Case clustering in pityriasis rosea: a multicenter epidemiologic study in primary care settings in Hong Kong

To investigate the epidemiology of pityriasis rosea in primary care settings in Hong Kong and to analyze for temporal clustering.

Retrospective epidemiologic study.

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).

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.

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).

Significant temporal clustering independent of seasonal variation occurred in our series of patients with pityriasis rosea. This may be indicative of an infectious cause.