|Classification and external resources|
Pruritus ani (also known as anusitis) is the irritation of the skin at the exit of the rectum, known as the anus, causing the desire to scratch. The intensity of anal itching increases from moisture, pressure, and rubbing caused by clothing and sitting. At worst, anal itching causes intolerable discomfort that often is accompanied by burning and soreness. It is estimated that up to 5% of the population of the United States experiences this type of discomfort daily.
The irritation can be caused by intestinal parasites, anal perspiration, frequent liquid stools, diarrhea, residual stool deposits, or the escape of small amounts of stool as a result of incontinence or flatulence. Another cause is yeast infection or Candidiasis. Some diseases increase the possibility of yeast infections, such as diabetes mellitus or HIV infection. Treatment with antibiotics can bring about a disturbance of the natural balance of intestinal flora, and lead to perianal thrush, a yeast infection, affecting the anus. Psoriasis also can be present in the anal area and cause irritation. Abnormal passageways (fistulas) from the small intestine or colon to the skin surrounding the anus can form as a result of disease (such as Crohn's disease), acting as channels which may allow leakage of irritating fluids to the anal area. Other problems that can contribute to anal itching include pinworms, hemorrhoids, tears of the anal skin near the mucocutaneous junction (fissures), and skin tags (abnormal local growth of anal skin). Anal itching is conjectured to be possibly caused by irritating chemicals in foods consumed, but no clinical evidence based on scientific inquiry has been asserted to demonstrate this.
Some authorities describe “psychogenic pruritus” or "functional itch disorder" , where psychological factors may contribute to awareness of itching.
The helminth (worm) Enterobius vermicularis (pinworm, or threadworm) can cause Enterobiasis and severe itching from migration of gravid females (from the bowel), leading to haemorrhage and eczema.
For pruritus ani of unknown cause treatment typically begins with measures to reduce irritation and trauma to the perianal area. Stool softeners can help prevent constipation. If this is not effective topical steroids or injected methylene blue may be tried. If the itchiness is secondary to another condition such as infection or psoriasis these are typically treated.
See also 
- James, William; Berger, Timothy; Elston, Dirk (2005). Andrews' Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0.
- Markell, KW; Billingham, RP (2010 Feb). "Pruritus ani: etiology and management.". The Surgical clinics of North America 90 (1): 125–35, Table of Contents. PMID 20109637.