Myself Dr Josh- and I’m happy to help with your question today.
I understand that you have received handjob and body slide at massage center and there was no actual penetration.
Well, most of the STIs are transmitted either through bodily fluids such as semen, vaginal fluids, blood, breast milk, or saliva or skin-to-skin contact.
Chances of getting STI's is high, when an infected person’s mucous membranes, bodily fluids, open lesions, or infected shedding skin cells come into contact with an uninfected person’s mucous membranes or open lesions.
As you said that you had a chronic skin condition on buttocks, and has it for years, so chances of transmission are slim.
There are STI's which can spread by skin-to-skin contact and include oral and genital herpes, HPV, and syphilis.
But, Skin-to-skin contact occurs when an infected site of one individual’s skin comes into direct contact with a mucous membrane or lesion on an uninfected person’s body.
In your case, as it a chronic skin condition, so chances of getting any infection are slim.
Though chances are slim from that incident, Still best bet is that you should get it evaluated from your primary doctor.
Hope this helped
If you have any further question then you can ask me.
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