She is probably molting at her age. She could have Mareks disease. Botulism is another disease that causes neurological symptoms. Here is a link for each disease: http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://www.thepoultrysite.com/diseaseinfo/19/botulism
http://www.thepoultrysite.com/diseaseinfo/19/botulism