Causes and Prevention of Leprosy: Transmission and Host Factors

Leprosy – or Hansen’s disease – has existed for thousands of years, yet it continues to affect hundreds of thousands of people every year. According to the World Health Organization, approximately 200,000 new cases are reported globally each year, with Brazil, India, and Indonesia bearing the largest share of the burden. Despite this, leprosy remains widely misunderstood – particularly around how it spreads, who gets it, and why some people are more vulnerable than others. Understanding the causes, transmission pathways, and host factors is key to both dispelling myths and building effective prevention strategies.

What causes leprosy?
How leprosy is transmitted
The respiratory route
Environmental and animal reservoirs
Who is most at risk? Understanding host factors
Genetic susceptibility
The immune response: tuberculoid vs. lepromatous leprosy
Prevention: what works and what is still in development
The role of the BCG vaccine
Emerging vaccine candidates

What causes leprosy?

Leprosy is caused by two slow-growing bacteria: Mycobacterium leprae and Mycobacterium lepromatosis. Of the two, M. leprae is the older and more well-known pathogen – first identified by Norwegian physician Gerhard Armauer Hansen in 1873, making it one of the earliest bacteria ever linked to human disease. M. lepromatosis, on the other hand, was only identified in 2008 from a case of diffuse lepromatous leprosy, and it is found primarily in the Americas.

Both organisms are obligate intracellular pathogens, meaning they cannot survive or replicate outside a living host’s cells. This is largely due to gene deletion and reductive evolution, which has made the bacteria heavily dependent on the host’s cellular machinery for nutrients and metabolic functions. M. leprae preferentially infects two cell types: macrophages and Schwann cells – the cells that form the protective myelin sheath around peripheral nerves. This nerve tropism is what makes leprosy particularly damaging, often resulting in peripheral neuropathy, loss of sensation, and, if untreated, permanent disability.

One unusual biological characteristic of M. leprae is its extremely slow replication rate. Its doubling time ranges from 12 to 14 days, compared to just 20 minutes for common bacteria like E. coli. This contributes to the disease’s long incubation period – typically ranging from 3 to 10 years, though symptoms can take up to 20 years to appear in some cases.

How leprosy is transmitted

One of the most persistent misconceptions about leprosy is that it spreads easily through casual contact. This is simply not true. You must have prolonged, close contact with someone with untreated leprosy over many months to contract the disease. Brief, incidental interactions – shaking hands, sitting next to someone, sharing a meal – do not transmit the infection.

The respiratory route

The primary mode of transmission is through the upper respiratory tract. The principal route involves aerosol spread of nasal secretions, which are then taken up through the nasal or respiratory mucosa. In practical terms, this means transmission is most likely when a healthy person repeatedly inhales droplets released when an untreated leprosy patient coughs or sneezes over an extended period of shared living or close contact. Leprosy is neither sexually transmitted nor passed from a pregnant person to their unborn child.

Once a person starts appropriate multi-drug therapy (MDT), they are considered non-infectious within 72 hours – an important public health fact that underscores why early diagnosis and treatment are central to breaking the chain of transmission.

Environmental and animal reservoirs

While human-to-human transmission is the primary pathway, it is not the only one. Aside from infected humans, environmental sources have also been identified as potential reservoirs – including soil from households of leprosy patients and habitats of red squirrels in the British Isles.

The most significant non-human reservoir is the nine-banded armadillo. Wild armadillos in the United States have harbored a natural infection with M. leprae for many decades, and recent reports indicate that zoonotic transmission from armadillos may account for up to 64% of leprosy cases among persons born in the United States. In the southern US states, people who handle or consume armadillo meat are at heightened risk. That said, the overall risk of contracting leprosy from armadillos remains very low for most people.

Who is most at risk? Understanding host factors

Here is a striking fact: around 95% of people cannot get leprosy because their immune system can fight off the bacteria. This means that even after significant exposure, the vast majority of individuals never develop the disease. This points strongly to the role of host factors – particularly genetics and immune response – in determining who gets sick and how severely.

Genetic susceptibility

There is now robust scientific evidence that genetics plays a meaningful role in leprosy susceptibility. Genetic studies have uncovered several gene variants that act as risk factors for leprosy – both for the disease itself and for the specific clinical form it takes.

A key area of focus is the Human Leukocyte Antigen (HLA) system, part of the Major Histocompatibility Complex (MHC) located on chromosome 6p21. Genetic factors in leprosy involve both HLA and non-HLA genes, and these affect the individual’s susceptibility to the disease, including how bacilli are transmitted and how clinical features develop. The HLA system is responsible for presenting pathogen-derived peptides to T cells – a critical step in activating an immune response. When an individual’s HLA system does not present M. leprae peptides in a way that properly activates the right lymphocyte response, that person becomes more susceptible to infection.

