River Blindness: Onchocerciasis
Onchocerciasis (also known as river blindness) is an eye and skin infection that is caused by a parasitic worm (onchocerca volvulus), transmitted by the bite of a black fly that lives and breeds on the banks of fast-flowing rivers and streams. While onchocerciasis is not a direct cause of mortality, the socioeconomic consequences extend beyond the infected individuals, affecting families and communities as a whole.
In West Africa, the risk of blindness due to onchocerciasis used to affect up to 50% of adults leading to whole communities abandoning their fertile river valleys due to the fear of the disease. Onchocerciasis is the second leading cause of infectious blindness (after trachoma) and the fourth leading cause of preventable blindness worldwide.
In 2010, the WHO’s Africa Regional Office estimated that there were 37 million people infected with the disease and more than 102 million people at risk of the disease in Africa alone. 99% of all cases worldwide are found in Sub-Saharan Africa, but isolated foci exist in Yemen and in six countries in Central and South America.
Did you know?
In the West African savannah, the risk of blindness due to onchocerciasis used to be so high along the breeding sites of the black fly that up to 50% of adults were blind in some areas. Such a high disease burden forced people to abandon their fertile river valley lands in fear of contracting the disease, which led to higher levels of poverty and famine.
In the 1970's, economic losses due to onchocerciasis were estimated at US$30 million, and because of the socioeconomic importance of the disease the WHO, along with other international partners, created the Onchocerciasis Control Programme in West Africa (OCP) in 1975. OCP was extremely successful and onchocerciasis is no longer considered a public health problem in 10 out of the 11 countries in which it operated.
Transmission cycle and symptoms:
When a black fly bites a human it transmits an immature larvae of the parasitic worm. In the human body, the larvae form nodules in the subcutaneous tissue where they mature to adult worms and reproduce. Worms can live up to 15 years and mature female worms release up to 1000 microfilariae a day. The microfilariae move through the human body. When the microfilariae die, the body’s inflammatory response leads to a variety of skin and eye conditions.
Many people experience itchy skin, rashes, nodules under the skin and vision impairment. If left untreated, the skin reactions that cause severe itching can lead to lesions and bacterial infections that can progress to the skin becoming rough and course with patchy depigmentation. Vision impairment only occurs after many years of severe infection and therefore is usually only present in individuals over the age of 30. The inflammation caused by larvae that die in the eye result in small opaque spots on the cornea. If left untreated, the cornea becomes permanently cloudy and all the anatomical parts of the eye, including the optic nerve, are damaged, leading to irreversible blindness.
When a black fly bites a human it transmits an immature larvae of the parasitic worm. In the human body, the larvae form nodules in the subcutaneous tissue where they mature to adult worms and reproduce. Worms can live up to 15 years and mature female worms release up to 1000 microfilariae a day.
Diagnosis and treatment:
The time between infection and detection of microfilariae is between three and 15 months. Though a physical examination can detect nodules, a biopsy of the skin (called a skin snip) is the most common laboratory diagnosis for identifying microfilariae. A microscope with a light is used to examine the eye for lesions or larvae. Once diagnosed, the safe, fast-acting, and effective treatment for onchocerciasis is a drug called ivermectin (Mectizan®).
Treatment is on an annual basis and usually continues for 10-15 years to ensure that all the microfilariae in circulation and being produced by the adult female worm are killed. In 1987 the pharmaceutical company Merck & Co. announced the creation of the Mectizan Donation Program to donates tablets for onchocerciasis control to all who needed it, for as long as needed.
Sustainable, community-led mass drug administration of ivermectin is currently the primary control strategy for disease and the WHO goal for controlling onchocerciasis is 2020.