- There are more cases of Hansen’s disease reported in Brazil than any other nation besides India. Even though the disease is highly prevalent, scientists are developing new studies to map out its occurrence with greater precision.
- One of the main objectives of the new studies is to prove that Hansen’s disease is zoonotic and can be transmitted to humans who illegally hunt wild armadillos and handle their meat for consumption.
- A study carried out by Brazilian researchers and published in May revealed a strong association in Brazil between direct contact with armadillos and a high risk of contracting Hansen’s disease.
- The study led to a new multinational project financed by the Leprosy Research Initiative in the Netherlands. It also emphasizes the role that conservation plays in containing the dissemination of zoonotic diseases and calls for new environmental and sanitation policies in Brazil.
Hansen’s disease (also known as leprosy) has afflicted humans for thousands of years — it has been cited throughout history, even in passages of the Bible. Yet science has still not determined whether it originated in Asia or in Africa.
But Hansen’s disease’s modern name has a very traceable history. In 1873, Norwegian physician Gerhard Armauer Hansen identified the bacillus that causes Hansen’s disease: Mycobacterium leprae. From that point on, the illness was named after the researcher and typically referred to as Hansen’s disease rather than leprosy in scientific literature.
More than a century and a half later, Hansen’s findings are still a hot topic in Brazil because they lie at the center of a public health crisis: This is the nation with the second-largest number of registered cases of the disease, ranking only behind India.
Part of the problem is the disease’s transmission, which happens when droplets of saliva from infected individuals come into contact with the airways of a healthy person. Coughing and sneezing facilitate the propagation of these bodily fluids.
The bacteria themselves are silent, making them harder to detect because the microorganism can take years, or even decades, to manifest itself. Once it has become active, it affects peripheral nerves and the skin, causing spots, lesions, tingling and even the loss of feeling in some parts of the body.
Another uncertainty lingers regarding Hansen’s disease. Even though many indicators point in this direction, Brazilian scientists still have not come to consensus in classifying it as a zoonosis — since transmission episodes are still considered anomalous and extremely localized.
But recent studies are seeking to connect the dots and increase the scope of classification, which may be crucial for creating prevention strategies across Brazil.
Following the tracks of Hansen’s disease
In tropical regions, scientific studies have shown that different armadillo species — like the nine-banded armadillo (Dasypus novemcinctus), the six-banded armadillo (Euphractus sexcinctus) and the Brazilian three-banded armadillo (Tolypeutes tricinctus) — play the role of zoonotic reservoirs. This term is used to define individuals that carry pathogenic organisms, and in many cases, those which cause Hansen’s disease.
These organisms become infectious vectors in a process known as spillover infection. In the case of armadillos, the infected content that they carry can pass to humans during the physical contact that happens during hunting. The risk increases during handling and consumption of the contaminated meat of the dead animal. Armadillo is commonly eaten in certain rural regions of Brazil.
Even though it is a customary practice in Brazil, hunting armadillos is prohibited anywhere inside the country and may be punished with fines and even imprisonment. Sentences range from six months to a year of prison and fines may reach 5,000 reais ($937) if the crime involves an endangered species.
“Some studies have proven the existence of zoonotic transmission between armadillos and humans in the United States. But no studies have been done that really prove [that relationship] in Brazil,” said Brazilian biologist and molecular ecologist Naiara Sales, who works at the University of Salford in the United Kingdom.
“The bacteria have been identified in armadillos. But we have not yet been able to determine if the same bacteria found in these animals are also found in humans. So, this question came up: Is this just something that happens? Or could there be zoonotic transmission?” Sales said.
Zoonotic diseases are those transmitted between animals and humans. They may also have only zoonotic origins, meaning they come from a pathogenic organism present in animals, jump to human bodies but, after establishing themselves there, no longer need the host’s presence.
According to biologist Paula Prist, a coordinator at the IUCN, it is crucial to investigate these diseases so they can be mapped out and outbreaks can be anticipated by science.
“When we have a new pathogen, like COVID-19 for example, which was unknown [before the pandemic], its origins need to be investigated: Who is the reservoir, who is the host and how transmission occurs. If it is a known zoonosis, monitoring will take place in order to observe potential recurring outbreaks or even new outbreaks in new regions,” Prist told Mongabay.
This is why researchers from many different institutions in 10 countries including Brazil will develop research projects financed by the Leprosy Research Initiative in the Netherlands over the next three years to identify zoonotic characteristics of Hansen’s disease in Brazil.
Started in April, the project has established a collaborative network to gather flesh and blood samples from different armadillo species across Brazil. The material collected will undergo analysis to detect possible bacteria.
“We want to sequence [the bacteria’s] genome. We are not only looking for information on a fragment of its genome, but other extensive data. Then later we will do a comparative analysis with the bacteria found in humans. It’s through this genetic similarity that we will be able to understand if zoonotic transmission actually occurs or not,” said Sales, who is part of the new project’s scientific team.
