Ivermectin does not have to be given topically to treat leg mites, the 1% injectable and the horse paste can be used too.For severe case - Ivermectin (pour on) - Rub onto the legs, wear gloves. Std 0.75ml = 25ml ea leg & 25ml on skin. Ivomec on daily 1 & day 10, rub some kind of oil on legs daily.
More info, including proper dosing here:
http://veterinarymedicine.dvm360.com/knemidocoptiasis-birds?id=&sk=&date=&pageID=6