Specific HLA gene variants have been associated with different leprosy outcomes. Research from Brazil found that HLA-DRB1*08 frequency was notably higher in lepromatous patients than in tuberculoid ones, indicating a role in susceptibility to the most severe form of leprosy. Beyond HLA genes, several other gene variants – including NOD2, LRRK2, and the PARK2/PACRG group – have been identified as influencing macrophage response to the leprosy bacillus.

The immune response: tuberculoid vs. lepromatous leprosy

Perhaps the most clinically significant host factor is the nature of an individual’s immune response once infected. Leprosy does not present as a single, uniform disease – instead, it exists on a clinical spectrum shaped by how the immune system reacts to M. leprae.

At one end of the spectrum, tuberculoid leprosy is characterized by resistance to bacilli proliferation due to a predominant Th1 immune response, which results in a strong cellular immune reaction. People with this form have fewer lesions, and their immune system actively limits bacterial spread. At the other extreme, lepromatous leprosy is characterized by the spread of bacilli due to a weak cellular immune response and a predominant Th2 response, which induces the production of antibodies – antibodies that, in this case, are largely ineffective against the intracellular bacteria. This form is significantly more severe, with multiple skin lesions and much greater bacterial load.

Individuals who mount a vigorous cellular immune response have the tuberculoid form, while those with a weaker response develop the more aggressive lepromatous form. This immune-driven spectrum is not random – it is heavily influenced by genetic makeup, as described above, but also by nutritional status and general health. Conditions that reduce immune function – such as malnutrition or other illnesses – can increase the risk of developing leprosy, though notably, HIV co-infection does not appear to significantly raise that risk in most cases.

Prevention: what works and what is still in development

Since leprosy is curable and only transmitted through prolonged, close contact with untreated cases, early diagnosis and prompt treatment form the cornerstone of prevention. WHO recommends contact screening – including household, neighbourhood, and social contacts – accompanied by a single dose of rifampicin as post-exposure prophylaxis (SDR-PEP) to reduce the risk of developing the disease among those who have been exposed.

The role of the BCG vaccine

There is currently no dedicated leprosy vaccine, but the BCG (Bacillus Calmette-Guérin) vaccine – originally developed for tuberculosis – has shown a meaningful protective effect against leprosy as well. Since the 1940s, clinical studies have consistently shown that BCG offers some level of protection against leprosy, though the range varies considerably – from 18% to 90% depending on the population and setting.

A large meta-analysis published in the Indian Journal of Dermatology brought some clarity to these varying figures. Across 22 studies, the summary protective effects from trials, cohort studies, and case-control studies were 43%, 62%, and 58% respectively – all statistically significant – confirming the protective association between BCG vaccination and leprosy. Notably, not a single study in the review reported a negative protective effect. The WHO officially recognized BCG as a leprosy vaccine in 2018.

Emerging vaccine candidates

The scientific community is not stopping at BCG. Several new vaccine candidates have emerged, including Mycobacterium indicus pranii (MIP), which has shown demonstrable protective efficacy lasting 8-10 years, and LepVax – an adjuvanted recombinant subunit vaccine that has shown reductions in bacterial count and delayed nerve function impairment in animal models, and has been found safe in healthy adults in early clinical studies.

Evidence continues to accumulate that BCG vaccination, active case detection, adherence to multi-drug therapy, and continued post-treatment surveillance are the strategies most consistently shared by countries that have achieved a substantial reduction in leprosy incidence – alongside broader socioeconomic improvement. The WHO’s Towards Zero Leprosy strategy for 2021-2030 frames these tools within a broader vision: zero infection and disease, zero disability, and zero stigma and discrimination.

The challenge ahead is not just medical. Leprosy disproportionately affects people living in poverty, and the barriers to early diagnosis – lack of access to healthcare, persistent stigma, and the disease’s long incubation period – remain significant. Tackling transmission requires addressing both the biology of the bacterium and the social conditions that allow it to persist.

What do you think? Given that 95% of people are naturally resistant to leprosy even after exposure, should public health strategies focus more heavily on identifying and treating the vulnerable 5% through targeted genetic or immune screening? And considering the varying efficacy of the BCG vaccine across different regions, how should high-burden countries prioritize vaccine policy alongside contact tracing and chemoprophylaxis?

References

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Table of Contents

What causes leprosy? 🔗

How leprosy is transmitted 🔗

The respiratory route 🔗

Environmental and animal reservoirs 🔗

Who is most at risk? Understanding host factors 🔗

Genetic susceptibility 🔗

The immune response: tuberculoid vs. lepromatous leprosy 🔗

Prevention: what works and what is still in development 🔗

The role of the BCG vaccine 🔗

Emerging vaccine candidates 🔗