Etiology of Leprosy: Causative Organism and Modes of Transmission

Leprosy, also known as Hansen's disease, is a chronic infectious disease caused by Mycobacterium leprae, a slow-growing bacillus that primarily affects the skin, peripheral nerves, and mucosal surfaces of the upper respiratory tract.

Causative Organism: Mycobacterium leprae

Mycobacterium leprae is an acid-fast, intracellular bacterium with a unique preference for cooler areas of the body, such as the skin and extremities. It has a remarkably slow replication rate, making diagnosis and disease progression difficult to track.

Modes of Transmission

Leprosy is transmitted primarily through prolonged close contact with an infected individual. The recognized modes of transmission include:

• Respiratory Droplets: The most widely accepted method of transmission occurs via inhalation of nasal secretions from infected individuals.
• Skin-to-Skin Contact: While less common, direct contact with an open lesion may lead to transmission.
• Vector Hypothesis: Some studies suggest insects might play a minor role in transmission, but conclusive evidence is lacking.

Pathogenesis of Leprosy

Upon entering the human body, M. leprae infiltrates macrophages and Schwann cells, leading to nerve damage and dermal manifestations. The immune response determines the clinical form of the disease:

• Tuberculoid Leprosy: Strong cellular immunity limits bacterial spread, resulting in localized skin lesions.
• Lepromatous Leprosy: Weakened immune response allows bacterial proliferation, leading to widespread skin nodules and nerve involvement.

Clinical Features

Leprosy presents with skin patches, nerve dysfunction, and sensory loss. Common manifestations include:

• Hypopigmented or erythematous skin lesions.
• Loss of sensation in affected areas.
• Thickened peripheral nerves leading to motor dysfunction.

Diagnostic Features

Diagnosis relies on clinical examination, skin smears, histopathology, and polymerase chain reaction (PCR) testing to detect M. leprae DNA.