Hi, glad you found us. One of our infirmary mods is kind of an EC expert--he has successfully treated a few bunnies with it, which not many people can say.
I would like to know why they think it is EC and not an inner ear infection. Inner ear infections are very common and cause head tilt and balance issues. One way to check is to look at the eyes while the rabbit is resting. If they are moving on a horizontal direction (toward the nose, then toward the tail), it is likely to be an inner ear infection. If they are moving up and down, it is likely to be EC-caused brain damage.
Here's some info
http://www.medirabbit.com/EN/Neurology/head_tilt.htm
http://www.medirabbit.com/EN/Neurology/otitis_interna_media.PDF
Our EC person is not happy with how the -bendazoles like panacur work in treating it. They are not very good at penetrating the blood-brain barrier, which is where most of the damage is caused by the parasite. He prefers the use of Marquis (ponazuril), which is much better. It was developed for horses to treat a similar parasite that attacks their brain.
Here are some other alternative treatments to Panacur:
http://www.medirabbit.com/EN/Neurology/cuniculi/pyrimethamine.htm
http://www.medirabbit.com/EN/Neurology/cuniculi/Starsky.htm
I don't like that your vet said to separate him and Sally. At this point, if Cookie does have EC, he's pretty far into the disease. Head tilt is not one of the first clinical signs of EC--more commonly, bunnies start having urinary tract problems, and hind leg weakness early in the disease. In addition, almost all domestic rabbits will already have been exposed to the parasite, most likely when they were still babies. Being exposed to an active infection is not likely to increase their risk of getting an active case themselves. The parasite lives dormant in the body until a stressful situation causes the immune system to stop keeping it under control, and then you get a full-blown infection. It's very important for Cookie psychologically to stay with Sally. Another of our members had a few bunnies that got EC (only one at a time), and she has them all living together (7 or so bunnies that all get along in a herd-like thing), and having the sick bunnies stay with the rest of them was very good for the sick bunnies' in terms of morale. The others did not get sick.
If our EC guy doesn't pop into this thread on his own in a few days, I will email him.
For now, I'd stay with the treatment you were given, but also look for a different vet, or present your vet with the info I gave you from the Medirabbit website. You will want to rule out an inner ear infection. If that can be ruled out, there is a blood test for EC. In addition, it would be good if you could get him on Marquis instead of Panacur if it is indeed EC.
How old is Cookie?