jaundice isn't a disease in and of itself, it's a symptom. Jaundice means the yellow coloring to the eyes, skin and gums that is seen when the liver is not functioning properly for some reason. This could be caused by eating a toxic plant or substance, a disease that damages the liver, etc.
You need to examine your hay and pasture for any plants that are unsafe for horses, check your feed for signs of mold, make sure that what you are feeding is appropriate for horses and make a note of any and all medications given to your horse recently and how much was given.
Then wait for your vet to get the lab results back.
Licensed Veterinary Technician