Cleaning cat’s messy rear

Common complaint especially among humans who live with long-haired cats.

The rear becomes messy.

Helpful video showing a veterinarian shaving that area.

Other suggestions.

1. If poop is soft, it’s more likely to get caught in fur. Some cat foods result in softer poop.  If that’s the situation, try different brands until you find one that results in reasonably firm poop. That won’t help you right now, but it will help in the future. 
2. Cats benefit from litter boxes in which they can easily turn around.  Double check to make sure your box is very roomy, and is in an area where cat feels comfortable.
3. Different types of litter might help.  I like “World’s Best Cat Litter.”
4. Wipe rear once a day with a large wet wipe made for cats.  Cats get wiggly when you touch their rear, so you can try distracting them with toys and treats, or one of you can hold him while the other wipes.  One way to hold a cat is to wrap them up in a very big towel, like a burrito, and expose the part on which you’re working. Do it fast, ha ha.  Cats generally don’t like to have their bottoms messed with.
5. Small round-tipped scissors can be used to trim dirty bits.  The important factor is that you have to make 100% sure you’re not cutting the skin.  Put a metal grooming comb or your finger between the skin and the part you’re trimming.  Cat skin is easy to cut due to its thin-ness, so do this with good lighting and caution. You can buy round-tipped trimming scissors for pets and a metal grooming comb on Amazon.
6. You can buy the same sort of clipper I use for shaving. It’s very quiet and easy to handle.  You can also use it to trim fur in other areas.  I use Wahl’s Bravura clipper. This is a quiet, strong clipper.
7. If cat has a bunch of poop accumulated on his rear, cat will probably need a partial bath to soak it off. You can buy hypo-allergenic, unscented pet shampoo. Use as directed on label.
I make house calls for sanitary trims, but if your cat is gentle, you can try doing it yourself.
Good luck!