How Common Is Leprosy Around the World and in the U.S.?

Leprosy, also known as Hansen disease, infects roughly 200,000 people worldwide each year, though the global rate has been falling for decades. Between 1990 and 2019, the age-standardized incidence dropped from about 1.5 per 100,000 people to around 0.65 per 100,000. In the United States, cases number only in the low hundreds annually, but the disease has not vanished, and recent evidence suggests it may be quietly establishing itself in parts of the Southeast. The story of leprosy’s prevalence is tangled up with poverty, animal reservoirs, diagnostic delays, and a bacterium that can survive in surprising places.

Global Numbers and Where Cases Concentrate

Leprosy is not evenly distributed. The vast majority of cases cluster in a handful of countries, with India and Brazil shouldering the heaviest burden. Despite global elimination campaigns that have driven case counts down dramatically since the 1980s, the disease remains a serious public health problem in many low- and middle-income countries.¹ A large analysis of global burden data from 1990 to 2019 confirmed that countries with lower socioeconomic development consistently had higher rates of new cases, higher prevalence, and greater disability from the disease.²

Men are affected more often than women across nearly every region, and the peak age for new diagnoses falls between the mid-twenties and mid-thirties, with prevalence highest in people around 35 to 40.² Interestingly, the age at which leprosy tends to first appear rises with a country’s development level. In the poorest nations, younger adults are hit hardest; in wealthier ones, diagnoses tend to come later in life, possibly reflecting differences in exposure timing and healthcare access.

One counterintuitive finding from the same global analysis is that high-income countries have actually seen unfavorable trends in recent decades. While incidence has been falling in the developing world, wealthier regions are not showing the same clean downward trajectory. This could reflect better surveillance catching cases that were previously missed, or it could signal that imported and locally acquired cases are maintaining a low-level presence in places that assumed the disease was gone.²

Leprosy in the United States

The U.S. reports roughly 150 to 200 new cases of leprosy per year. Most are diagnosed in people born outside the country who acquired the infection before arriving. A detailed study of 131 cases in Washington State over two decades found that 95% were in foreign-born individuals, 72% were male, and about a third involved people of Micronesian or Marshallese origin. Compared to U.S.-born residents, people born in Oceania had a leprosy rate more than a thousand times higher.³

But the picture is shifting. Florida has seen a rise in cases among people with no history of foreign travel, no known contact with a leprosy patient, and none of the traditional risk factors that usually explain the disease in a non-endemic country. This pattern, combined with a declining share of cases among foreign-born individuals, has led researchers to argue that leprosy has become endemic in parts of the southeastern United States.⁴ Central Florida, in particular, has emerged as a hotspot, with case reports describing patients whose only plausible exposure was living in the region.

This does not mean leprosy is about to sweep through American suburbs. The absolute numbers remain tiny. But the trend matters because it challenges the assumption that every U.S. case can be traced to travel abroad. If the bacterium is circulating locally, even at very low levels, clinicians need to consider it as a diagnosis even in patients who have never left the country.

How Leprosy Spreads

Leprosy is caused by Mycobacterium leprae, an extremely slow-growing bacterium that cannot be cultured in a laboratory dish. It spreads primarily through prolonged close contact with an untreated patient, most likely via respiratory droplets from coughing and sneezing, though direct skin contact may also play a role.⁵ The key word is “prolonged.” Brief encounters with an infected person pose extremely low risk. The incubation period is famously long, averaging three to five years but sometimes stretching to two decades, which makes tracing the source of any individual case nearly impossible.

What puzzles researchers is that leprosy keeps appearing in people who have no identifiable contact with another human case. Several explanations have been proposed: exposure to infected armadillos, contact with contaminated soil while gardening, and even water sources harboring viable bacteria.⁶ The fact that *M. leprae* can survive outside a human host for extended periods in environmental niches complicates the traditional person-to-person transmission model considerably.

