Onchocerciasis, also known as river blindness, is a disease caused by infection with the parasitic worm Onchocerca volvulus. Symptoms include severe itching, bumps under the skin, and blindness. It is the second-most common cause of blindness due to infection, after trachoma. The parasitic worm is spread by the bites of a black fly of the Simulium genus. Usually, many bites are required before infection occurs. These flies live near rivers, hence the common name of the disease, river blindness. Once inside a person, the worms create larvae that make their way out to the skin, where they can infect the next black fly that bites the person. There are a number of ways to make the diagnosis, including placing a biopsy of the skin in normal saline and watching for the larva to come out, looking in the eye for larvae, and looking within the bumps under the skin for adult worms. A vaccine against the disease does not exist. Prevention is by avoiding being bitten by flies. This may include the use of insect repellent and proper clothing. Other efforts include those to decrease the fly population by spraying insecticides. Efforts to eradicate the disease by treating entire groups of people twice a year are ongoing in a number of areas of the world. Treatment of those infected is with the medication ivermectin every six to twelve months. This treatment kills the larvae but not the adult worms. The antibiotic doxycycline weakens the worms by killing an associated bacterium, Wolbachia, and is recommended by some as well. The lumps under the skin may also be removed by surgery. According to the Centers for Disease Control and Prevention, as of 2017, about 20.9 million people were infected with onchocerciasis, and an estimated 1.15 million have some vision loss from the infection. Most infections occur in sub-Saharan Africa, although cases have also been reported in Yemen and isolated areas of Central and South America. In 1915, the physician Rodolfo Robles first linked the worm to eye disease. It is listed by the World Health Organization (WHO) as a neglected tropical disease. In 2013 Colombia became the first country to eradicate the disease.
Cause Onchocerciasis is a parasitic infection caused by the roundworm species Onchocerca volvulus. The larvae of O. volvulus enter a human host when an infected female adult fly from the genus Simulium bites them. After that, it can take up to three months for the worms to mature under the skin of their host. The worms mainly get nutrients for growth in humans from blood, but they have also been seen to rely on other bodily fluids such as cerebrospinal fluid, and urine. It is common to see nodules formed in the skin where the adult worms reside and mate. However, these worms will often travel throughout the body using blood vessels in connective tissues and even settle behind the cornea.
Life cycle The life of the parasite can be traced through the black fly and the human hosts in the following steps:
A Simulium female black fly takes a blood meal on an infected human host and ingests microfilariae. The microfilariae enter the black fly's gut and thoracic flight muscles, progressing into the first larval stage (J1.). The larvae mature into the second larval stage (J2.) and move to the proboscis and into the saliva in its third larval stage (J3.). Maturation takes about seven days. The black fly takes another blood meal, passing the larvae into the next human host's blood. The larvae migrate to the subcutaneous tissue and undergo two more molts. They form nodules as they mature into adult worms over six to 12 months. After maturing, adult male worms mate with female worms in the subcutaneous tissue to produce between 700 and 1,500 microfilariae daily. The microfilariae migrate to the skin during the day, and the black flies only feed during the day, so the parasite is in a prime position for the female fly to ingest it. Black flies take blood meals, ingest these microfilariae and restart the cycle.
Signs and symptoms The larvae can move through the body without triggering a response from the host's immune system, so some people who are infected with the parasite experience no symptoms; the Global Burden of Disease Study estimated that in 2017 there were at least 20.9 million people infected worldwide, of which 14.6 million had skin disease symptoms and 1.15 million experienced symptoms that impacted vision. After a blackfly bite, it can take 12–18 months for the larvae to develop into mature adult worms that will produce their larvae, which is what leads to the development of symptoms. Almost all the clinical manifestations of onchocerciasis are due to localized host inflammatory responses to dead or dying microfilariae (larvae). The signs and symptoms of onchocerciasis are usually divided into two categories, skin and eye symptoms. Skin symptoms will develop years before any vision problems. These symptoms include:
Intense itching Swelling Inflammation Depigmentation Hyperpigmentation Rash Nodules under the skin Skin atrophy Hanging groin (folds of inelastic atrophic skin in the groin associated with enlarged lymph nodes) Eye symptoms include:
Vision impairment, low vision, or permanent blindness. Clouding of the cornea Light sensitivity Lesions on eyes Glaucoma Eye pain Eye redness Eye symptoms provide the common name associated with onchocerciasis, river blindness, and may involve any part of the eye from conjunctiva and cornea to uvea and posterior segment, including the retina and optic nerve. The microfilariae migrate to the surface of the cornea. Punctate keratitis occurs in the infected area. This clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time, the entire cornea may become opaque, thus leading to blindness. Some evidence suggests an immune response to bacteria present in the worms causes the effect on the cornea.
Mazzotti reaction
The Mazzotti reaction, first described in 1948, is a symptom complex seen in patients after undergoing treatment of onchocerciasis with the medication diethylcarbamazine (DEC). Mazzotti reactions can be life-threatening, and are characterized by fever, urticaria, swollen and tender lymph nodes, tachycardia, hypotension, arthralgias, oedema, and abdominal pain that occur within seven days of treatment of microfilariasis.
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