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The biology and control of human onchocerciasis
Published on March 30, 2021 47 min
A selection of talks on Ophthalmology
Photoreceptor protection: significance of docosanoids and elovanoids 1: docosahexaenoic acid (DHA)
- Dr. Nicolas Bazan
- School of Medicine, Louisiana State University Health, New Orleans, USA
Hello and welcome. I'm Ed Cupp, and during the next 50 minutes or so, I'll discuss the biology and control of human onchocerciasis, a major vector-borne disease and formerly the number two cause of infectious blindness on a global basis.
Let's begin by looking at the lifecycle of the etiological agent Onchocerca volvulus, as depicted by the CDC figure. First, it's worth noting that the disease is commonly referred to as 'river blindness' in Africa and 'Robles disease' in the Americas. Onchocerca volvulus is a filarial nematode, which means it belongs to that group of parasitic worms that utilize a blood-sucking arthropod as an intermediate host and vector. In this case, the vector intermediate hosts are members of the genus Simulium, or insects commonly referred to as 'black flies'. We see them depicted here in the diagram by numbers 1 and 5. Humans are the only vertebrate hosts for Onchocerca volvulus. This is a very important aspect from an epidemiological and control perspective, because it means that only Homo sapiens serves as the natural host and reservoir of the parasite. The adult female worms are sedentary and (as we'll see shortly) usually occur in nodules. They may grow to a length of a meter or more. Male worms are much smaller and move between nodules to inseminate the female worms, which then produce an embryonic stage referred to as a microfilaria. This stage is modal, and moves throughout the skin to position itself to be taken up by a vector black fly in a blood meal. This movement through the body causes skin and ocular disease. Parasite development in the vector can be measured in days, whereas development in the human host takes months, as the parasite molts twice and eventually reaches sexual maturity.