly store glycogen in several organs of the body .

• Horses with N / N genotype will not have glycogen branching enzyme deficiency and cannot transmit this glycogen branching enzyme deficiency variant to their offspring .

• Matings between two carriers result in a 25 % chance of producing a

GBED-affected foal .

• Horses with GBED / GBED genotype will have glycogen branching enzyme deficiency , a fatal condition .

MYHM : Myosin-heavy chain myopathy ( MYHM ) is a muscle disease that results in two distinct clinical disease presentations , immune-mediated myositis ( IMM ) and nonexertional rhabdomyolysis . Both presentations involve muscle loss or damage and are linked to the same genetic variant .

• Horses with N / N genotype will not have increased susceptibility for a myosin-heavy chain myopathy and cannot transmit the MYHM variant to their offspring .

• Horses with N / My genotype may develop a myosin heavy chain myopathy . They may transmit this MYHM variant to 50 % of their offspring . Matings to horses with the N / N genotype will result in a 50 % chance of producing a foal that may develop myosin-heavy chain myopathy .

• Horses with My / My genotype may develop a more severe form of a myosin-heavy chain myopathy . They will transmit this MYHM variant to all of their offspring .

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INFORMATION : https :// vgl . ucdavis . edu / then SEARCH and / or : CONTACT THE REGISTRAR : qhorse @ aqha . com . au • P : 02 6762 6444

AQHA JULY / AUGUST ISSUE 2023