Onchocerca volvulus is a nematode that is found in the tropics. The harmful parasite can cause river blindness in humans.
What is Onchocerca volvulus?
The term “Onchocerca” comes from Greek and translates as “tail” or “hook”. The Latin term “volvulus” means “to roll” or “to turn”. Onchocerca volvulus belongs to the filariae, which form a superfamily of nematodes. It is considered a parasite that affects humans and causes disease. The history of Onchocerca volvulus can be traced back to 1890. In that year, the German helminthologist and zoologist Rudolf Leuckart (1822-1898) received a concoction of worms from Ghana, Africa, for identification at his institute in Leipzig. The specimens came from the bodies of two African patients and represented tumors the size of pigeons. These growths contained nematodes, the females of which were twice as long as the males. In addition, a large number of embryos were located near the nodal cavity. Without publicizing the discovery, Leuckart sent his own specimen and description to the British tropical physician Patrick Manson (1844-1922), who reported the nematode at a London congress in 1891. A written report also appeared in a tropical medicine textbook in 1893. Therefore, the years 1891 and 1893, respectively, are considered the discovery period of Onchocerca volvulus. However, the worm did not receive its name until 1910 by Railliet and Henry, who used the Greek-Latin word combination to describe a “twisting hooked tail.”
Occurrence, distribution and characteristics
Onchocerca volvulus is found primarily in tropical areas from West Africa to Angola. In addition, the nematode is found in East Africa, Central Africa, South and Central American countries such as Brazil, Ecuador, Colombia, Venezuela, Guatemala, and Mexico, as well as in isolated regions in Yemen. The parasite prefers to live in humid regions along fast-flowing rivers. Typical characteristics of Onchocerca volvulus include its filamentous narrow shape. Its diameter is below one millimeter. While males grow to be about 23 to 50 centimeters long, females can reach up to 70 centimeters. The larvae, also called microfilariae, have a length between 220 and 280 micrometers. In human skin, the nematode is able to survive for 15 to 17 years. Onchocerca volvulus represents a parasite whose only end host is humans. In affected endemic regions, sometimes almost 100 percent of the population can be infected. The nematode uses the females of the blackfly (Simulium damnosum) as an intermediate host. The mosquito ingests the microfilariae during the biting process. Within the mosquito, a moult of the larvae takes place, which subsequently reach an infectious stage. When biting again, the blackfly transmits Onchocerca volvulus to humans. Within the organism, the onchocerci migrate through connective or adipose tissue over a period of two years. In some cases, they also pass through the eyes when they reach the head region. After about a year, the nematodes form balls or nodules called onchocercomes. In this way, they lay their larvae in the subcutaneous connective tissue or deeper tissue layers. Microfilariae are deposited by the female onchocerci in skin nodules and tissue crevices. From these sites, they can travel to other areas of the skin. In the early stages, the larvae infest the legs of humans. A few years later, they move on to the upper sections of the body such as the eyes and head.
Diseases and symptoms
One disease caused by Onchocerca volvulus is onchocerciasis, also called river blindness. It predominantly affects tropical areas of Africa and South America. It is estimated that approximately 200 million people worldwide are infected with the nematode. About 10 percent of all affected people go blind as a result. The name river blindness can be traced back to the fact that the disease occurs in most cases near rivers. This is where the larvae of black flies grow up, which serve as intermediate hosts for Onchocerca volvulus. The typical symptoms of onchocerciasis include the appearance of painless nodules within the subcutis.Later, the microfilariae cause skin inflammation, which is noticeable by severe itching. In addition, the elastic parts of the connective tissue are destroyed, which in turn results in the development of so-called old man’s skin or paper skin. Furthermore, the development of a leopard skin pattern due to hyperpigmentation is possible. Subcutaneous onchocercomas are usually found in the iliac crest, sacrum, ribs, shoulders, neck and head. Larger nodules reach 10 centimeters in circumference and can be seen on the skin. It takes years for the microfilariae to reach the eye. However, then there is a risk of visual impairment and even blindness caused by them. Indications are sclerosing keratitis and corneal opacity. Onchocerciasis is usually diagnosed by the physician through a skin biopsy. During this procedure, the physician removes 2 to 3 millimeters of tissue from the skin and performs a microscopic examination. If the microfilariae emerge from the skin sample, the result is positive. To treat onchocerciasis, the patient is given antiparasitic drugs such as ivermectin, albendazole or diethylcarbamazine. These induce larval decay and result in the release of antigens.
