Spread of Onchocerciasis
Onchocerca volvulus,
the worm responsible for Onchocerciasis, is spread by the means of blackfly (Simulium species). Specifically, humans
become infected when blackflies deposit Onchocerca
volvulus larvae into the skin when biting to extract blood. Usually many
bites are required before infection in a human occurs, so people who travel for
short periods of time to areas where the blackfly is found have a low chance of
becoming diseased compared to permanent residents in Onchocerca volvulus ridden regions. The blackfly is usually found
in rural environments consisting of rushing streams as this is where they lay
their own larvae, resulting in the disease’s common name “river blindness”.
Therefore the disease is most intensely transmitted throughout regions having a
fast-flowing water source.
The life cycle of Onchocerca volvulus through both its hosts is below:
- When a blackfly feeds on an infected human, the larvae in the person’s body tissue enter and infect the blackfly.
- The larvae develop in the fly to a stage that is infectious to humans. An infected blackfly will drop the larvae when biting a human and these will then penetrate the new host’s skin.
- The larvae form nodules (lumps) in the subcutaneous tissue, where they mature into adult worms. These nodules usually hold up to 5 adult worms at a time.
- As adults, female worms produce thousands of new larvae daily. These leave the nodules and move through the body, and when they die they cause a variety of conditions which we associate with Onchocerciasis.
- When another blackfly bites an infected person it ingests the new larvae, and the cycle recommences.
The life cycle of Onchocerca volvulus through both its hosts is below:
- When a blackfly feeds on an infected human, the larvae in the person’s body tissue enter and infect the blackfly.
- The larvae develop in the fly to a stage that is infectious to humans. An infected blackfly will drop the larvae when biting a human and these will then penetrate the new host’s skin.
- The larvae form nodules (lumps) in the subcutaneous tissue, where they mature into adult worms. These nodules usually hold up to 5 adult worms at a time.
- As adults, female worms produce thousands of new larvae daily. These leave the nodules and move through the body, and when they die they cause a variety of conditions which we associate with Onchocerciasis.
- When another blackfly bites an infected person it ingests the new larvae, and the cycle recommences.
Diagram showing the life-cycle of Onchocerca volvulus through both hosts
Picture showing A blackfly taking a blood meal from a human