Prospective case-control study of chlamydia, legionella and mycoplasma infections in patients with pityriasis rosea.

A double-blind placebo-controlled trial reported the benefit of erythromycin in treating pityriasis rosea (PR), a postulated mechanism being the eradication of bacteria susceptible to erythromycin. The aim of this study was to investigate the association between PR and Chlamydia pneumoniae, C. trachomatis, Legionella longbeachae, L. micdadei, L. pneumophila, and Mycoplasma pneumoniae infections. We recruited 13 patients aged seven to 46 years (mean: 26.8 years) diagnosed to have PR in a primary care setting in 18 months. Lesional histopathology was arranged for atypical cases. Clotted blood was collected at initial presentation and four weeks later. Controls were 13 paired age-and-sex-matched patients requiring blood collection for non-dermatological diseases. Serology tests were performed in parallel but were read "blinded" on the acute and convalescent specimens of patients and the control subjects. The serology profiles were not diagnostic of active infection by any of the bacteria studied for all 13 patients. Two patients had four-fold increase in IgG titres against C. pneumoniae, with IgM being negative. Two patients had IgM detectable against L. pneumophila serotype 6 and M. pneumoniae respectively, with no significant rise of the specific IgG. These patients had no symptom or sign of chest infection. The seroprevalence and IgG titres of the study patients for the bacteria investigated were insignificantly different from those of control subjects. We conclude that the bacteria investigated in this study do not play a significant role in the pathogenesis of PR. We believe that anti-inflammatory and immunomodulatary effects might contribute towards the action of erythromycin, if any, in PR.