HYPP

["horse Connect - Donna Paton" <donna@webexpress.net.au>]

Ellen,

I have dug out an article on HYPP.  This is a cut version of what it is says
about it.  I hope this help a little.  Another thought maybe your horse had
arthritis?

Donna P.
www.horseconnect.com.au
donna@webexpress.net.au


A horse does not catch HYPP, it is inherited along with the good
conformation and the kind temperament and the cow sense and the white sock
on the near hind foot.  The plan of how an animal will look is contained in
it's genes.

The allele for HYPP is known and it is dominant.  This means that only one
copy of the HYPP gene is inherited from one of it's parents is necessary for
a horse to be affected by the illness.  Is also known as "Impressive
Syndrome".  Genetic tests and studies of the pedigrees of affected horses
provide strong evidence that this popular and successful sire is the source
of the illness in modern Quarter Horse.  It can also be a problem in Humans.
Because there are so many steps involved in muscle control and contraction,
many possible causes had to be eliminated by researches before the culprit
could be identified.

Affected horses had no problems with neuromuscular transmission so neither
the nervous system nor the brain was at fault.  All the machinery for muscle
contraction was operating normally.  The release of calcium ions occurred at
the right time and the myofibril responded in the right way.  The active
transport of Na+ and K+ ion pumps function normally in HYPP individuals so
they knew the cause must be the other Na+/K+ system that forms channels.

Electromyographs, which record electrical discharges in muscles, showed
abnormalities in HYPP affected horses even between attacks.  Their resting
or unstimulated muscle cells are significantly depolarised compared with
muscle cells of unaffected horses.  Muscle cells of HYPP horses have a lower
concentration of K+ than usual and about four times as much Na+ as the cells
of unaffected horses.

All of this suggests that there are more positive ions than there should be
entering the muscles of HYPP animals.

During an attack the muscle membrane becomes inexecutable and cannot respond
to a message from the nerves.  The muscle becomes paralysed. The symptoms
include muscle tremors, muscle weakness and collapse.  The casual factor is
known to be an increase in the amount of potassium in the serum.  The
clinical history that leads to a paralytic episode may involve rest after
exercise, fasting or stressful events such as transport or competition.  All
of these things as well as feeding molasses or Lucerne hay which are rich in
potassium can raise the concentration of potassium sufficiently to bring on
an attack in an affected horse.

Reading:
The Impressive syndrome by Andrew Dart- The Australian Quarte Horse Magazine
July/August 42-44

Canadian Journal of Veterinary Research (Research 56: 62 - 66) - Inheritance
of Myotoinc discharges in American Quarter Horses and their relationship to
Hyperkalemic periodic paralysis

American Journal of Physiology - Joel G. Pickar