Outbreak of erythema nodosum of unknown cause - New Mexico, November 2007-January 2008.

Erythema nodosum (EN) is a form of panniculitis, which has been associated with several infectious and noninfectious etiologies. EN clusters have been associated with outbreaks of Coccidioides immitis, Histoplasma capsulatum, and Yersinia pseudotuberculosis infections. In December 2007, a physician in a rural New Mexico community of approximately 10,000 persons reported to the New Mexico Department of Health (NMDOH) that 13 patients had been diagnosed with EN since mid-November. No EN outbreak had ever been detected in this community, and since 2006, only one diagnosis of EN had been made at the local health-care facility. NMDOH initiated an investigation to confirm the existence of the outbreak, determine the underlying etiology, and implement control measures. This report describes the results of that investigation. Twenty-five EN cases were identified. Seventeen of 20 patients who answered a standard questionnaire reported being at a construction site with crowded and dusty conditions before EN onset. Nine of 15 chest radiographs were abnormal. Serologic test results were interpreted as negative for mycotic agents and inconclusive for Mycoplasma pneumoniae infection. No etiology of the outbreak could be found. During an EN outbreak, timely (acute and convalescent) specimen collection (ideally from case-patients and control subjects to determine baseline seropositivity) and sensitive tests (e.g., polymerase chain reaction [PCR]) are essential to differentiate among possible causes of EN.