First, as regards the sexual exposure you have described, heterosexually syphilis remains rare, and even among the most sexually active women and your description of your partner's lip and skin problems don't sound much like syphilis. So now the question is whether your partner has secondary syphilis, reflected in her skin problems; or if the oral lesion was a chancre of primary syphilis. Or does she have late syphilis, i.e. are the skin and lip problems caused by something else? Do you have more detail about her positive blood test? Most important, her RPR or VDRL titer? And what syphilis stage has been diagnosed by the clinic? If she has either primary or secondary syphilis, or any stage of syphilis that is under a year in duration, then you indeed were at risk. Kissing statistically is very low risk, but not zero.

If the clinic believes she has early syphilis, kissing and the brief oral sex put you you should be treated with long acting (benzathine) penicillin. Up to half of syphilis strains in the US and other industrialized countries are resistant to azithromycin. (If azithromycin IS active against her syphilis strain, it probably would have protected you, but not necessarily. The drug remains in the system for 7-10 days after the last dose, but 500 mg is too low a dose.)

Going to your opening question, syphilis typically is sexually transmissible during the primary and secondary stages and, in the absence of symptoms, during the first year after acquiring it. After a year, sexual transmission is rare if it occurs at all. People with late or tertiary syphilis generally do not have oral, genital or anal lesions and their sex partners are not at risk (although it still can infect a fetus in utero, hence congenital syphilis can occur when pregnant women have late syphilis).

Assuming you did not register at the sexual health clinic when your partner did, you should do that now -- then follow their advice about preventive treatment, which will depend primarily on your partner's syphilis stage as well as their evaluation of her rash and lip lesion.

I'm also curious where you are? Syphilis rates, especially heterosexually, are highly variable in different countries and clinical settings.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD