Summary about Disease
Onchocerciasis, also known as river blindness, is a parasitic disease caused by the filarial worm Onchocerca volvulus. It's transmitted to humans through the bite of infected blackflies (genus *Simulium*). The disease is endemic in parts of Africa, Latin America, and Yemen. It causes severe itching, skin disfigurement, and visual impairment, including blindness.
Symptoms
Intense itching (pruritus)
Skin rashes and discoloration
Skin nodules (onchocercomata) containing adult worms
Eye lesions, including blurred vision, photophobia (sensitivity to light), and eventually blindness
Lymphadenitis (swelling of lymph nodes), particularly in the groin
Hanging groin (elastic tissue of the groin loses elasticity due to inflammation)
Causes
Onchocerciasis is caused by infection with the parasitic worm Onchocerca volvulus. The worms are transmitted to humans through the bite of female blackflies infected with the parasite larvae (microfilariae). When a blackfly bites an infected person, it ingests the microfilariae. These microfilariae develop within the blackfly and are then transmitted to another person during a subsequent bite. Once inside the human body, the microfilariae mature into adult worms, which live in nodules under the skin and produce more microfilariae. It is the microfilariae migrating through the body that cause the itching, skin lesions, and eye damage.
Medicine Used
The primary medicine used to treat onchocerciasis is Ivermectin. Ivermectin paralyzes and kills the microfilariae but doesn't kill the adult worms. Therefore, it needs to be administered annually or semi-annually for the lifespan of the adult worms (10-15 years). Doxycycline has been shown to sterilize or kill the adult worms but treatment lasts multiple weeks.
Is Communicable
Onchocerciasis is not directly communicable from person to person. It is transmitted only through the bite of an infected blackfly.
Precautions
Avoid blackfly bites:
Wear long-sleeved shirts and long pants.
Use insect repellent containing DEET or picaridin.
Avoid areas near rivers and streams where blackflies are most prevalent, especially during peak biting hours (dawn and dusk).
Chemoprophylaxis: In endemic areas, regular ivermectin treatment is recommended as a preventive measure.
Vector control: Insecticide spraying in blackfly breeding sites can help reduce the blackfly population.
How long does an outbreak last?
An "outbreak" in the sense of a sudden, short-term surge in cases is not typically how onchocerciasis presents. Onchocerciasis is endemic in certain regions. Transmission occurs year-round, with the duration of the risk being essentially continuous in these areas, unless control programs are effectively implemented to reduce blackfly populations and/or parasite transmission. The "outbreak" effectively lasts until the disease is controlled through mass drug administration and vector control. The disease itself lasts for the lifespan of the adult worms in a person (10-15 years) without treatment.
How is it diagnosed?
Skin snip biopsy: A small sample of skin is taken and examined under a microscope for the presence of microfilariae. This is the most common diagnostic method.
Nodules: Palpation of subcutaneous nodules and, if present, excisional biopsy with microscopic examination.
Ophthalmological examination: Slit-lamp examination to detect microfilariae in the cornea or anterior chamber of the eye.
Antibody detection: ELISA tests can detect antibodies against Onchocerca volvulus antigens in blood samples, but these tests may not be specific enough to differentiate between current and past infections. PCR-based methods are also available in some research settings.
Timeline of Symptoms
Incubation period: Microfilariae appear in the skin within 1 year.
Early: Itching (pruritus) can begin months to a year after infection.
Progressive: Skin changes (rashes, discoloration) develop over months to years. Nodules form after several years.
Late: Eye lesions and visual impairment can occur after many years (10-15 years or more) of chronic infection. Blindness is a late-stage complication.
Important Considerations
Mass Drug Administration (MDA): Ivermectin MDA programs are crucial for controlling onchocerciasis in endemic areas. These programs aim to interrupt transmission by reducing the microfilarial load in the human population.
Community Participation: Successful control programs rely on community participation and health education.
Co-infection: In areas where onchocerciasis is co-endemic with Loa loa (another filarial worm), ivermectin treatment can cause severe adverse reactions in individuals with high Loa loa microfilarial loads. Pre-treatment screening for Loa loa may be necessary in these areas.
Sustainable Control: Long-term sustainability of control programs is essential to prevent resurgence of the disease.
Reinfection: Even after successful treatment, reinfection is possible in endemic areas if blackfly populations are not controlled.