Cellulitis (blisters) all over my hen's body !

[Bruwen chicks]

Children's liquid Benadryl is safe for use in chickens once in 24 hours. Regular adult Benadryl or dosing more than once or overdosing can kill a chicken. However it doesnt have the same effect in birds as it does with mammals. In chickens, it's good for keeping the trachea open for breathing purposes if bitten by a poisonous insect like a bee, spider, or even a scorpion or snake.

I was thinking possibly fire ants, but the hen would be dead and covered with them. I also thought it might be feather follicle mites, but other birds would be infected with them as well.
I'm not sure what it is but leaning towards Marek's disease.

Thank you very much for this information, i really appreciate it.