Why Most Exposed People Never Get Sick

Even among people living in close contact with an untreated leprosy patient, only a small fraction ever develop the disease. The reason is largely genetic. Your immune system’s particular wiring determines whether *M. leprae* gains a foothold or gets shut down immediately. Research has identified numerous gene variants that influence susceptibility, many of them related to immune function, but some discovered through broader genomic approaches that were not predicted by immunology alone.⁷

Among people who do develop leprosy, the clinical picture varies enormously based on the type of immune response their body mounts. At one end of the spectrum are patients whose immune systems produce a strong, targeted response that walls off the bacteria and limits damage to a few skin patches and nerves. At the other end are patients whose immune response fails to contain the bacterium, allowing it to multiply and spread throughout the body.⁸ This spectrum is not just a quirk of classification. It determines how contagious a patient is, how much nerve damage they accumulate, and how they respond to treatment.

A study of patients who experienced leprosy recurrence found that those who relapsed were significantly more likely to carry double copies of known risk-gene variants compared to patients who stayed disease-free, suggesting that some people are genetically stacked against clearing the infection entirely.⁹

The Armadillo Connection

Nine-banded armadillos are the only well-established non-human reservoir for *M. leprae*, and they are common across the southern United States. A genetic study in the Southeast found that more than 40% of human leprosy patients in the region were infected with the same *M. leprae* strains found in local armadillos, strongly implicating zoonotic transmission.¹⁰ Both direct contact with armadillos (handling or hunting them) and indirect exposure (living in areas where armadillos forage) have been identified as risk factors.¹¹

Armadillos make unusually good hosts for *M. leprae* because their body temperature is lower than that of most mammals, and the bacterium thrives in cooler tissue. The animals can carry enormous bacterial loads without showing obvious signs of illness for long periods. In states like Texas, Louisiana, and Florida, where armadillo populations overlap with human settlement, this creates an ongoing, low-level source of infection that is difficult to control through standard public health measures aimed at person-to-person spread.

Hiding in the Soil

One of the more surprising discoveries in leprosy research is that *M. leprae* can survive for months in the environment, sheltered inside tiny single-celled organisms called free-living amoebae. Laboratory experiments showed that the bacterium remained alive and capable of causing infection in mice after spending more than eight months inside amoebic cysts, structures that amoebae form to protect themselves when conditions are harsh.¹² These amoebae are found in soil and water everywhere.

Field work in leprosy-endemic areas of India confirmed that soil and water near the homes of leprosy patients contained viable *M. leprae*, and its survival was associated with the presence of amoebae in those samples.¹³ This finding has significant implications. If the bacterium can persist in soil between human (or animal) hosts, it could explain why new cases keep appearing in communities even after active patients have been treated. It also raises questions about whether gardening without gloves in certain areas carries a real, if very small, risk.

Why Diagnosis Often Takes Too Long

Leprosy’s early symptoms are easy to ignore. A pale or reddish skin patch, a patch of numbness, some tingling. In a disease with an incubation period measured in years, early signs are painless and easily mistaken for eczema, fungal infections, or nothing at all. In a large study across five Indian states, 90% of patients who delayed seeking care said they simply did not know their symptoms could be leprosy and assumed the problem would go away on its own.¹⁴ When patients waited more than three months to see someone, their odds of being diagnosed with significant disability were about 60% higher than for those who came in sooner.

Stigma compounds the delay. Fear of social rejection keeps people from seeking help, and this effect is especially pronounced for women. Research in Bangladesh found that fear of stigma was a reliable predictor of delayed diagnosis, and that women in some communities were abandoned by their families after a leprosy diagnosis.¹⁵¹⁶ The cruelty here is circular: stigma delays care, delayed care causes disability, and visible disability reinforces the stigma.

Even when patients do seek care, the diagnosis can be tricky. Standard methods rely on clinical examination, skin biopsies, and microscopy, all of which require expertise that may not be available in the communities where leprosy is most common. Newer molecular techniques like quantitative PCR offer better sensitivity, especially for cases with very few bacteria, but they require laboratory infrastructure that is not always accessible in the field.¹⁷ Point-of-care tests that can be done without equipment are in development and could help close the gap, particularly for early detection in remote areas.¹⁸

What COVID Did to Leprosy Detection

The COVID-19 pandemic did not increase leprosy infections, but it severely disrupted the systems that find them. In Brazil, which reports more cases than any country outside India, the number of leprosy diagnoses dropped by more than 40% in 2020 compared to the previous five-year average, a decline of over 11,000 cases in a single year.¹⁹ Among children under 15, the drop was even steeper, close to 57%. Since childhood cases are a key marker of ongoing community transmission, this masked the true spread of the disease at a critical time.

