The following pretty much applies with any anal to oral contact, weather its analinguis or "ass to mouth":
I allways try to clean myself out with a plain water enema prior to engadging in this activity. Not a bad idea for your partner to do so as well especially if they expect you to preform analinguis on them.
Though on occassion sometimes I use a small dillution of soap or even shampoo in the mix as well.
There are still however some health conserns.
There are many health problems that can result from practicing unsafe anal to oral contact, because of the presence of bacteria, viruses or parasites on or in the anus or rectum.
These include Hepatitis A, Hepatitis B, Hepatitis C, intestinal parasites, chlamydia, human papillomavirus (HPV), gonorrhea, herpes, and other sexually transmitted diseases.
Applying the mouth to his cock after it has been up your back door can inadvertently introduce the bacterium Escherichia coli ("E. coli") into the urethra, causing a urinary tract infection, not to mention reintroducing higher levels of it into the wrong end of you digestive track resulting in some frequent trips to the little girls room for you later, the same thing can happen to the girl if you decide to eat her out after.
Frequent ass to mouth with casual partners increases the health risks associated with the practice.
If the couple knows that each of them has a healthy background, there is less risk of catching one of the serious viruses or parasites.
Parasites may be in the feces if poorly cooked meat was consumed.
Hepatitis A traces in feces only apply if the infected person has eaten contaminated food.
Hepatitis C is rare although possible if the receiver has trace amounts of infected blood through the anus or feces.
If you have wounds or open sores in your mouth or anus, or if the giving partner has wounds or open sores on or in their penis, this poses an increased risk of STD transmission.
Brushing the teeth, flossing, undergoing dental work, or eating crunchy foods such as potato chips relatively soon before or after performing ass to mouth also increases the risk of transmission, because all of these activities can cause small scratches on the inside of the lips, cheeks and palate.
These wounds, even when they are microscopic, increase the chances of contracting STDs that can be transmitted orally under these conditions. Such contact can also lead to infections from common bacteria and viruses found in and around the genitals and anus.
Though its not a bad idea to use listerine very soon after. Not only to avoid the idea of getting "butt breath" lol, but to help kill any unwanted bacteria. Make sure not to do prolonged washing though, some of the natural bacterial fuana of your own mouth is good for you to have.
Saftey first, kinky second is my motto.