Case report: Secondary syphilis masquerading as tinea corporis

Case: A 60 year old man with a past history of alcohol dependence and liver cirrhosis presented with a 10 centimeter-wide annular red scaly plaque on his posterior thigh. The plaque had started as a small papule, which then slowly expanded over the next 6 months. He was otherwise asymptomatic. Initial differential diagnoses included tinea corporis, erythema annulare centrifugum and mycosis fungoides.
Investigations: A punch biopsy was performed revealing numerous plasma cells. Syphilis serology was subsequently ordered and returned positive. Screening for HIV, chlamydia and gonorrhea proved negative. On further questioning the patient reported that his last sexual contact was with a local female in Thailand one year prior. He recalled a painless penile ulcer developing a few months afterward, which self-resolved.
Treatment and progress: The patient was referred to the Infectious Diseases unit and responded well to a single dose of I.M. benzathine penicillin, showing clinical improvement with partial resolution of the plaque.
Discussion: This case highlights the difficulty in diagnosis of syphilis, given its protean clinical manifestations. Syphilis should not be forgotten as a differential diagnosis of red scaly rashes, especially as its incidence is on the rise in Australia. The presence of numerous plasma cells in biopsies should alert the clinician to the possibility of syphilis and the need to order syphilis serology. Early diagnosis is vital as syphilis is an easily treated disease in its early stages, but progression to tertiary syphilis may have serious consequences.

Dr. Gwyneth Natalie Wong