Meanwhile, the proportion of new cases classified as the more severe, multi-bacterial form increased by about 8%, suggesting that the cases being detected during the pandemic were those that had progressed further before someone noticed.¹⁹ Those thousands of undiagnosed patients did not stop being infectious. They continued living in their communities, potentially transmitting the disease, and accumulating the nerve damage that early treatment could have prevented. The pandemic years created a backlog that leprosy programs in affected countries are still working to clear.

Leprosy Reactions and Lasting Damage

Even after treatment begins, leprosy can flare dangerously. These episodes, called leprosy reactions, are immune-mediated events that can cause sudden inflammation in the skin and nerves. There are two main types. Type 1 reactions involve a surge of cell-mediated immunity that inflames existing skin lesions and peripheral nerves, potentially causing rapid loss of sensation and muscle function.²⁰ Type 2 reactions are driven by immune complexes and produce painful red nodules along with systemic symptoms like fever and joint pain.²¹

Reactions are the main reason leprosy still causes disability. The bacterial infection itself responds well to antibiotics, typically a combination of drugs taken for six to twelve months depending on the type of disease.²² But reactions can occur before, during, or even years after treatment, and if they are not recognized and managed quickly with corticosteroids or other immunomodulators, the nerve damage becomes permanent. This is why early diagnosis matters so much. It is not that leprosy is hard to cure. It is that the window for preventing irreversible harm is narrower than people realize.

BCG and the Question of a Vaccine

There is no dedicated leprosy vaccine, but the BCG vaccine, developed for tuberculosis, provides partial cross-protection. A large trial in Malawi estimated that BCG reduced the risk of leprosy by at least 50%, and the protection appeared to be even stronger against the more severe, multi-bacterial forms of the disease.²³ Because BCG is widely given to children in countries where leprosy is common, it likely contributes to the overall decline in cases, even though it was not designed for that purpose.

Several groups are working on leprosy-specific vaccines and post-exposure prophylaxis strategies. Single-dose rifampicin given to close contacts of newly diagnosed patients has shown some promise in reducing new cases, and research is ongoing into combining it with BCG or newer vaccine candidates. The challenge is that leprosy’s long incubation and low attack rate make vaccine trials slow and expensive, since you need enormous numbers of participants followed for many years to see whether an intervention made a difference.

Red Squirrels and Medieval Persistence

Armadillos are not the only animals carrying leprosy bacteria. On Brownsea Island in southern England, red squirrels were found to be infected with *M. leprae* strains whose genetic sequences most closely matched those recovered from buried humans in 13th-century England and 14th-century Denmark.²⁴ A medieval red squirrel skeleton from nearby Winchester carried a similar strain. The implication is striking: *M. leprae* may have persisted in British wildlife for centuries after human leprosy disappeared from the region, essentially circling in a quiet animal reservoir while the disease vanished from clinical view.

Ancient DNA studies have found remarkable diversity of *M. leprae* strains in medieval European skeletons, suggesting the disease was far more widespread and genetically varied in the past than it is today.²⁵ Some of these medieval strains fall on the same branches of the bacterial family tree as strains now found in armadillos and red squirrels, which hints at a long history of interspecies transmission. Migrations of human populations across Eurasia during the first millennium likely shuffled different *M. leprae* lineages across the continent, introducing new strains and fueling medieval leprosy epidemics.²⁶

There is a second species capable of causing leprosy in humans, *Mycobacterium lepromatosis*, whose genome is remarkably similar to that of *M. leprae* despite the two having diverged from a common ancestor roughly 14 million years ago.²⁷ So far it has been found almost exclusively in patients from Mexico and has been linked to a particularly severe form of the disease, but a survey of over 200 leprosy biopsies found it in only a handful of cases, all Mexican. Whether *M. lepromatosis* exists in animal reservoirs or environmental niches remains unknown. Its discovery underscores how much about leprosy’s ecology is still being worked out, even for a disease that has been recognized for thousands of years.

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