Relentless Hives

[Upcountryday]

Four out of the last five years my 8 yo cremellow gelding has had continual hives, usually from April to October. We've done all the vet rountines, and his allergy is apparently "summer airborn allergens". He takes TriHist daily. This has been his worst year ever, advancing to dripping welts & much edema.
Last year however was a good year, no hives. The only difference was that he was getting a small flake of alfalfa daily as well as his usual grass hay. He unfortunately, being the delicate flower that he is, was unable to handle the rich green hay, and started having chronic tummyaches. Question - I don't know what the chemical breakdown is for different hays/legumes, but could the alfalfa contain something that prevented hives or perhaps boosted his system ? What ?? Now feeding grass/forage mix hay, and Equinesaver vitamins mixed w/meds in 1/2 lb grass hay complete feed. My steed is n easy keeping Peruvian Paso. Your help/ideas appreciated ! Day

[EquineOnly]

It's unlikely that the hay/alfalfa contained anything that prevented or cured the hives.

Hives are an acute allergic reaction triggered by agents that are inhaled, consumed, injected or come in contact with the skin. Instigators include but are not limited to vaccines, systemic or topical drugs, plants, topical sprays, pollen, or insect bites or stings. In many cases, the cause of a hives episode is never determined.

So it's more likely that you changed something else in his routine that kept him away from whatever he is/was allergic to.