You can give 250 mg (1/2 of a 500 mg tablet) orally daily for 5 days, according to Plumbs Veterinary Drug Book.
I have a closed flock as well, and one of my 7 year old hens has become the first I have had to show Mareks signs. It is still unkown, but she even had the vaccine as a chick.
I have a closed flock as well, and one of my 7 year old hens has become the first I have had to show Mareks signs. It is still unkown, but she even had the vaccine as a chick.