Onchocerca volvulus
Encyclopedia
Onchocerca volvulus is a nematode that causes onchocerciasis
or "river blindness" mostly in Africa. Long-term corneal inflammation, or keratitis
, leads to thickening of the corneal stroma which ultimately leads to blindness. Humans are the only definitive host for O. volvulus. The intermediate host or vector is the black fly
(Simulium
). Onchocerca volvulus is one of four filarial nematodes that cause subcutaneous filariasis
in humans. The other three filarial nematodes are Loa loa
(the African eye worm), Mansonella streptocerca
and Dracunculus medinensis
(the guinea worm).
O. volvulus, along with most nematodes, share an endosymbiotic relationship with the bacterium Wolbachia
. In the absence of Wolbachia, larval development of the O. volvulus is disrupted or ceased.
The life cycle of O. volvulus begins when a parasitised female black fly of the genus Simulium takes a blood
meal. The microfilariae form of the parasite found in the dermis
of the host is ingested by the black fly. Here the microfilariae then penetrates the gut and migrates to thoracic flight muscles of the black fly, entering its first larval phase (J1). After maturing into J2, the second larval phase, it migrates to the proboscis
where it can be found in the saliva
. Saliva containing stage three (J3S) O. volvulus larvae passes into the blood of the host. From here the larvae migrate to the subcutaneous tissue where they form nodules and then mature into adult worms over a period of six to twelve months. After maturation, the smaller adult males migrate from nodules to subcutaneous tissue where they mate with the larger adult females, which then produce between 1,000 and 3,000 microfilaria
e per day. The normal adult worm lifespan is up to fifteen years. The eggs mature internally to form stage one microfilariae, which are released from the female's body one at a time and remain in the subcutaneous tissue.
These stage one microfilariae are taken up by black flies upon a blood meal, in which they mature over the course of one to three weeks to stage three larvae, thereby completing the life cycle.
The normal microfilariae lifespan is 1–2 years; however, their presence in the bloodstream causes little or no immune response until death or degradation of the microfilariae or adult worms.
Onchocerciasis
Onchocerciasis , also known as river blindness and Robles' disease, is a parasitic disease caused by infection by Onchocerca volvulus, a nematode . Onchocerciasis is the world's second-leading infectious cause of blindness. It is not the nematode, but its endosymbiont, Wolbachia pipientis, that...
or "river blindness" mostly in Africa. Long-term corneal inflammation, or keratitis
Keratitis
Keratitis is a condition in which the eye's cornea, the front part of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves impaired eyesight.-Types:...
, leads to thickening of the corneal stroma which ultimately leads to blindness. Humans are the only definitive host for O. volvulus. The intermediate host or vector is the black fly
Black fly
A black fly is any member of the family Simuliidae of the Culicomorpha infraorder. They are related to the Ceratopogonidae, Chironomidae, and Thaumaleidae. There are over 1,800 known species of black flies . Most species belong to the immense genus Simulium...
(Simulium
Simulium
Simulium is a genus of black flies, which may transmit diseases such as onchocerciasis . It is a large genus with several hundred species, and 41 sub-genera....
). Onchocerca volvulus is one of four filarial nematodes that cause subcutaneous filariasis
Filariasis
Filariasis is a parasitic disease and is considered an infectious tropical disease, that is caused by thread-like nematodes belonging to the superfamily Filarioidea, also known as "filariae"....
in humans. The other three filarial nematodes are Loa loa
Loa loa
Loa loa is the filarial nematode species that causes Loa loa filariasis. It is commonly known as the "eye worm". Its geographic distribution includes Africa and India....
(the African eye worm), Mansonella streptocerca
Mansonella streptocerca
Mansonella streptocerca, , is the scientific name of a human parasitic roundworm causing the disease of streptocerciasis...
and Dracunculus medinensis
Dracunculus medinensis
Dracunculus medinensis is a nematode that causes dracunculiasis.Dracunculiasis, also known as Guinea worm disease, is caused by the large female nematode, Dracunculus medinensis, which is among the longest nematodes infecting humans. The adult female is primarily larger than the adult male. The...
(the guinea worm).
O. volvulus, along with most nematodes, share an endosymbiotic relationship with the bacterium Wolbachia
Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...
. In the absence of Wolbachia, larval development of the O. volvulus is disrupted or ceased.
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
meal. The microfilariae form of the parasite found in the dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
of the host is ingested by the black fly. Here the microfilariae then penetrates the gut and migrates to thoracic flight muscles of the black fly, entering its first larval phase (J1). After maturing into J2, the second larval phase, it migrates to the proboscis
Proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...
where it can be found in the saliva
Saliva
Saliva , referred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most other animals. Saliva is a component of oral fluid. In mammals, saliva is produced in and secreted from the three pairs of major salivary glands,...
. Saliva containing stage three (J3S) O. volvulus larvae passes into the blood of the host. From here the larvae migrate to the subcutaneous tissue where they form nodules and then mature into adult worms over a period of six to twelve months. After maturation, the smaller adult males migrate from nodules to subcutaneous tissue where they mate with the larger adult females, which then produce between 1,000 and 3,000 microfilaria
Microfilaria
The microfilaria is a stage in the life cycle of certain parasitic nematodes in the family Onchocercidae. In these species, the adults live in the circulatory system of vertebrates while the early larval stages develop in blood-feeding arthropod vectors...
e per day. The normal adult worm lifespan is up to fifteen years. The eggs mature internally to form stage one microfilariae, which are released from the female's body one at a time and remain in the subcutaneous tissue.
These stage one microfilariae are taken up by black flies upon a blood meal, in which they mature over the course of one to three weeks to stage three larvae, thereby completing the life cycle.
The normal microfilariae lifespan is 1–2 years; however, their presence in the bloodstream causes little or no immune response until death or degradation of the microfilariae or adult worms.