Onchocerciasis
Onchocerciasis, commonly known as river blindness, is a parasitic disease caused by the filarial worm Onchocerca volvulus, primarily affecting individuals in Africa, Latin America, and the Arabian Peninsula. The disease is transmitted through the bites of black flies from the Simulium genus, which introduce worm larvae into the human body. Symptoms often manifest several months after infection and can include painful skin nodules, severe rashes, and, in serious cases, vision impairment leading to blindness. The World Health Organization identifies onchocerciasis as the second-leading infectious cause of blindness worldwide, with more than 90% of cases concentrated in specific regions.
Diagnosis typically involves analyzing skin samples or nodules, while treatment primarily consists of ivermectin, which helps control the infection and prevent further health deterioration, although it does not eliminate existing adult worms. Preventive measures have included community-directed treatment programs and insecticide spraying to reduce black fly populations. Despite significant efforts, the disease remains a public health challenge in endemic areas, necessitating ongoing awareness and intervention strategies.
Onchocerciasis
- ANATOMY OR SYSTEM AFFECTED: Eyes, lymph nodes, skin, tissue, vision
- ALSO KNOWN AS: River blindness
Definition
Onchocerciasis is a parasitic infestation by filarial worms (Onchocerca volvulus) that affects persons in Africa, Latin America, and the Arabian Peninsula. Worm larvae enter the body from black-fly bites, causing nodular skin swelling that may progress to a harmful eye disease known as river blindness. According to the World Health Organization (WHO), more than 90 percent of infected people live in thirty-one African countries and two Latin American countries, and onchocerciasis is the second-leading infectious cause of blindness worldwide.
Causes
Black flies (genus Simulium) feed on the blood of infected people and ingest microfilariae, the embryos of worms, which then mature into larvae in the gut of the fly within seven days. The larvae are then deposited into other persons through fly saliva. The larvae develop into adult worms and then live and reproduce in firm nodules in the subcutaneous and deeper layers of skin. Adult worms produce numerous microfilariae, which travel from the parent nodule and move throughout the skin. The presence of the microfilariae, dead and alive, causes the body to have a powerful immune response, leading to a severe inflammatory reaction that damages surrounding skin and eye tissue.
Risk Factors
Persons who live in Africa, Latin America, and the Arabian Peninsula near streams and rivers, the breeding habitat for the black fly, are at the greatest risk for developing onchocerciasis.
Symptoms
Symptoms of onchocerciasis may not appear until three to fifteen months after infection. Early indicators include skin nodules that contain two or more adult worms. The migration of microfilariae causes a severe rash and painful, hot, or swollen skin. Lymph nodes in the neck and groin can become enlarged. Chronic infection may lead to thickened, pigmented, or depigmented skin, often in a lizard or leopard pattern. If the microfilariae migrate to the eye, the body’s immune system responds by destroying the eye tissue, leading to deteriorating vision. People infected with only a few larvae may not experience any noticeable symptoms at all.
Screening and Diagnosis
The diagnosis of onchocerciasis is commonly made by analyzing skin snips that contain microfilariae or by excision of the nodule containing adult worms. A dipstick test for the presence of an antigen in urine or tears has been in development. Antibody tests and slit lamp examination of the eye are additional screening methods.
Treatment and Therapy
In 1987, the World Health Organization collaborated with pharmaceutical company Merck to develop ivermectin, the most effective treatment for onchocerciasis. Ivermectin is administered annually or biannually for ten to fifteen years, although treatment recommendations can vary. Community-directed treatment programs have been established to provide treatment at no cost. Damage done to the skin and eyes cannot be reversed, but treatment prevents further deterioration by killing the microfilariae. Although treatment does not kill the adult worms, it prevents them from reproducing.
Prevention and Outcomes
WHO has initiated several prevention programs, including the Onchocerciasis Control Programme (1974-2002), the African Programme for Onchocerciasis Control (1995-2015), and the Onchocerciasis Elimination Program for the Americas (1992-). These programs' preventive measures included spraying insecticides to prevent blackfly breeding in affected areas and distributing ivermectin treatment to persons and communities in need. In the twenty-first century, the WHO's Expanded Special Project for Elimination of Neglected Tropical Diseases has concentrated on fighting onchocerciasis with community-directed treatment with ivermectin.
Bibliography
"Clinical Overview of Onchocerciasis - Filarial Worms." CDC, 14 May 2024, www.cdc.gov/filarial-worms/hcp/clinical-overview/onchocerciasis.html. Accessed 31 Oct. 2024.
Expanded Special Project for Elimination of Neglected Tropical Diseases, espen.afro.who.int. Accessed 31 Oct. 2024.
"Onchocerciasis." World Health Organization, 11 Jan. 2022, www.who.int/news-room/fact-sheets/detail/onchocerciasis. Accessed 31 Oct. 2024.
"Onchocerciasis (River Blindness)." World Health Organization (WHO), www.who.int/health-topics/onchocerciasis#tab=tab‗1. Accessed 31 Oct. 2024.
Stingl, Peter. "Onchocerciasis: Developments in Diagnosis, Treatment, and Control." International Journal of Dermatology, vol. 48, 2009, pp. 393–396.
Taylor, Mark J., et al. "Lymphatic Filariasis and Onchocerciasis." The Lancet, vol. 376, no. 9747, 2010, pp. 1175-1185, doi:10.1016/S0140-6736(10)60586-7. Accessed 31 Oct. 2024.
Udall, Don N. "Recent Updates on Onchocerciasis: Diagnosis and Treatment." Clinical Infectious Diseases, vol. 44, no. 1, 2007, pp. 53-60, doi.org/10.1086/509325. Accessed 31 Oct. 2024.