Exploration of tongue dorsum sampling to support clinical diagnosis of leprosy patients in the Comoros: A cross-sectional study

Background The accuracy of the WHO-endorsed clinical leprosy diagnosis depends on the expertise of health care workers. For molecular confirmation of clinically diagnosed patients, skin biopsies have the highest sensitivity to detect Mycobacterium leprae . As less invasive tongue swabs showed promising results for qPCR-based M. tuberculosis detection, this study investigated the presence of M. leprae on the tongue dorsum of clinically diagnosed leprosy patients. Methodology/Principal Findings During the activities of the (BE-)PEOPLE study, 499 clinically diagnosed, consenting patients from the Comoros were recruited. Samples collected included skin biopsies from active lesions, nasal swabs, tongue swabs, and, in some cases, tongue scrapes. M. leprae DNA was quantified with the RLEP qPCR assay and human mitochondrial DNA was quantified as sample adequacy control (SAC). On 18.1% (90/498) of tongue swabs and 13.2% (12/91) of tongue scrapes M. leprae DNA was detected. In only six patients tongue scrapes outperformed the tongue swab based on the number of bacilli/sample. Except for three paucibacillary (PB) patients, all 102 positive tongue samples were from multibacillary (MB) patients. Only patients with a RLEP-positive skin biopsy and positive bacteriological index (BI) yielded M. leprae DNA on the tongue scrape. The skin biopsy samples had a sensitivity of 92.5% (248/268) for MB and 74.3% (130/175) for paucibacillary (PB) patients. Nasal swabs were positive for 60.2% (162/269) of MB but only 0.87% (5/229) of PB patients. Conclusion/Significance This is the first study to identify M. leprae bacilli on the tongue dorsum of clinically diagnosed leprosy patients by RLEP qPCR. Due to low positivity rates, tongue sampling has limited added value over skin biopsies and nasal swabs for the microbiological confirmation of leprosy. However, the mouth in general and the tongue specifically remain interesting sampling sites to gain further insights on the distribution of M. leprae bacilli in the body and potential transmission modes.