"---Factors such as prolonged wetting by rain, high humidity, high temperature,---"
I fit in the high humidity and high temperature category. My horses never had issues until we moved to Middle Tennessee.
My metabolic horse still never had issues until he became insulin resistant two years ago. It has really played havoc with his immune system.
I am experimenting with Vitamin E and won't know until July & August get here if it is truly working as that is when "everything allergy" hits my dogs and horses really hard.
It does seem to be helping a little bit in reducing the reaction to tick and midge fly bites on my Arab, but I could be Wishing On A Star and only think I am seeing something positive
Vitamin E is water soluble and easily dumps from the system so I am not out anything but money if this doesn't work. Not that I can afford to lose money but at this point, I am willing to try anything within reason that doesn't involve allergy shots