2 Answers

Information here >>>http://www.medicinenet.com/anal_itching/article.htm

The cause of itchy bottom is not always known. However, it can sometimes be a symptom of another problem or underlying condition.
Infection

An itchy bottom may be a sign your body is trying to deal with an infection. The infection may be:
bacterial - such as the streptococcal bacteria that causes streptococcal infections or the staphylococcal bacteria that causes staphylococcal infections
fungal - such as the Candida albicans fungus that causes vaginal thrush (an infection that causes irritation and swelling of the vagina)
parasitic - such as threadworms (small worm parasites that infect the intestines) or scabies (tiny mites that burrow into the skin)
viral - such as the herpes simplex virus that causes cold sores
An itchy bottom can sometimes be a symptom of a sexually transmitted infection (STI).
If you have had unprotected sex and you think there may be a chance you have an STI, you should visit your local sexual health clinic for advice, testing and, if necessary, treatment.
Gastrointestinal conditions

Gastrointestinal conditions affect your digestive tract (your mouth, throat, stomach, intestines and anus).
Gastrointestinal conditions that may cause an itchy bottom include:
haemorrhoids (piles) - swellings in and around your anus that contain enlarged and swollen blood vessels
anal fistula - where a small channel (tract) develops between your anal canal (the last section of the large intestine) and the surface of your skin, near the anus
anal fissure - a tear or ulcer (open sore) that develops in the lining of the anal canal
sphincter incompetence - where the sphincter (the ring of muscle that opens and closes your anus) stops working properly, causing bowel incontinence (leaking stools)
long-term diarrhoea - passing loose, watery stools
long-term constipation - an inability to completely empty your bowels
Anal and bowel cancer

In rare cases, an itchy bottom can be a symptom of a gastrointestinal cancer, such as anal cancer or bowel cancer.
Although most cases of itchy bottom are not caused by cancer, it is important your GP rules out all possibilities.
Anal cancer is rare, with around 930 cases diagnosed in the UK each year.
In the UK, bowel cancer affects more than 36,500 people a year, and is the second most common type of cancer in women and the third in men. Over 80% of cases occur in people 0 years of age or over.
If you do have cancer you are likely to have other symptoms as well such as:
weight loss
changes to how often you go to the toilet
Read more about anal cancer and bowel cancer.
Skin conditions

Some dermatological (skin) conditions can affect any area of skin on your body, including the skin around your anus.
Skin conditions that can cause itchy bottom include:
psoriasis - where red, flaky, crusty patches of skin develop because your skin cells reproduce too quickly
contact dermatitis - where your skin reacts to certain substances, known as allergens, causing your skin to become inflamed (red and swollen)
lichen sclerosus - is a long-term skin disorder that causes itchy or sore white spots to develop on the skin around the genitals
atopic eczema - where your skin becomes red, dry and flaky.

Read more  >http://www.nhs.uk/Conditions/itchybottom/Pages/Causes.aspx