Case Clustering in Pityriasis Rosea: A Multicenter Epidemiologic Study in Primary Care Settings in Hong Kong

Abstract

OBJECTIVES: To investigate the epidemiology of pityriasis rosea in primary care settings in Hong Kong and to analyze for temporal clustering. DESIGN: Retrospective epidemiologic study. SETTING: Six primary care teaching practices affiliated with a university. Patients Forty-one patients with pityriasis rosea, 564 patients with atopic dermatitis (negative control condition), and 35 patients with scabies (positive control condition). METHODS: We retrieved all records of patients with pityriasis rosea, atopic dermatitis, or scabies diagnosed in 3 years. We analyzed temporal clustering by a method based on a regression model. RESULTS: The monthly incidence of pityriasis rosea is negatively but insignificantly correlated with mean air temperature (gamma s = -0.41, P =.19) and mean total rainfall (gamma s = -0.34, P =.27). Three statistically significant clusters with 7, 6, and 7 cases were identified (P =.03), occurring in the second coldest month in the year (February), the second hottest month (July), and a temperate month (April), respectively. For atopic dermatitis (negative control condition), the nonclustering regression model was selected by Akaike information criteria. For scabies (positive control condition), 1 cluster of 20 cases was detected (P =.03). CONCLUSIONS: Significant temporal clustering independent of seasonal variation occurred in our series of patients with pityriasis rosea. This may be indicative of an infectious cause.