What to do with a trained horse. Mentally fit or blown. Fine line.

This horse has hives caused from allergic reaction to a new fly spray Read to end for cheap homemade fly spray The young client of mine who takes care of this horse was in a panic and contacted me. I have seen this several times from using new fly spray. She did get vet advice to hose off and treat. Scheduling vet to see in am, Hives were almost gone in morning. What I do when trying a new fly spray is always spray in a small area and then see if they have a reaction in 24 hours. If no reaction