It sounds like you’re on the right track – once worms & flies are out of the picture, itchiness comes down to nutrition, allergies & skin sensitivity, in my experience. These are long and annoying processes, though. Here’s my 2 cents:
I have 3 geldings with varying degrees of itchy butt. One has allergies, so his butt is seasonally itchy, and antihistamines help. One has a sensitive gut and gets the runs from diet changes or stress, and while we’re working on his nutrition, the butt-itch comes from having liquid manure dry on his skin. He needs washing & skin moisturizer/protector.
The last one just has sensitive skin and a perpetually dirty sheath. I use coconut oil – a big, cheap tub from Costco – on all 3 to clean, moisturize & protect skin. I wear a latex glove and melt it in my hand and apply it on their dock, butt, and for the dirty sheath guy, I use it to clean his sheath anywhere from weekly to monthly. He is so appreciative, he stands with one leg hiked up to give better access.
You mentioned that your guy has a gunky sheath and the vet doesn’t recommend more frequent cleaning. I adore my vet, but I haven’t found him particularly helpful for getting to the bottom of non-life-threatening dermatology problems. I’ve had great luck with this protocol, and my boys no longer cranky and itchy, and it’s a very simple fix.
The vets & armchair vets who insist that sheaths shouldn’t be cleaned frequently – well, I care a lot more about what my horse thinks. I see him every day. The internet, I can turn off. :lol:.
