Written by Samhita Uppalapati
Abstract
Leprosy, a fairly rare disease, afflicting around only 208,000 people worldwide (Miller), is caused by the intracellular pathogen Mycobacterium leprae . The pathogen, M. leprae, is a slow-growing bacillus discovered by Armauer Hansen in 1873, which is why the disease is also often called Hansen’s disease. The bacterium primarily targets the skin, peripheral nerves, and mucosal surfaces of the respiratory tract, resulting in a range of clinical manifestations and symptoms.
Intracellular: within the cell
Peripheral nerves: nerves that branch out from the brain and spinal cord and connect to all parts of the body
1. Causes and Transmission:
The bacteria M. leprae, is transmitted primarily through respiratory droplets released by individuals with untreated lepromatous or multibacillary leprosy. Coughing, sneezing and more will all spread the disease, however casual contact will not. However, transmission through these droplets remains one of the bigger questions in leprosy, as often, a significant portion of people who come in contact with them still end up not contracting the disease. Additionally, it is thought that contact with the infected person must be very prolonged, over the course of many months, for the disease to spread. (CDC) Therefore, genetic susceptibility and the host immune response play pivotal roles in determining the outcome of M. leprae exposure.
1.1 Genetic Factors and Immune Response:
Genetic predisposition significantly influences susceptibility to leprosy. Variants in genes associated with the human immune system, particularly those involved in the recognition and response to M. leprae, contribute to individual susceptibility or resistance. The spectrum of leprosy, ranging from tuberculoid to lepromatous forms, makes clear the critical role of the body’s immune response. Tuberculoid leprosy is characterized by a strong cell-mediated immune response, effectively controlling the infection, while lepromatous leprosy exhibits a weakened immune response, permitting bacterial proliferation.
1.2 Pathogenesis:
M. leprae primarily infects macrophages and Schwann cells within the peripheral nerves. The bacterium's unique cell wall composition and ability to reside intracellularly render it less susceptible to standard antimicrobial therapies. As a result, the infection progresses insidiously, with an extended incubation period ranging from several months to years. The bacteria's predilection for peripheral nerves leads to nerve damage, sensory loss, and the characteristic deformities associated with leprosy.
Macrophage: A type of white blood cell that digests foreign microorganisms
Schwann Cells: a type of cell that surrounds neurons and assists in regeneration of peripheral nerves (physiopedia)
2. Clinical Manifestations and Symptoms:
Leprosy can present itself in a spectrum of forms, the 2 ends being paucibacillary (tuberculoid) and multibacillary (lepromatous) forms. The former, Tuberculoid leprosy is characterized by well-defined, hypopigmented skin lesions, and peripheral nerve involvement may lead to sensory and motor deficits. (Ramos-e-Silva and Rebello). In contrast, lepromatous leprosy manifests as widespread skin lesions with a higher bacterial load, resulting in systemic involvement, deformities, and potentially severe complications. The lesions themselves are also more prevalent than tuberculoid, and are far more contagious.
Figure 1: Example of skin lesion
Eichelmann. “Tuberculoid Leprosy.” Research Gate, Online Image, 4 July 2022. Accessed 24 Jan. 2024.
Systemic Involvement: Involving the entire body
Overall, while Leprosy may not be of major concern to those living in more developed countries, it still affects many people across the world and can have very severe effects, so it is important to gain further knowledge and awareness regarding it, as well as how to recognise it.
Works Cited
CDC. “Transmission | Hansen’s Disease (Leprosy) | CDC.” Www.cdc.gov, 15 Jan. 2019, www.cdc.gov/leprosy/transmission/index.html#:~:text=Scientists%20currently%20think%20it%20may. Accessed 23 Jan. 2024.
Medline Plus. “Systemic: MedlinePlus Medical Encyclopedia.” Medlineplus.gov, 2 Feb. 2023, medlineplus.gov/ency/article/002294.htm#:~:text=Systemic%20means%20affecting%20the%20entire. Accessed 23 Jan. 2024.
physiopedia. “Schwann Cell.” Physiopedia, 2022, www.physio-pedia.com/Schwann_Cell#:~:text=Schwann%20cells%20(SCs)%20are%20a. Accessed 24 Jan. 2024.
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