The best solution in my opinion is that you should lick and suck the vagina and her vulva instead of anus. It will give you and your partner much more pleasure than licking her anus.
Use a dental dam, or shower together beforehand?
A hot bed of disease unless the person is willing to go through a disinfection routine. It's a definite risk.
For starters, porn stars go through frequent testing for STDs and colonic cleansings before this type of sexual activity. Porn stars aside, look for typical sexual barriers similar to non-spermacidal condoms to keep a separation but still allow pleasure.