Information on Droopy tail, walking like a "Penguin" and panting

Hi all - I had a 15 month old hen come up with the above symptoms yesterday afternoon and I searched the forum for any possible causes. I tried the warm water for eggbinding to no avail, and she got bad enough last night that the flock rejected her. Today I took her to the vet and it turns out she had Mercks so I put her down.
This is purely for Information purposes because no one seemed to have any definitive answers. Here were her full symptoms: droopy tail, walking without good balance - which progressed through the day to having to use her wings for balance, panting during the day and then when it was cooler only panting after the labor of walking, voracious appetite, drinking a lot of water, very alert eyes and personality, comb was hot and swollen and her legs were hot and showing signs of paralysis by this morning, lots of poop, did not lay yesterday but did have an egg today.
The rest of the flock is fine so far, but good chance they will come down with it. She was the Boss hen and was up in all the business of the other hens.