The disease affects the skin and the eyes, and it is one of the leading causes of preventable or avoidable blindness, second only to trachoma (a bacterial infection of the eye caused by Chlamydia trachomatis), notes the Centers for Disease Control and Prevention (CDC). (1) The World Health Organization (WHO) lists onchocerciasis as one of 20 “neglected tropical diseases” (along with trachoma). (2) There is no preventive medicine or vaccine against onchocerciasis, but antiparasitic drugs used to treat the active disease in people have helped eliminate river blindness from multiple countries and regions, according to the Carter Foundation. (3) The blackfly is typically found near fast-running streams and rivers in intertropical zones. (4) When the blackfly bites someone who already has onchocerciasis, it can pick up microscopic Onchocerca worm larvae called microfilariae. These larvae will develop in the fly for more than a week, at which point they’ll become infectious to humans.
How Onchocerciasis Is Transmitted and Spreads
If the infected fly then bites a person, it will usually leave behind the worm larvae, which enter the body through the bite wound. The larvae grow in subcutaneous tissue (below the skin) and develop into adults called filariae. The filariae live in nodules (nests) in their human host’s subcutaneous tissues — though nodules can sometimes have up to 50 worms in them, most of these nests only have three to five worms. Adult worms can live for up to 10 to 15 years in the nodules; females are able to produce up to 1,000 microfilariae in a single day and millions over the course of their lives. These larvae live up to two years in people and may migrate through subcutaneous skin to other areas of the body, where they may be sucked up by unsuspecting blackflies. If the microfilariae do not get taken up by blackflies, they will die and release bacteria, causing an inflammatory reaction (and associated symptoms) in the area they perished in. (1,5,6) People with river blindness may be asymptomatic, at least for the first 24 months while the microfilariae develop. Initial symptoms typically include various skin changes, including:
Bumps under the skin that may be over 1 inch in diameter (these bumps are the nodules)Skin irritation and itching, which may be localized at first and then spread to the whole bodyAreas of swelling on the skinThinning of the skinEnlarged groinPatches of skin with pigment or color changes (7,8)
Over time, the skin may waste away and lose elasticity, eventually resulting in a hanging groin (a sac of thinned skin containing hardened lymph nodes in the groin area). The aforementioned pigmentation issues may also cause a condition known as leopard skin, particularly in the lower legs. (9)
How Does Onchocerciasis Affect the Eyes?
Onchocerciasis is known as river blindness because of the effects the disease can have on the eyes. The microfilariae can travel into the tissues of the eye. The larvae can affect all types of ocular tissues, from the eyelid and conjunctiva to the cornea and sclera and beyond. When the microfilariae die, they release Wolbachia bacteria, which live symbiotically with the worms and are actually necessary for adult females to reproduce. The Wolbachia bacteria elicit the immune response and inflammation associated with onchocerciasis, causing itching, light sensitivity, eye redness, and cataracts (clouding of the front surface of the eye). In more severe cases, the worms and bacteria can cause lesions, optic atrophy (worsening of vision caused by the deterioration of optic nerves), and glaucoma. The disease can ultimately cause vision impairment and blindness. (7,8,10)
What Is a Skin Snip Biopsy?
If it appears that you may have the disease based on your symptoms, your doctor will conduct a type of skin biopsy called a snip. If you have microfilariae in your skin, they will emerge from the snips when placed in a saline (saltwater) solution. Alternatively, your doctor may run blood tests to look for antibodies for the parasite. Neither test is 100 percent accurate in diagnosing onchocerciasis, however. The snips may not always reveal the parasites and a positive blood test doesn’t necessarily mean you are still infected with the parasite, according to the CDC. (1,8) In Africa, doctors have found skin patch testing to be an effective alternative for onchocerciasis diagnosis. The test involves placing onto the skin a patch with the drug diethylcarbamazine, which can kill onchocerciasis and cause a type of hypersensitivity reaction that reveals the disease. (10) However, the widespread death of the microfilariae in the body produced systemic and potentially severe side effects, including rashes, fevers, body pains, uveitis (an inflammation of part of the eye), and even anaphylaxis. Today, onchocerciasis is treated with the drug ivermectin (Stromectol). Rather than kill the macrofilariae (the adult worms), ivermectin kills the microfilariae (or larvae) and sterilizes the adult female worms in the body for up to six months, preventing them from releasing more microfilariae into the body. (10) The drug is given as a single dose every 6 to 12 months over the course of a decade or more, until the last of the adult worms die naturally, though treatment may be longer if the person treated is reinfected by subsequent blackfly bites. Research shows that ivermectin can help prevent optic atrophy when treatment starts early in the disease progression, as well as reduce visual field loss and corneal inflammation. (11) If the disease has progressed to the point of affecting the eyes, surgery may be necessary. Possible surgical techniques for onchocerciasis-related issues include:
