Pinworms are not visible anywhere near as often in snake feces, as they normally get a strain which comes from rodents and is therefore considerably smaller than a human's would be. People generally get a strain more commonly found in hoofstock like cattle and horses.
Best way to detect them is a fecal smear on a microscope slide. While they aren't microscopic they are indeed very tiny! They don't usually cause itching in the way they do in mammals, and so you can't go by a snakes' behavior. Itching in mammals is rectal, a cloaca isn't a mucus membrane and doesn't produce histamine the way a rectum does.
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