Hello everyone! We purchased 7 hens for our granddaughter about a month ago. Two are already laying & the other 5 are about 6 months old. All was well til' one of the laying hens began stretching her neck out & making a wheezing noise. Feed store gave us duramyacin (sp.?) & said to give it to all of them for 7 days in their water The sick hen died by morning. It's been 4 days & all the others appear to be fine. We are so attached to all of them & don't wanna lose anymore. Any preventative measure suggestions?