Hansen's disease (Leprosy)

Leprosy is a chronic infection primarily affecting the skin and peripheral nerves caused by Mycobacterium leprae, one of the last bacterial species of medical interest that cannot be cultured in vitro. Belonging to the genus Mycobacterium and family Mycobacteriaceae, it is a rod-shaped bacillus that appears immobile on microscopic analysis. It is a microaerophilic and slow-growing bacterium that ideally develops at a temperature between 30° and 35°C. The bacilli cluster in highly infected tissues to form clumps of bacilli (known as globules) that can number in the hundreds. A new species of mycobacteria, known as M. lepromatosis, isolated from two deceased patients infected with disseminated lepromatous leprosy, was also identified in 2008. This type of leprosy was first described in 1852 in Mexico by the scholars Lucio and Alvarado and is named Lucio's Phenomenon after the former. The results of whole genome sequencing of the M. lepromatosis species conducted during the 2008 studies revealed that M. leprae and M. lepromatosis are quite similar in terms of phylogenetics. In fact, both are derived from the same ancestor and diverged more than 13.9 million years ago.

The exact system of transmission of leprosy is still unknown.
It is known that leprosy spreads through contact between infected patients and healthy individuals, affecting individuals of all age groups. Several hypotheses have been proposed in the literature: transmission by the respiratory route, particularly the upper tract, or transmission through insects.
Humans are the main reservoir of infection, but nevertheless, infection among African monkeys and armadillos has been reported in Louisiana.

Leprosy is endemically ubiquitous in tropical countries, particularly in underdeveloped and developing countries, with more than 200,000 new cases each year. A considerable number of cases occur in Southeast Asia, the Americas, Africa, the Eastern Pacific, and the Western Mediterranean The largest number of reported cases occurs in Brazil, Indonesia, and India, where in particular more than 60 percent of all new cases are reached. The epidemiology of leprosy is difficult to understand because of the proposed continuous variations in its definition, its long incubation period, and changes in control activities that cause a delay in case detection.

The various presentations of the disease are related to the patient's immune response, infectious load, and delay in diagnosis.
Leprosy primarily affects the skin and peripheral nerves. Skin lesions are usually the first visible sign, which is why it is considered a dermatologic disease. These lesions are usually macules, hypopigmented or erythematous papules, or even nodules and plaques, the same color as the skin or slightly reddened.
Secondary infections caused by trauma resulting from sensory loss in turn cause tissue loss. Bone and cartilage, in fact, are absorbed by the infection by shortening and deforming.
As a result of nerve damage, patients may complain of loss of sensation in skin lesions, hands, or feet. They may have facial or limb pain or mention a numbing sensation, like "ants running under their skin" of the affected areas.
Tissue damage, however triggered by infiltration of the bacterium, is largely due to immune system reactions.
If left untreated, leprosy can progress causing permanent damage to the skin, nerves, limbs and eyes.

Early diagnosis of the injury is critical for timely and correct implementation of treatment. This helps prevent the consequences of the disease that could lead to physical disabilities with immeasurable impact on the individual's social and personal life, then adding to the stigma attached to the disease.

The diagnosis of leprosy in a patient who has yet to complete a full course of treatment is established on the manifestation of at least one of three basic symptoms:
- a pale or reddish patch of skin with localized loss of sensation;
- a thickened or enlarged peripheral nerve associated with loss of sensation and/or weakness of the muscles supplied by that nerve;
- evidence of acid-fast bacilli in a slit skin smear.

Depending on the number of lesions, WHO has classified leprosy into:

- multibacillary disease, when the affected individual has more than 5 skin lesions or more than 1 nerve involvement or positive skin smear at any site.
- paucibacillary disease, diagnosed if there are fewer skin lesions or no nerve involvement, or if there is a single nerve involvement with negative skin smears at all sites.
This classification is useful therapeutically because paucibacillary cases are treated for 6 months while multibacillary cases are treated for 12 months using different treatment regimens.
Several other classifications have included different aspects such as clinical, immunological, and histopathological. For example, the so-called Ridley-Jopling classification of leprosy originated in 1966, is based on clinical features, histopathology and bacteriological analytical index and classifies leprosy into five groups:

- Tuberculoid (TT)
- Borderline tuberculoid (BT)
- Mid-borderline (BB)
- Borderline lepromatous (BL)
- Lepromatous (LL)

The Indian scenario adds another variant which is a pure neurotic type characterized only by nerve thickening and sensation impairment without skin lesions and is confirmed by nerve biopsy.
The diagnosis is further strengthened by histopathological examination of the skin biopsy. This indicates that once a full course of medication is taken, the patient is "cured" and thus no longer considered a case of leprosy.

Multidrug therapy revolutionized the duration of treatment by bringing it to a short fixed period. Multidrug therapy (dapsone, rifampicin, with or without clofazimine) has been recommended since 1982 as the standard treatment of leprosy, with duration:

- 6 months for patients with paucibacillary leprosy
- 12 months for patients with multibacillary leprosy.

Worldwide use of leprosy drugs began in the 1980s, and their free access since 1995 has contributed to the dramatic decline in the number of new patients.
However, it must be remembered that after pharmaceutical therapy, the disease can recur due to insufficient treatment, resistance, persistence or new infections.

Despite many efforts to develop a universal active vaccine involving DNA techniques, no specific and effective solution against leprosy is available. The BCG (bacillus of Calmette-Guérin) antitubercular vaccine provides some protection against leprosy, but it is not commonly used to prevent infection.
Although the risk of contracting leprosy is low today, the best way to prevent it is to avoid close physical contact with untreated sick people.