I dont know what Thomas is allergic to, but it appears to be seasonal. I double checked dates, and it was the end of August/beginning of September last year, when he had a terrible outbreak of hives.
Poor biscuit. I was giving him Benadryl, but switched him to Zyrtec last night. That actually seems to be working a bit better. I really don’t want to have to give him steroids if I don’t have to, but I have a supply just in case. (And yes, I consulted his vet about all of this.)