A chronic infectious disease, leprosy is far from having disappeared from the surface of the globe. It is still prevalent in many parts of the world, with about 200,000 new cases diagnosed each year.

Leprosy is still a global public health issue. While effective treatments exist, the disease still claims victims among some of the world's poorest populations. Leprosy is a very old disease, described for thousands of years in many parts of the world. According to the classification established by the World Health Organization (WHO), it is one of the Neglected Tropical Diseases (NTDs). These diseases mainly affect poor communities and have a deleterious impact, not only on health but also on society and the economy.

1. What is leprosy? Definition

Leprosy is an infectious disease caused by a slow-growing bacterium called Mycobacterium leprae. This bacterium, which has similarities to the tuberculosis bacillus, was identified in 1873 by the Norwegian bacteriologist and dermatologist Gerhard Armauer Hansen. He gave his name to the leprosy bacillus, which is still sometimes called "Hansen's disease". There are two types of leprosy: paucibacillary leprosy (one to five insensitive skin lesions) and multibacillary leprosy (more than five insensitive skin lesions).

Leprosy is a long-standing incurable, mutilating disease that attacks the skin, peripheral nerves, eyes, limbs and mucous membranes of the upper respiratory system. Because of the stigma, it can cause, leprosy is considered a shameful disease, synonymous with rejection. Many patients are still discriminated against.

2. Leprosy in the world: a disease that is still very present

According to WHO figures, which take into account official data reported by 161 countries, 202,256 new cases of leprosy were registered worldwide in 2019 across 116 countries. Of the new cases, 38.9 percent were women and 7.4 percent were children. The majority of new cases detected were in Southeast Asia, the Americas, and the African continent. The most affected countries are Brazil, DRC (Democratic Republic of Congo), Nepal, India, Indonesia, Mozambique, and Tanzania. Leprosy is also endemic in many other countries such as Angola, the Central African Republic, and Madagascar.

The WHO estimates that 3 to 4 million people in the world today live with visible disabilities or malformations due to leprosy. This is difficult to assess due to the nature of the disease, which often leads to social exclusion within the affected communities.

3. The transmission of leprosy: a disease that is not very contagious

Leprosy is transmitted through droplets (cough, sputum) and nasal secretions. While leprosy was long thought to be a highly contagious disease, it is now known that close, frequent, and long-lasting contact with a person carrying the mycobacterium responsible for leprosy is necessary to become infected.

If leprosy is not very contagious, the danger lies in its very long incubation period, 5 years on average. A person can even remain asymptomatic for 20 years while transmitting the disease. The spread of leprosy is therefore extremely difficult to control, which partly explains why the disease is still so prevalent in some countries.

4. The symptoms of leprosy

As we have seen, a person can be infected with the leprosy bacillus and remain asymptomatic for several years. Once the first symptoms appear, they also evolve slowly, but it is precisely at this point that intervention is necessary to avoid major sequelae. The disease may be mild, with only a few affected areas, or much more severe, with symptoms that are not only cutaneous. A distinction is usually made between tuberculoid (milder) leprosy, lepromatous leprosy, and borderline leprosy.

The symptoms of leprosy are as follows:

Discolored, usually flat patches of skin that may appear lighter than the surrounding skin.
Raised nodules on the skin, which may be larger or smaller, appear on different areas of the body, from the face to the genitals.
Thick, dry skin
Painless ulcers on the soles of the feet
Painless swelling or bumps on the face or earlobes
Loss of eyebrows or eyelashes

When there is nerve damage, other symptoms may appear such as:

Numbness in the affected areas of the skin
Muscle weakness or paralysis (especially in the hands and feet)
Nerve enlargement
Eye problems that can lead to blindness

5. The diagnosis of leprosy

The diagnosis of leprosy can be made by several tests. While the presence of skin lesions characteristic of leprosy and a loss of sensitivity can already point to a diagnosis, this must be confirmed by various tests.

Skin biopsy

Bacteriological examination of samples taken from the skin lesions or nasal mucus.

A PCR test

A serological examination to look for antibodies against phenolic glycolipid PGL-1, but the examination is often not very sensitive.

An early diagnosis, before the appearance of visible malformations, is obviously fundamental in order to start the treatment as soon as possible and thus avoid sequels. Patients excluded from the health system are often not detected, or until too late.

In its Global Strategy for Leprosy Control (2021-2030), the WHO insists on the challenge of diagnosing the disease, stating that there are still too many late detections. According to WHO figures, the proportion of people with ID2 (grade 2 disability) at the time of diagnosis is still far too high. Grade 2 disability means that patients already had one or more visible deformities, or lesions, sometimes with already a severe decrease in visual acuity. "Causes may include lack of diagnostic capacity, lack of point-of-care diagnostic tests to detect leprosy infection or disease, inadequacies in case detection and contact tracing programs, lack of community awareness, and stigma associated with leprosy," says the World Health Organization.

6. Complications of leprosy

Complications of leprosy can be multiple, and potentially very debilitating. Due to peripheral neuropathy and nerve damage, the patient may lose the ability to feel pain, but also heat or cold, and thus injure themselves.

Muscle damage can cause significant deformities, especially in the extremities.

The loss of fingers and toes clawed fingers.

Plantar lesions can be very painful and make walking difficult.

Erythema nodosum leprosum (ENL), is a serious immune complication of the disease. In addition to the painful swellings, the patient experiences general malaise with fever.

The eye damage may progress to glaucoma and blindness.

In lepromatous leprosy, renal failure may be observed.

In men, erectile dysfunction and impaired fertility.

7. The treatment of leprosy

Today, it is possible to treat leprosy and even cure it. It is an infectious disease like any other, even if it is still seen as a shame. It was the development of an antibiotic, dapsone, in 1941 that marked a real breakthrough in the treatment of leprosy. When the bacterium began to show resistance to this treatment, two other drugs were combined with it. Since 1981, WHO has recommended the use of multidrug therapy (MDT), which consists of the administration of three antibiotics (dapsone, rifampicin, and clofazimine). This is a long treatment, six months for paucibacillary leprosy and twelve months for multibacillary leprosy. The person ceases to be contagious rapidly after the start of treatment. If this treatment stops the evolution of the disease and cures the patient, it does not allow the lesions already present to disappear. Corticosteroids can be prescribed to treat inflammatory reactions.

It is also important to remember that leprosy disappeared from the West before the advent of antibiotics but was lost in less socially and economically advantaged regions of the world due to several factors: the difficulty of access to health centers, promiscuity, the vulnerability of populations to other infections, the difficulty of education and awareness.

8. Leprosy vaccine: research continues

In its Guidelines for the Diagnosis, Treatment, and Prevention of Leprosy, WHO recommends preventive treatment of contacts of a confirmed patient with a single dose of rifampicin. This chemoprophylaxis reduces the risk of leprosy in contacts by 60%, but it is not a substitute for an effective vaccine. However, there is no vaccine for leprosy.

In the United States, the LepVax project at the Infectious Disease Research Institute (Idri) has not yet resulted in the commercialization of an effective vaccine, but research is ongoing. Because of the similarities between the pathogens that cause leprosy and tuberculosis, BCG is used in some countries to control leprosy, but the results are not consistent and the tuberculosis vaccine will never be a substitute for a leprosy vaccine.