We were on vacation last week with a house sitter. She texted me that my 2 year old hen was limping, then another text that she was getting worse. My neighbor came over to check her out but could not find anything. We arrived home and saw that she cannot stand, She is alert. We have isolated her, have given her food and water. The other 5 hens seem healthy, but worried. Does anyone have an idea of what might be causing this?