The causative agent of Onchocerciasis, also known as “River blindness”, is a by a parasitic worm Onchocerca volvulus. It is a filarial nematode worm that is transmitted to humans by blackflies of the genus Simulium. They are tissue-dwelling nematode parasites that live as adults in the circulatory system or connective tissues of vertebrate hosts. Female worms produce live microfilariae (pre-larvae) instead of laying eggs. They have indirect life-indirect life cycles which consist of the transmission of larvae by arthropod intermediate hosts (blood or tissue feeding insect vectors). A key reason of why the worm is so virulent to humans is its ability to circumvent the innate immune response. The microfilariae that the filariae produce are capable of arresting complement action in the innate immune response. The microfilariae that the filariae produce are slow down complement action in the innate immune response. This inhibits the innate immune response and also disrupts the adaptive immune response allowing the parasite to continue to thrive.
The microfilariae develop in the fly over a period of two weeks and then migrate to its mouthparts. The infectious larvae are then transmitted to a new human host during the next blood meal.The incubation varies from host to host and by the amount of bites from infected vectors. The generally accepted incubation period ranges from 3 months-2 years. This is considered the time from initial infection to the time that the mature filariae produce larvae. The larvae then form nodules and begin maturing into adult world. This process can take up to a year. Nodules may contain various male and female worms where they live coiled up mating together.. Normally nodules only contain 3-5 adult worms, but in extreme cases there has been up to 50 adult worms found inside a single nodule. Females measure 33 to 50 cm in length and 270 to 400 μm in diameter, while males measure 19 to 42 mm by 130 to 210 μm. Once inside the nodules of the skin the adult , the female worms produce progeny microfilariae in numbers of up to 1000 per day for a lifespan of 15 years. The microfilariae, measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that may reach 2 years. The microfilariae migrate throughout the body. The death of the microfilariae in sensitive organs leads to the major symptoms of Onchocerciasis. Major symptoms generally appear nine months to two years after initial infection. The worms are normally found in the skin and in the lymphatics of connective tissues but also sometimes found in peripheral blood, urine, and sputum.
Onchocerciasis manifests into three different forms; subcutaneous nodule formation; dermatitis; and blindness. These nodules tend to form in subcutaneous tissues over bony areas as a form of protection from the immune system. These areas include areas such as the hips, pelvis, ribs, shoulder blades, and skull. If ruptured it is possible that these nodules will trigger a very painful immune response.
Blindness is the most well-known system of Onchocerciasis . As the microfilariae try to make it towards the skin in order to continue the life cycle, some of the organisms make it into the cornea of the eye. The microfilariae cannot survive in the eye and end up dying. The corpses of the microfilariae trigger an inflammatory response that leaves scar tissue in the eye. This makes blindness very gradual but also inevitable without proper treatment. Since blindness take a while to develop, to see anyone under the age of 30 to exhibit serious ocular deficiency. This scar tissue causes multiple issues within the eye including punctate (snowflake) keratitis, sclerosing keratitis, and iridocyclitis.