I can tell you for a fact that N/H horses can die from an attack. We have had friends that had horses die when they had their very first attack.
Alfalfa is a HUGE no-no for HYPP horses. It is one of the worst triggers. Molasses and sweat feed are major causes. Corn syrup would scare me.
Here is a excerpt from the UC DAVIS Vet School website.
Causes of an Attack: |
Environmental factors can actually cause an attack of muscle weakness. Owners of HYPP-positive horses should be aware that external stimulus and events could increase the chance of paralysis onset. These factors include dietary changes, fasting, general anesthesia, and concurrent illness and exercise restriction.
Prevention and Control of HYPP Attacks:
Dietary management is extremely important in the management of affected horses. Dietary adjustments include (1) avoiding high potassium feeds such as alfalfa hay, brome hay, canola oil, soybean meal or oil, and sugar molasses and beet molasses, and replacing them with timothy or Bermuda grass hay, grains such as oats, corn, wheat and barley, and beet pulp; (2) feeding several times a day; and (3) exercising regularly and/or being allowed frequent access to a large paddock or yard. Due to the high water content of pasture grass, a horse is unlikely to consume large amounts of potassium in a short period of time if kept on pasture. If the horse is experiencing problems on its present diet, it is recommended to feed a diet containing between 0.6% and 1.5% total potassium concentrations.
I hope this helps.