It is also known as rimming.
This act is the least safe of all the oral sex acts, as there is very high chance of passing assorted bacterial and fungal infections from it. E coli, candidias and Hepatitis A are possibilities. An enema can help, but the micro flora and fauna found in the intestines are so ubiquitous that no safe enema can possibly eliminate them all.
A Dental Dam is strongly advised when performing this act.