What is onchocerciasis and its symptoms?

Onchocerciasis or river blindness is a disease caused by a tropical worm, which causes changes in the skin or eyes. Reducing or eliminating the sources of transmission will make it possible to end this scourge.

Onchocerciasis is a tropical disease caused by a worm called Onchocerca volvulus. Black flies transmit it in large areas, mainly rural, of sub-Saharan Africa and two foci in America (in Brazil and Venezuela, in specific areas). Millions of people are affected, and half a million of them are blind. As black flies breed in freshwater areas, this disease is also referred to as ‘river blindness.

The symptoms produced by this parasite can be cutaneous, with various types of signs on the skin, from itching with small bumps to the alteration of the color and elasticity and thickness of the skin. The appearance of nodular lumps is another common manifestation. But it is undoubtedly the possibility that the worm affects the most worrying eye since it can cause irreversible blindness in affected people.

Sporadic travelers to worm transmission areas are rarely infected, as multiple bites by infected flies are usually required over more than a year for infection to occur. It is a disease that mainly affects the local population in rural areas dedicated to agriculture, with significant repercussions on their lives due to the stigma and disability it produces.

The disease is suspected when compatible skin or eye symptoms appear in people living in endemic areas. For diagnosis, skin biopsies and slit lamps, ophthalmologic examinations are usually performed. Treatment is done with ivermectin for long periods until the patient is symptom-free. Eradication programs with massive administration of ivermectin could allow the complete disappearance of the disease in the future since man is the only host of this parasite.

Causes of onchocerciasis

The Onchocerciasis is a tropical disease caused by a parasite called Onchocerca volvulus. It is a cutaneous filaria, a nematode worm that affects the skin and the eyes. Onchocerciasis is also called ‘river blindness’ because it is transmitted by the repeated bites of infected black flies (of the genus Simulium ) that breed in rivers, rapids, and streams, especially in rural areas of Africa. It is the second infectious cause of blindness globally: half a million people are blind due to ocular Onchocerciasis.

There are an estimated 37 million infected people globally and about 120 million at risk of infection in sub-Saharan Africa. There are also some foci of affectation in Yemen and Central and South America. However, in this continent, transmission has been eliminated or interrupted, and only two foci remain, one in Brazil and the other in Venezuela. One could distinguish between a savanna form of Onchocerciasis –especially in West Africa– where mainly ocular involvement occurs, and another jungle form, with a higher frequency of skin disease.

The black fly is not very effective at transmitting the worm, requiring a stay of at least 12 months in an endemic area and repeated bites to become infected. Therefore, it is rare for a sporadic traveler to acquire the infection. Humans are the only host for this worm that also needs a bacterium called Wolbachia to survive and reproduce.

When the fly infected by larvae bites a person, it deposits larvae on the skin, 6-12 months become adults. Female adult worms are 20-80 cm, and males 3-5 cm. The females live in nodules under the skin or in the muscles, and the males migrate from one nodule to another. The females produce larvae (microfilariae), which also migrate through the subcutaneous or ocular tissues. A single female can produce between 1,000 and 3,000 microfilariae per day. When a black fly bites an infected person, it carries off the larvae and can infect another person.