five years hiding

before the skin shows

Problem statement

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.

Why this matters

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.

What’s been tried and why it hasn’t worked

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.

What would unlock progress

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.

Entry points for student teams

A modeling team needs no lab at all: take the WHO TPP's >=81% sensitivity and >=99% specificity as variables rather than givens and work out, from the published LPEP contact-tracing and SDR-PEP effectiveness numbers, at what performance a test actually changes who gets prophylaxis — how many contacts a 95%-specific test wrongly clears, what the blanket-PEP strategy costs per case averted versus a test-and-treat one, and whether the TPP's thresholds are the binding ones or whether specificity alone decides. A bench team could build the multiplex lateral flow prototype combining anti-PGL-I IgM with 2–3 host cytokine markers (IP-10, CCL4, CRP) on one finger-stick strip, using purified PGL-I antigen and monoclonal antibodies from BEI Resources' leprosy reagent collection, since the engineering problem — quantitative discrimination on a cheap strip with ambient-stable reagents — is testable on characterized reagents before any patient sample exists. A third team could develop the simplified isothermal assay targeting M. leprae RLEP elements and characterize its limit of detection and inclusivity by spiking BEI's M. leprae genomic DNA (strains NHDP, Br4923, Thai-53) into a nasal-swab matrix; whether nasal carriage tracks infection risk is a multi-year household-contact cohort in an endemic country, so the semester deliverable is the assay plus that cohort study's design. The two bench doors need a BSL-2 molecular lab and a BEI Resources account — free but registration-gated, held by a faculty PI, and the M. leprae materials sit under NIAID's Leprosy Research Support contract. Relevant disciplines: biomedical engineering, immunology, infectious disease, diagnostics design, decision analysis.