I live in Utah as well, and something in the air has aggravated my allergies horribly this year. We've had odd weather the past two years, with the late thaw and flooding last year and the very dry weather (not to mention wildfires) this year. It is possible that we have an odd surge of particularly obnoxious weeds due to the weather. I am more suspicious of environmental issues than of your ponies
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Some weather websites post allergen reports. You might see if local pollen counts are high on days your allergies flare up before going to a lot of effort bathing your collection.