No experience with any of my hounds, but having more allergies than you can shake a stick at, I would re-examine all possibilities. Yes, something sprayed in a neighboring yard could have caused the problem. Do you take Chase for walks? Have you been to any events, M&Gs, Vet's office (immediately preceding the outbreak)? Changed washing detergents? Cleaning agents in the house? Any changes in her water? Food? Treats? Toys? Bedding? Food bowls? Shampoo? Flea treatment? Doing any remodeling in or outside of your home? Paint? Wallpaper? Trees, shrubs, flowers, ground cover? What I'm getting at is examining EVERYTHING as if she had been kept in a theoretical sterile bubble. I know this is a repeat of Claudia's suggestions..................................
I pray that you get some answers soon!