Minimally invasive sampling to identify leprosy patients with a high bacterial burden in the Union of the Comoros

The World Health Organization (WHO) endorsed diagnosis of leprosy (also known as Hansen’s disease) entirely based on clinical cardinal signs, without microbiological confirmation, which may lead to late or misdiagnosis. The use of slit skin smears is variable, but lacks sensitivity. In 2017–2018 during the ComLep study, on the island of Anjouan (Union of the Comoros; High priority country according to WHO, 310 patients were diagnosed with leprosy (paucibacillary = 159; multibacillary = 151), of whom 263 were sampled for a skin biopsy and fingerstick blood, and 260 for a minimally-invasive nasal swab. In 74.5% of all skin biopsies and in 15.4% of all nasal swabs, M. leprae DNA was detected. In 63.1% of fingerstick blood samples, M. leprae specific antibodies were detected with the quantitative αPGL-I test. Results show a strong correlation of αPGL-I IgM levels in fingerstick blood and RLEP-qPCR positivity of nasal swabs, with the M. leprae bacterial load measured by RLEP-qPCR of skin biopsies. Patients with a high bacterial load (≥50,000 bacilli in a skin biopsy) can be identified with combination of counting lesions and the αPGL-I test. To our knowledge, this is the first study that compared αPGL-I IgM levels in fingerstick blood with the bacterial load determined by RLEP-qPCR in skin biopsies of leprosy patients. The demonstrated potential of minimally invasive sampling such as fingerstick blood samples to identify high bacterial load persons likely to be accountable for the ongoing transmission, merits further evaluation in follow-up studies.Author summary: Leprosy is the oldest infectious disease known to humankind. We still do not succeed in curbing its transmission, with more than 200,000 new patients detected worldwide each year. Identifying persons with a high burden of bacteria is key to curb transmission. To identify these persons, bacteria are counted in invasive and painful samples like slit skin smears and skin biopsies. We evaluated whether we can use less invasive samples, like fingerstick blood or nasal swabs, to determine the bacterial load. We found that the level of antibodies against M. leprae (αPGL-I IgM) in fingerstick blood correlates well with the bacterial load determined in skin biopsies from the same leprosy patient. Therefore, a high level of antibodies against M. leprae in fingerstick blood might identify persons who pose a potential risk for transmission of leprosy and could be prioritized for contact screening, which is essential for control of the disease.