AQHA Magazine July August 2019 AQHA July-Aug 2019 Final | Page 25

PG.23
ARTICLE: DR JENNIFER STEWART - EQUINE VETERINARIAN AND CONSULTANT NUTRITIONIST
PHOTOS: PROFESSOR DEREK KNOTTENBELT, LIVERPOOL UNIVERSITY IN THE UK.
www.drjenniferstewart.com
www.jenquine.com
Quarter horses it is due to an underlying muscle
disorder. Known as equine polysaccharide storage
myopathy (EPSSM or PSSM) the muscles of affected
horses store up to 4 times the normal amount of
glucose. In the majority of cases, horses should initially
be thought to have ER. But, PSSM must be considered
if over time multiple episodes of ER occur despite
appropriate management.
PSSM is due to a genetic mutation of the GYS1 or
glycogen synthase-1 gene that originated early in
the evolution of the horse. Glycogen = sugar and
synthase = synthesis i.e. the GYS-1 gene controls
sugar production, and the GYS-1 mutation results in
abnormal sugar storage in the muscle. There are 2
types of PSSM – in PSSM1 the genetic mutation causes
a build-up of abnormal sugar and in PSSM2, abnormal
sugar also builds up, but we don’t know why – i.e. the
horses don’t have the GYS1 mutation. Genetic testing
is the gold standard. Around 28% of cases that are
diagnosed with PSSM by muscle biopsy do not have
the GYS1 mutation – i.e. they are PSSM2. PSSM2 is
more common in higher performance horses, such as
barrel-racers, reining and cutting horses, compared to
the higher PSSM1 in halter horses.
Both types of PSSM horses have similar clinical signs,
typically occurring shortly after the beginning or
the end of exercise – although some horses can be
affected even when not exercised. It can show up as a
failure to lengthen when asked, reluctance to engage
the hindquarters, decreased performance or a loss of
speed. There may be slight, sometimes unnoticeable
cramping, chronic back pain, resistance to having hind
feet picked up or, in more severe cases, the horse may
be reluctant to move; the back and hindlimb muscles
may be firm and/or painful, the gait short and stiff. In
other horses the only signs may be excessive sweating,
increased heart and respiration rate, symptoms of colic
and dark brown-reddish-coloured urine.
JULY/AUGUST ISSUE 2019