You can use it, but it's no longer approved for use on livestock, so might be better to use permethrin spray or permethrin dust. You will also need to treat your coop.
https://www.backyardchickens.com/t/1134783/permethrin-spray
You can get this a Tractor Supply:
For poultry mix 5 ml (one...
And try not to feel too bad about the bath... I only know about bathing being bad for sick birds because I lost one by doing the *exact* same thing.
-Kathy
Sorry you lost her.
Get yourself some 10% permethrin spray and treat the coop, perches, nesting boxes, and all of your birds. If it's too cold were you live, dust all of the birds with just permethrin powder instead of using the spray.
-Kathy
Yes, permethrin is what people should use. People need to stop believing that DE, wood ash, etc will treat mites and treat with something that's known to work. I know there is lots of conflicting info on the web, and often it takes lots of time to wade thru it, but do take the time and research...
DE will *not* lice or mites. Permethrin powder or spray is what should be used to treat an active infestation. Not doing so could cost a bird it's life.
-Kathy