Case Report We present a case of primary syphilis in the third trimester of pregnancy requiring penicillin desensitisation. A 34-year old woman was contacted by provider referral as a syphilis contact. She was 30 weeks pregnant. Both HIV and syphilis serology were negative at booking. She had presented to a community clinic with a sore vulva one week before and had empirical aciclovir for possible genital herpes. In our clinic, examination showed a small, non-indurated ulcer. Dark-ground microscopy was not done. A syphilis antibody screen was requested and reported positive six days later. On recall, repeat examination showed a larger, indurated ulcer. Treatment for primary syphilis was advised before confirmatory testing. However, the patient reported a possible reaction to penicillin. This was also documented by her general practitioner but the reaction was unknown. The next day she was admitted for penicillin desensitisation and the first dose of benzathine penicillin. Urgent referral to foetal medicine was made. Treponema pallidum was later detected by PCR on a vulval swab. Syphilis serology was reported as RPR 1:8 and TPPA 1:80. HIV serology and HSV PCR were negative. A second dose of benzathine penicillin was administered a week later, followed by 45 minutes of observation. After delivery at term, the neonate received 10 days of benzyl penicillin.

Discussion Learning points: 1. Exclude syphilis in anyone with genital ulceration, particularly in pregnancy. 2. Consult Immunology for advice on desensitisation regimen. 3. Write a syphilis birth plan as recommended by new BASHH guidelines