There are no lesions and no chancres have formed, though I’ve read that syphilis, as the great imitator, can often resemble friction burn and that the so-called “clinical presentation” of a single indurated chancre is only present in less than 50% of patients. I have also read that macules that present pretty much exactly like the one I have are very common in early secondary syphilis. I also know that while the average for symptom appearance is 21 days, 70 is well within the window.

Should I worry? Do you see many instances of receptive syphilis transmission? How common is it to have symptoms appear two months after exposure? I am terrified of transmitting something to my girlfriend.

The sex worker was a pre-op MTF transgendered woman.

I am planning on going to get follow up testing at a clinic tomorrow, but am a bit worried it won’t show an infection even if it were there as I’m finishing a course of Amoxicillin for an unrelated infection. Would the blood tests still show a positive result if I had just finished a course of antibiotics? The erythematous macule most likely appeared the literal day that I began the antibiotics. 

Mostly, my concern lies with whether or not the macule that won’t go away on my penis could have been syphilis in the first place, and if I could have infected my girlfriend prior to taking the course of antibiotics.

Male-male sex is the highest risk scenario for syphilis. (A pre-op MtF transgender woman is anatomically male and available data suggest similar risks and HIV rates as men who have sex with men [MSM].) That said, oral sex is low risk. (By the way, do you really mean receptive fellatio, i.e. you performing oral sex on your partner? Or insertive, i.e. your penis in her mouth? Of course only the latter would risk syphilis of the penis.) There are no data on the per-exposure risk for either of these exposures, however.

But perhaps most important, the syphils blood tests pick up the large majority of new infections by 5 weeks, so your negative test at that time is very reassuring. Further, it is rare for the first symptoms to appear 2 months after exposure. It could happen if you had asymptomatic primary infection (e.g. an unnoticed sore, the chancre), but that's awfully unlikely if your penis was exposed. The reason for only 50% of cases with noticeable chancres is because of exposures in not easily visitble sites, like inside the vagina, rectum, or sometimes mouth. With a penile exposure, probably almost all chancres are readily visible, along with enlarged, tender lymph nodes in the groin, with onset typically 10-20 days after exposure. There's more chance of an unnoticed chancre after oral exposure, but still relatively unlikely (compared with rectum, vagina, etc).

A "red patch" starting at around 2 months is compatible with secondary syphilis, if indeed a chancre had been missed. However, to have a secondary syphilis rash only of the penis would be very unusual. Finally, there's the amoxicillin, which is highly active against syphilis. It would have cured syphilis for sure -- if the red patch developed while on amoxicillin, or persisted more than a few days while taking it, that proves a cause other than syphilis.

Finally, it may interest you to know that the large majority of genital dermatitis, rashes, etc are caused by conditions other than STDs. On the bookshelf 3 feet from where I'm typing is a book titled Genital Dermatology Atlas. As the name implies, it is filled with clinical photos of genital skin problems. Of its 300 pages, only 15 cover all STDs.

Considering all these factors -- a relatively low risk exposure, no chancre, negative blood test at 5 weeks, only a single "red patch", and apparent persistence of that patch with amoxicillin -- there is no realstic chance you have syphilis. You could have another blood test now; the expected negative result would prove for sure you do not (and did not) have it.

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

HHH, MD

Thank you much for your timely and detailed reply. Just a few points of clarification and follow-up questions:

1. I should clarify that both receptive and insertive oral took place, but that you are correct in that I was only thinking of the latter due to the location of the macule. I suppose I could have had an unnoticed chancre in my mouth and this macule could be a secondary symptom, but to you this would seem unlikely as a) I’m paranoid and probably would have noticed and b) I likely would have noticed the presence of an abnormality on her genitalia, correct?

2. On the extremely off chance that these symptoms are syphilis, would all blood tests still show the infection now that I have finished a course of antibiotics? I am getting mixed information from what I’ve read, and my greatest fear is that I have exposed my girlfriend in the interim and that if I get tested now, it will come up negative even if I was previously infected. Additionally, I’m admittedly a little unclear as to whether or not the initial blood test was after 4 weeks or 5 weeks now that I’ve thought it over. Is efficacy much different from week 4 to week 5?

 3. Lastly, would you have any guesses as to what might cause a patch of skin like the one mentioned if not an std? I know diagnosis is probably not something you tend to do on a forum such as this, but some range of differential diagnoses might do me a world of good in easing anxiety as the main question I keep returning to is “what else could it be?”

Thank you again for your time and your measured response. You all seem to be helping a lot of anxious people.

2) Antibiotics don't affect blood test results so quickly. If you had syphilis, the blood test already done probably would have been positive; and another blood test at this time certainly would be.

3) As I said above, the large majority of genital skin problems are not STDs. But there are many possibilities, including at least 5-6 reliatively common conditions. The single most common probably is lichen planus, which typically has a lavender hue, and also often is associated with white/lavender streaks in the mouth, usually inside the cheeks. You can google LP for typical photos. But this isn't a diagnostic service and I won't go further in trying to judge the cause. Best to see a doctor. But I'll reiterate that there is no chance you have syphilis. Let it go!