The most common cause is parasitic infections, especially pinworms, which live in the intestinal tract for a long time and crawl out of the anus especially at night, often causing itching and diarrhea symptoms. 2. The main reason for this is that internal hemorrhoids can form nuclei as they develop, and as they grow they can cause irritation to the surrounding mucosa of the lower rectum and cause itchy symptoms. 3. In addition to these causes, anal papillomas and perianal abscesses can also cause itchy symptoms in the anus. In particular, systemic allergic reactions can also lead to the appearance of this symptom.