Five Years Hiding Before the Skin Shows

Leprosy infects over 170,000 new people per year, including 9,400 children, with an estimated 4 million additional undiagnosed cases. Single-dose rifampicin post-exposure prophylaxis (SDR-PEP) reduces leprosy risk in contacts by 50-60%, and the LPEP programme has shown that tracing and treating 170,000 contacts could near-eliminate leprosy within a generation if scaled globally. But PEP is currently given blindly to all contacts without knowing infection status, because no field-deployable test exists to detect subclinical *M. leprae* infection. The WHO's 2023 TPP calls for a point-of-care test with >=81% sensitivity and >=99% specificity usable in household and outdoor settings — a test that would transform contact-tracing from mass prophylaxis into targeted intervention.

Leprosy causes irreversible nerve damage, limb deformity, and blindness if not treated early. In 2024, 289 children were diagnosed with Grade 2 disability — indicating years of missed detection during which irreversible damage accumulated. The average incubation period of 2-5 years (range 1-20+) provides a window for intervention, but only if infection can be identified. Without a diagnostic, enhanced PEP regimens (PEP++) being tested for higher-risk contacts cannot be rationally allocated, and the distinction between infected contacts who need treatment and uninfected contacts who don't remains invisible.

*M. leprae* has never been successfully cultured in vitro — unlike virtually every other bacterial pathogen — making standard assay development approaches impossible. Anti-PGL-I antibody tests detect multibacillary patients reliably but miss paucibacillary patients, who mount cell-mediated rather than humoral immune responses. The NDO-LID and LID-1 fusion antigen lateral flow tests show similar multibacillary bias. Multi-biomarker approaches combining 5 markers (CCL4, CRP, IL-10, IP-10, anti-PGL-I IgM) can detect both forms but have not been simplified to field-deployable formats. Transcriptomic signatures (RISK4LEP 4-gene panel, AUC 0.86) can predict leprosy development 4-61 months before onset but require laboratory RNA extraction and analysis. The fundamental challenge is that subclinical *M. leprae* infection is immunologically ambiguous — distinguishing it from cross-reactive responses in endemic areas requires >=99% specificity, which no current biomarker achieves alone.

The WHO TPP sets stringent parameters: finger-stick blood or nasal swab, <10-100 microliters, zero-infrastructure conditions (households, outdoor settings), capital costs <$2,000. Meeting this likely requires a multiplex approach combining pathogen-specific markers (anti-PGL-I) with host immune response markers (cytokines, transcriptomic signatures) on a single lateral flow or microfluidic platform. A student contribution could address the biomarker combination problem: systematically testing which minimal set of commercially available markers best discriminates subclinical infection from cross-reactive immunity in an endemic contact population.

A team could design a multiplex lateral flow prototype combining anti-PGL-I IgM detection with 2-3 host cytokine markers (IP-10, CCL4, CRP) on a single finger-stick test strip, evaluating whether the combination overcomes the multibacillary bias of serology alone. The engineering challenge is maintaining quantitative discrimination on a low-cost platform with ambient-temperature-stable reagents. Alternatively, a team could develop a simplified nucleic acid detection assay targeting *M. leprae* RLEP elements in nasal swab specimens using isothermal amplification, testing whether nasal carriage correlates with infection risk in household contacts. Relevant disciplines: biomedical engineering, immunology, infectious disease, diagnostics design.