According to her, this work could result in important conclusions for Brazilian science. “Hansen’s disease is still not considered to be a zoonotic disease in Brazil. There is a correlation, there are some indications [to the contrary], but we don’t have concrete evidence.”
New discoveries could aid in medical treatment
Specialists say the proof of zoonotic aspects of Hansen’s disease could, among other technical factors, result in more precise diagnoses and treatments.
“It is a difficult disease to diagnose. Sometimes people have symptoms, even in the more advanced stage, but there are few indications [as to the forms of contagion],” Sales said. “Understanding the relationship between the disease and hunting of the animals could help health care professionals suspect [this form of transmission]. They could ask if the person takes part in the activity or if they live with someone who does.”
According to Sales, this will allow for faster treatment for patients, which could in turn help to interrupt transmission cycles.
One of the studies that led to the new project on Hansen’s disease zoonosis in Brazil was published in May 2025 and involved researchers in Brazil and overseas from a number of different institutions, including Rondônia Federal University, the University of São Paulo (USP) and the Institute for Wildlife Conservation (ICAS), among others.
Through biogeographic analyses showing how illegal hunting contributes to prevalence of the disease, scientists hope to provide the data necessary for public health agencies to take action in critical locations experiencing contamination.
According to the investigation, many case studies “consistently show a strong association between direct contact with wild armadillos and an elevated risk of contracting leprosy, especially among hunters and consumers of these animals.”
The study analyzed some 326,000 cases of Hansen’s disease reported in Brazil by its SUS Unified Health System between 2013 and 2022 by crossing the information with data on 554 wild armadillos hunted in 175 municipalities. Social media posts about hunting activities were used in the study to compete the mapping.
The M. leprae bacterium was identified in 376 armadillos collected in 97 municipalities, representing a 38.5% prevalence rate. “These were used to build spatial models assessing hunting-related infection risk and integrated as a variable into a generalised linear model alongside socioeconomic, climatic, and environmental predictors to evaluate their effects on human leprosy prevalence,” the study describes.
Data on malnutrition, population density, vegetation, socioeconomic conditions and even rainfall variation in different regions were considered in the study.
Alisa Aliaga-Samanez, a researcher in ecology and public health in Malaga and one of the co-authors of the study, told Mongabay the results could lead to important technical tools for many different countries in the Americas where Hansen’s disease exists.
“Creating these models was important for the entire continent, and especially for Brazil. Before this study, we really didn’t have clear knowledge as to whether hunting really was a significant route of transmission.”
Conservation is key in fighting infections
According to the study, the factors that most contribute to the existence of armadillo hunting are high population density and the ability to own firearms. The study also points to environmental degradation as a factor able to increase risk of infection.
Contamination by hunting of the animals shows as the second greatest indicator of prevalence of Hansen’s disease in humans, representing around 25% of the cases in all of Brazil. And this number jumps to 40% in areas with “high deforestation rates.”
These conclusions strengthen previous evidence about how changes to habitat like deforestation and the conversion of biomes to land used for pasture can increase human exposure to potentially infected wildlife.
According to the specialists, these new studies are aiming to leverage the revision of national public policy so that illegal hunting can be reduced and sanitation controls be increased in regions at risk. Scientific information on zoonotic risk is spread to encourage people to report cases of Hansen’s disease more consistently in Brazil.
According to Prist from IUCN, the preservation of biodiversity is absolutely fundamental if contact between new pathogenic organisms and humans is to be avoided. As she explained, about 75% of documented emerging infectious diseases come from wild animals.
“In general, new diseases arise when there is contact between human beings and animals. This means that biodiversity serves as a barrier: The more biodiverse a region is, the fewer species who are [zoonotic] reservoirs will live there and fewer of the pathogens they carry. When an area is deforested, species who transmit diseases can be ‘selected’ — and in this way, they become dominant together with their pathogens.”
The One Health concept is gaining recognition around the globe today — an approach that supports the connection between human, animal and environmental health. Supported by a single pillar, these three must all be cared for in order to maintain balance.
“The climate crisis is something very serious. It will be catastrophic for our health. We are on a path leading to a situation where these pathogens will be more and more prevalent and bring with them greater risks. We are not prepared for it,” Prist said.
Banner image: A nine-banded armadillo (Dasypus novemcinctus) in a park in Campo Grande, Mato Grosso do Sul. Image by Wikimedia Commons
Citation:
Aliaga-Samanez, A., Deps, P. D., Fa, J. E., Real, R., Guégan, J., Oliveira, M. A., … El Bizri, H. R. (2025). Wildlife hunting and the increased risk of leprosy transmission in the tropical Americas: A pathogeographical study. Infectious Diseases of Poverty, 14(1). doi:10.1186/s40249-025-01301-z
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