So you are wise to try to intervene before it gets to that point.

I’m sure some of it was the long hair issue but when I switched their dry food to Purina One for Sensitive Systems the puking practically stopped. They must have been allergic to something in their other food.

something easily remedied.

and who once had a cat with autumn pollen allergies, I'd consider environmental possibilities besides food and litter. But, you have to start somewhere, so food and litter are good places to begin.

Is there a particular place that he likes to spend most of his time? A blanket or other fabric there that might be an irritant or allergen? Someplace that he sits a lot since it's his butt that's affected? An allergy to some kind of house mite? All homes have them, no matter how clean, and people can get allergies to them so maybe cats can, too.

Also, has the vet x-rayed the area for anything in Arthur's intestines or anus that could be causing the problem?

Poor guy. And poor you. Good luck to both of you on tracking this down.

When he took a really good, close look he said it's obviously NOT a prolapse issue, it's 'exterior' swollen tissue. The fecal sample came back clean of blood, worms or parasites. He said he is stumped but wouldn't disagree about it being an allergy. I'll keep an eye on him and document what I feed him and if there's any reaction. Arthur's behavior, litter box and eating habits haven't changed at all but I'm determined, with the help of my vet, to figure this out.

I finally managed to find some kibble that doesn't contain any sea food. A lot of dry cat food seems to be out of stock, everywhere.

Next on the list is wheat. There are some foods that have a limited number of ingredients that might be helpful. I seem to remember that soft food gives fewer problems than dry food.
Here’s hoping that you find the culprit quickly.

My rescue girl kept getting idiopathic utis. Nothing showed in urine test & he even did an ultrasound.

So I switched to Worlds Best litter & the utis stopped.

Some years later, she had another couple of utis & different vet had me add Pet Kelp to her food because of the added nutritional support & recent research showing glucosamine (1/8-1/4 teaspoon / day) is great for preventing lower itis in kitties.

Problem solved. Needless to say, she hates it. I eventually stopped adding it to her food & had forgotten about it.

Have you started growing wheat grass for them to snack on?

Maybe probiotics?

Switch to a gentle laundry detergent?

Poor Arthur. Sending good vibes to you both. 💓💓💓

I'm glad you are having him tested for allergies.