Onchocerciasis, also known as river blindness because the larvae of the blackfly vectors breed in fast flowing rivers, is a parasitic disease caused by the filarial nematode worm Onchocerca volvulus and transmitted to humans, the only natural hosts of the adult parasite, by black flies of the genus Simulium. It is endemic in Africa, and in 13 foci in six countries of the Americas where it was introduced through the slave trade. In the Americas, it is estimated that over half a million people live in 13 foci in Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela where transmission has been documented.
Life cycle of the parasite
The adult worms usually live within the subcutaneous tissues, frequently encapsulated within nodules where they copulate and produce larvae (microfilariae), which are released from the uterus of the fertile female and migrate into the skin. When a suitable black fly vector bites an infected person it takes some microfilariae with its blood meal. The larvae migrate from the midgut of the insect vector into the haemocel and eventually to the thoracic muscles into the head and proboscis, where they mature into infective L3 infective larvae. Within the insect vectors the cycle takes between 7 and 10 days. When the fly bites another person the infective L3 stage larvae penetrate into the bite wound, migrate to the subcutaneous tissues and eventually mature into adults. The adults can live within subcutaneous nodules for up to 12 years. Life cycle of O. volvulus.
Signs and Symptoms
The disease can cause severe skin disease and visual alterations which in severe cases can lead to blindness.
Skin manifestations can be caused both by the adult worms and by microfilariae. The adult worms encapsulate within subcutaneous nodules which are more frequently found in the head, the scapular region or the pelvis.
The microfilariae in the skin ellicit an inflammatorus reaction, dermatitis, which can cause severe itching, loss of skin elasticity and pigment, given the appearance of early aging. This has a psychological and social impact on the affected patient and its community.
Microfilariae migrate into the anterior chamber of the eye, where they eventually die, causing an inflammatory reaction of the cornea, punctuate keratitis, which, if not diagnosed and treated promptly can lead to corneal opacification and eventually to blindness. Blindness attributed to onchocerciasis is considered as the world’s fourth leading cause of preventable blindness after cataract, glaucoma and trachoma.
The disease is diagnosed by the identification of adult worms within subcutaneous nodules, by finding microfilariae in skin snip biopsies of infected patients or by ocular examination of the anterior chamber of the eye. Microfilariae can be seen actively moving by microscopic examination of small skin biopsies taken from infected patients and incubated in saline solution.
Treatment, Prevention and Control
The drug of choice for treating onchocerciasis is ivermectin (Mectizan®). The drug kills the microfilariae in the skin and suppresses the production of microfilariae by adult female fertile worms for a period of up to 6 months. Though ivermectin does not affect the viability of the adult worms, the sustained distribution of a single dose of ivermectin according to weight or height to the eligible population (children above 15 kg weight or 90 cm height and adults, excluding pregnant women or the severely ill) with coverage rates of at least 85% during a period of 10 to 12 years eventually leads to the interruption of the transmission cycle. Therefore, this treatment not only prevents the severe manifestations of skin or ocular disease in infected people, but it is also a very cost-effective public health intervention. Onchocercal nodules when present must be surgically removed.
Elimination of onchocerciasis in the Americas region
In 1991 the 35th Directing Council of the Pan American Health Organization (PAHO) approved Resolution CD35.R14, also called “Strategic Plan for the Elimination of Onchocerciasis in the Americas”, which established the goal of eliminating onchocerciasis as a public health problem in the region by the year 2007.
As a result, the Onchocerciasis Elimination Program for the Americas (OEPA) was created in 1992 with the purpose of providing both technical and financial support to the onchocerciasis elimination programs in the region. OEPA receives financial support from the Carter Center and Lions Club International.
The basic strategy for achieving elimination is through Mass Drug Administration (MDA) of ivermectin (Mectizan®) twice a year to at least 85% of all eligible population, complemented by health education and promotion of community participation, that lasts for at least 10 consecutive years. Mectizan® is donated by Merck’sMectizan Donation Program.
The minimum coverage in all the 13 foci in the region was achieved in 2002, and has been maintained since then. As a result, new cases of onchocercal blindness were eliminated since 2007. However, ocular morbidity still occurs, mainly in the Amazon region of Southern Venezuela and Northern Brazil inhabited by the Yanomami Amerindians, also known as the Yanomami area.
In 2008 PAHO’s 48th Directing Council of PAHO approved Resolution CD 48.R12 which established the goal of interrupting onchocerciasis transmission throughout the region by 2012.
By march 2014 onchocerciasis transmission was considered to be eliminated in 7 foci of the Region: Escuintla, Huehuetenango and Santa Rosa in Guatemala; north of Chiapas and Oaxaca in México; Esmeraldas in Ecuador and López in Colombia. Interruption of transmission has been achieved in another 4 foci including Northcentral and Northwest in Venezuela, South of Chipas in Mexico and Central in Guatemala; where post treatment surveillance is being implemented. The Yanomami area in the Amazon, shared by Brazil and Venezuela is considered to be the greatest challenge to completing regional interruption of transmission by 2012. To implement a bilateral border cooperation agreement to achieve elimination is urgently needed.