Tasmin is right too, although a newer treatment for EC is the antiparasitic (originally for horses) called Marquis. The typical intestinal worm treatment is one of the bendazole drugs, like fenbendazole (Panacur). There may not be any improvement of EC on a fenbendazole drug, because they cannot cross into the brain where the EC parasite really causes the most damage.
A course of fenbendazole would certainly not hurt. If it makes the weight loss and poop issues go away, that would be great. If not, other things need to be looked at.
An additional strategy is to use Marquis, and see if that gets rid of the intestinal symptoms AND the spinal issues. If it gets rid of the intestinal symptoms, but not the spine, you can guess that the intestinal issues were parasites and the spine is something other than EC. If it gets rid of both, that's great, you've caught EC early, just keep up the MArquis treatment to get rid of it totally. If it doesn't do anything, there's something else causing each of the issues.
Another common spinal thing that happens with bunnies and causes spinal twisting is spondylosis. This is like arthritis plus abnormal bone growth along the spine, and is pretty easy to diagnose with an x-ray. Things like joint supplements (glucosamine, chondroitin, hyaluronic acid) can be given to slow the progress of this, and long-term analgesic therapy can help with the pain.
Other options for what might be causing the GI issues have already been looked at.
An EC titer will tell you how the disease is changing over time--if you take a titer, then treat with Marquis, and take a titer again, any change in the EC titer may indicate an active disease. If it does down, the Marquis is working and Annabelle has an active case of EC. If it stays the same, you can probably assume that she doesn't have EC. The problem with the titer is that it has to be compared to values obtained from the same animal over time (at least this is how I understand it). The baseline levels of EC antigen in the blood (what the titer tests for) may vary from rabbit to rabbit, even if they're healthy. If the EC titer rises in a rabbit between two tests, you can assume that the parasite is increasing its hold on the animal. If it lowers, a treatment for EC is working. If it stays the same throughout treatment with an anti-EC drug, EC is probably not the issue.
I hope this makes sense. I think it would be good to get a titer done, treat with Marquis for at least 2 weeks, and then get another titer. This could tell you if EC is an issue, if intestinal parasites are there, etc. IE if the titer decreases following the 14 days of treatment, EC may be causing issues with Annabelle. If it doesn't change, it's probably something else. This is probably the best way to tell whether or not you bunny has EC.