Corneal transplant to replace a scarred cornea with a healthy donor corneaGlaucoma surgery, to reduce eye pressureCataract removalRetinal surgeries (7)
New Treatments for Onchocerciasis
New treatments for onchocerciasis, which scientists are still studying, target the symbiotic Wolbachia bacteria rather than the parasitic worms. The drug doxycycline (Vibramycin), which kills Wolbachia, appears to cause both long-lasting — and possibly permanent — sterilization of adult worms and early worm death. Clinical trials suggest a single dose of ivermectin along with daily doxycycline treatment for six weeks may be the treatment of choice for onchocerciasis. (10) Researchers are also studying the effectiveness of other antibiotics, including rifampin (Rifadin) and azithromycin (Zithromax), to kill Wolbachia bacteria within the parasites, notes the American Academy of Ophthalmology. (11)
Prevalence of Onchocerciasis (River Blindness)
The WHO estimates that onchocerciasis globally affects nearly 21 million people, though this may be underestimating the effect. Furthermore, another 240 million people live in areas that put them at risk of infection. (13) More than 99 percent of people infected with onchocerciasis live in 31 countries in sub-Saharan Africa, including Cameroon, the Democratic Republic of the Congo, Ethiopia, Ghana, Kenya, Malawi, Nigeria, Uganda, the United Republic of Tanzania, and many others. The rest of the people infected with onchocerciasis live in Brazil, the Bolivarian Republic of Venezuela, and Yemen. (14)
Efforts to Eradicate Onchocerciasis
Onchocerciasis occurs in tropical regions around the world. It was once endemic in sub-Saharan Africa and 13 regional areas across six countries in Latin America, specifically Brazil, Colombia, Ecuador, Guatemala, Mexico, and Venezuela. From 1974 to 2002, the WHO, along with the World Bank, the United Nations Development Programme (UNDP), and the Food and Agriculture Organization ran the Onchocerciasis Control Programme (OCP) to stop the continued transmission of the disease in 11 select countries in Africa. (14) For many years, the OCP focused on controlling the vectors of the disease — the blackflies. The program sprayed insecticides from helicopters and airplanes over blackfly breeding sites to kill the flies’ larvae. In 1987, Merck, the pharmaceutical company that produces Mectizan, launched its Mectizan Donation Program, in which the company would donate the drug to all who needed it for as long as the drug is needed. Thanks to these combined efforts, the WHO and its partners prevented 600,000 cases of blindness and reclaimed 25 million hectares of abandoned arable land for settlement and agricultural production in those African countries. (14) Running from 1995 to 2015, the African Programme for Onchocerciasis Control built on the OCP’s efforts, targeting endemic African countries that the OCP did not cover. The WHO then launched the Expanded Special Project for the Elimination of Neglected Tropical Diseases in Africa in 2016 to tackle a number of neglected tropical diseases in Africa, including onchocerciasis. On the other side of the Atlantic Ocean, the Onchocerciasis Elimination Program of the Americas (OEPA), launched in 1992, has successfully stopped the transmission of the disease in all but two of the six original endemic Latin American countries (and all but 2 of the 13 original endemic areas). In 2013, Columbia became the first country in the Americas to eliminate river blindness; Ecuador, Mexico, and Guatemala followed suit in 2014, 2015, and 2016, respectively. The Bolivarian Republic of Venezuela and Brazil are the only two areas left in the Americas where onchocerciasis transmission still occurs. In 2020, these countries treated about 22,000 people with Mectizan. (3,16,17) Instead, prevention of the disease involves protecting oneself from the blackflies that carry O. volvulus, such as by avoiding intertropical areas with fast-running streams and rivers, especially during the day, when blackflies are most likely to bite.
Apply and reapply a spray, lotion, towelette, or liquid repellent containing 20 to 30 percent DEET or 20 percent Picaridin on exposed skin, according to manufacturers’ directions.Apply sunscreen to your skin before the repellent (preferably at least 20 minutes before).Presoak or spray your outer layer of clothing, shoes, boots, and gear with permethrin (Elimite), which kills insects on contact (but do not apply it to your skin).Wear neutral-colored clothing that covers your legs and arms, as well as a hat with a mesh net covering the face and neck.Ensure that your doors and windows seal properly so that insects are unable to get inside. (18)
Learn More About Everything You Need to Know About Bug Bites and Disease The IAMAT also notes that it’s important to avoid relying on methods that have little to no proven effectiveness for preventing insect-borne illnesses, such as:
Vitamin B1 supplementsGarlicCitronella plants and citronella-based products, including candlesAlternative repellent devices, such as ultrasonic and electrocuting devices and odor-infused trapsWristbands, ankle bands, and neckbands that contain insect repellentsProducts that combine sunscreen and insect repellents (19)
