This is the best summary I found (thanks Shaka!)
"There are three forms of grass sickness, subacute, acute and chronic. Although not all horses show all the signs, the major symptom is paralysis from the horses gullet downwards. In the acute strand, symptoms are a lot mroe severe, come up without warning and the horse will often sadly die within 2 days. Muscle tremors and damp sweat patches also come up. Gut paralysis is very serious, it leads to signs of colic, like rolling, looking at flanks, kicking belly, droooling etc. Stomach liquid may start to pour from the nose that smells really rank and they can get constipated.
In subacute, symptoms are similar to acute but are less severe. Horses can have swallowing dificulty, sweating, muscle tremors, quick weight loss and have mild to medium colic. Some die and some may have to be put down within a week, but some move on to chronic.
In chronic grass sickness, the symptoms occur more slowly and only a few cases show mild, intermittent colic. There is some loss of appetite, one major symptom is rapid weight loss which can lead to the horse becoming skin and bone. Horses have been knownto recover from this. "