Diagnostic value of PCR-based detection of Sarcoptes scabiei in scabies outbreak situations

Abstract text

Introduction: Scabies, a dermatological condition caused by reactions to the mite Sarcoptes scabiei, is an important public health issue in institutions such as nursing/ residential homes, childcare facilities, prisons and hospitals. Scabies outbreaks occurring among patients, visitors and staff are common, subject to diagnostic delay and hard to control. Verifying the clinical suspicion remains highly challenging. Time-consuming microscopic examination and dermoscopy are recommended to visualize the parasite to verify human scabies. However, microscopy has a suboptimal sensitivity, dermoscopy has low specificity and is affected by the dermatologist"s experience.

Objectives: To evaluate a new laboratory-based molecular assay as a diagnostic method for scabies, we compared PCR to microscopy and dermoscopy in a prospective study.

Patients & Methods: 163 symptomatic patients, aged 9 weeks to 99 years, and 369 asymptomatic contacts were examined. All patients underwent clinical and dermoscopic examination. To obtain skin scales, adhesive tape was applied to suspicious lesions before and after skin scraping. Tapes were mounted onto slides to enable microscopy prior to DNA extraction and Sarcoptes scabiei real-time PCR targeting the mitochondrial cytochrome c oxidase subunit 1 gene (cox1). Specificity of the PCR assay was verified by using DNA extracted from other common mite species.

Results: Mite or mite products were detected by microscopy in only 8% of the patients. By dermoscopy, mites were detected in 88% (95% CI: 82·3 – 93·0) and Sarcoptes scabiei DNA was detected by PCR in 87% of patients (95% CI: 80·0 – 91·4). No cross-reactivity to other relevant mites was detected. Scabies PCR offers an improvement in sensitivity to verify scabies compared to microscopy and performed equivalently to dermoscopy (p-value 0·628). Skin scraping before applying adhesive tape significantly increased success (87% vs 56%; p <0·001).

Conclusion: Our PCR-based method from skin scrapings is nontraumatic, non-expert-dependent and reproducible. S. scabiei PCR is specific, sensitive and as successful as dermoscopy, but requires no specific equipment or expertise on-site. This PCR-based diagnostic method has the potential to accelerate diagnosis and treatment, ultimately limiting or preventing scabies outbreaks in care homes or other close communal environments. PCR-based verification can also be used as an objective diagnostic criterion for scabies in therapeutic trials.