From the description of clinical signs it appears that dehydration is responsible for the sunken eye appearance. When the eyes have this appearance dehydration is at least 10%. A 1500 lb bull that is 10% dehydrated needs 17 gallons of fluid each day to replace his fluid loss in addition to any continuing loss due to the diarrhea. This bull may need to have the fluid pumped into the rumen if he is not drinking. There are many causes of diarrhea in adult cattle. Parasites are a common cause of diarrhea. Several bacteria such as salmonella can also cause diarrhea. Johne’s disease is another disease that can cause chronic diarrhea that is unresponsive to treatment. Over eating can also cause diarrhea. If the bull doesn’t respond to your treatment it may take some additional diagnostic investigation to determine the cause of the diarrhea