From Jeffrey D. Klausner, MD, MPH – Former Director of San Francico City Clinic’s STD Services:

Oral-anal sex can transmit shigella, amebiasis, cryptosporidium, giardia, and hepatitis A. All of these illnesses are transmitted via contaminated fecal matter. You can use a cut-up condom, household plastic wrap or a dental dam for rimming to lessen your risks. Your best bet, though, is to have both partners wash thoroughly with soap and warm water both before and after sex.