Onchocerciasis, commonly known as river blindness, is caused by the parasitic filarial worm Onchocerca volvulus. The pathogenesis of onchocerciasis involves several key steps:
1. Transmission
The O. volvulus larvae are transmitted to humans through the bite of infected female black flies (genus Simulium), which typically breed in fast-flowing rivers and streams. When an infected fly takes a blood meal, it injects the larvae (microfilariae) into the human host.
2. Development and Migration of the Worm
Once inside the human body, the microfilariae migrate through the subcutaneous tissue and eventually develop into adult worms. Adult O. volvulus worms can live for many years (up to 15 years or more) and typically reside in nodules called onchocercomas, which form in connective tissue.
3. Immune Response
The presence of adult worms and their microfilariae can elicit a strong immune response. Humans host a complex immune reaction, including the activation of T-helper cells, B cells, and the production of various cytokines.
4. Pathology Due to Microfilariae
The microfilariae released by adult worms migrate throughout the body and can cause significant tissue damage. They are known to reside primarily in the skin, eyes, and lymphatic system. The most prominent pathological effects occur when microfilariae invade the eyes, leading to:
- Dermatitis: As microfilariae migrate through the skin, they cause intense itching, inflammation, and skin lesions (often referred to as "skin onchocerciasis").
- Ocular Damage: When microfilariae invade ocular tissues, they can lead to a range of serious eye problems, including keratitis, chorioretinitis, and ultimately, blindness. The immune response to these invading microfilariae contributes to the damage to ocular tissues.
5. Long-term Consequences
Long-term infection with O. volvulus can lead to various complications:
- Skin Changes: Chronic onchocerciasis can result in thickened, discolored skin known as "leopard skin," as well as depigmentation.
- Visual Impairment: The chronic inflammatory response and direct damage from microfilariae can lead to severe visual impairment, and if untreated, can progress to permanent blindness.
Summary
The pathogenesis of onchocerciasis involves transmission by black flies, development of adult worms and microfilariae, immune responses that lead to tissue damage, and the potential for long-term consequences, including skin disease and blindness. Effective treatment typically involves antiparasitic medications (such as ivermectin) that target the microfilariae and a public health approach to reduce transmission.