Mexican butt-worms are what you pick up in mexico after eating from a taco cart or tamale stand that is less than sanitary. They are also different from other butt-worms because they wear sombreros.
Did you hear about Steve's trip to Tijuana? He got a bad case of the mexican butt-worms after eating at a filthy taco stand.
A Butt Worm (or pinworm) is a parasite often found in feces, and the eggs can be spread about very easily upon contact. Having Butt Worms often leads to itchy/crawly sensations