An itching sensation in the anus can be caused by pinworms.
Gastrointestinal worms multiply by notching the skin in the anus and laying eggs. The person scratching the itch collects eggs on his hands. If he puts his hands in his mouth or touches the food, the eggs reach the stomach and there they hatch.
The easiest way to diagnose this condition is to wipe the area with toilet paper and examine the paper.
If the worms exist then they will appear on the paper as white worms in the length of several millimeters.
Medicine has medicines that annihilate the pinworms.
The natural (and long – about 7 weeks) way is to extinct them is to be aware of the issue and not scratch the area with bare hands. The worms will not reproduce and will eventually become extinct from the digestive system.