As the others have said and you have already figured out, the Corid isn't going to work.. I treated mine last year with Sulmet before I knew about Di-Methox. I recommend the Di-Methox if you can find it and try one last time with that. It should kill any Cocci if there are any left and will also help if there is an underlying infection going on. Following this treatment I would give them a probiotic. I personally use a human one called FloraGen 3 as it has Lactobacillus acidophilus, over 7.5 billion, Bifidobacterium lactis, over 6 billion and Bifidobacterium longum over 1.5 billion. This is per each capsule. I open a capsule and sprinkle it over their feed, once, maybe twice and that's it. Since there is no worry of an overdose with probiotics, using this isn't a worry. I use it on my own and have had fantastic results or wouldn't recommend it. I know, sounds like a commercial, sorry.
My biggest problem with my birds when the Marek's hit me last year was the coccidiosis and a very resistant form of it as well. I also lost lots and lots to the Marek's paralysis, but the wasting from the disease and the resistant cocci in already compromised birds was a disaster. I lost around 100 birds last year...I hear you.
For the other problems that go along with this...like the others have said, have your med kit ready at all times. I have Duramycin-10, Tylan 50 and 200, Di-Methox and Sulmet, in case the Di-Methox doesn't work. I also have poultry vitamins at all times. I Use the Durvet Poultry and Electrolytes. The salts are a little bit more than the Sav-A-Chick but if you have a sickened bird losing water through diarrhea, the extra can be the saving of them because it helps the body retain more fluids. With a respiratory infection, I do not use this. I switch to Sav-A-Chick with electrolites. You do not want the added water retention with one of them. You should also have activated charcoal just as a matter of course. It won't do a thing for the Marek's, but it will save them if they live and get into something they shouldn't later.
I am writting this on the fly, right now...if this has already been said, I'm sorry for repeating. I just quick skimmed the answers. Now I will mention this as I have no idea what your future plans are....if you are going to replace the birds lost this year, I would suggest you get cleaning now before they arrive. If you are getting them from a hatchery, make sure the hatchery uses the 3 Marek's vaccines. If you plan on getting them from breeders, I would highly recommend you buy hatching eggs instead. I would lay in a supply of Oxine as well and wash/rinse any eggs before you bring them home. hatch them and vaccinate all the chicks and the remaining birds you have by the time these new ones have hatched. I will find a link later for you to read, but there is strong evidence to show that revaccination supplies a lot better chance for your birds to live a long life. To be honest, it's a crap shoot no matter what. I just believe that giving my birds, or anyones, for that matter, a fighting chance. When the first vaccine is given between 1-3 days of age, wait 4-6 weeks and then do it again. Keep them totally away from any surviving birds for 6-8 weeks. Biosecurity is going to be your friend from now on.
One last thing...if your birds have Marek's, then you have to realize you are now a carrier of it as well. I tell people to think as if they have the disease and act accordingly. You can spread this by wearing your chore clothing out in public, by wearing shoes that have been where your birds have walked in your yard, and so on. Serious stuff and you need to be aware you can cause the deaths of other's flock of birds. Also, your birds are now sick for the rest of their lives. The are weakened and can and will catch anything you might bring home from a Fair, bird show, grain store, even a grocery store if someone else wears their chicken clothes into the store. As I said, serious stuff.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2643530/
Found it. I'm very sorry this has hit you and so hard as well. I have a mutated form of it here and it's not a lot of fun, but I do have survivors and am breeding for immunity with my survivors. It can be done and your birds that survive will give you all of the pleasures of a 'normal' flock, whatever that is and eggs and all of the rest. They will need more care and I, myself, treat with an antibiotic if I even suspect a respiratory disease. I know you've heard that you shouldn't, but you no longer have a normal flock of birds. Life after marek's is just different and you can do this.
