Onchocerca volvulus

Onchocerca volvulus is a species of roundworm that is classified within the Nematoda phylum. This species causes the disease onchocerciasis, more commonly known as river blindness. The life cycle of this species is dependent upon an intermediate host, typically the black fly, and a definitive host, which is always a human. Its lifecycle begins when a black fly ingests microfilariae from a human host by consuming blood. The microfilariae that were present in the skin of the human host now travel through the fly, penetrating through its stomach and moving to the muscle tissues of the fly. In this area, the microfilariae develop into stage one and stage two larvae. Once they reach stage two, the larvae will travel to the proboscis, where they remain in the fly’s saliva and develop into stage three larvae. Once the larvae reach this stage, they can infect a human the next time the black fly feeds on blood, traveling through the blood stream to the subcutaneous layer of the skin.

The third stage Onchocerca volvulus larvae will form small nodes within this layer of the skin and mature into adult worms. This process takes about six to twelve months, after which time males will migrate from their protective nodes to those of female worms in order to breed. Females can lay up to one thousand microfilariae eggs per day, which develop into stage one microfilariae within the subcutaneous layer of the host’s skin until a black fly consumes them, continuing the cycle of the species.

The average lifespan of adult worms is about fifteen years, while the lifespan of the microfilariae is only one to two years. However, neither adults nor juveniles produce symptoms of infection until death, making it difficult for hosts to seek treatment. If a host experiences a long-term infection in the eye, they may contact keratitis, also known as corneal inflammation. If this occurs, the corneal stroma progressively thickens and often causes blindness.

Image Caption: Photomicrograph (glycerine mount) of the microfilarial pathogen en:Onchocerca volvulus in its larval form. Credit: MarcoTolo/Wikipedia