Mode of transmission of onchocerciasis
The parasites that cause onchocerciasis are transmitted from human to human through the bites of blackflies, which belong to Simulum species (figure below). Blackflies breed in fast-flowing rivers and streams, with good vegetation nearby. Unlike mosquitoes and sandflies, they bite during the day when people are active in the area.
What type of water does the blackfly need to breed? How does this differ from the water required by the mosquito vectors of malaria?
Blackflies need fast-running water to breed, unlike Anopheles mosquitoes, which breed in shallow stagnant water collections.
The adult worms mate in the infected person, and the eggs hatch into microscopic worms called microfilaria, which burrow through the body tissues. The person's immune system attacks the microfilaria, causing inflammation and damage in the surrounding tissues. Sight defects and eventually blindness develops when the microfilaria are embedded in the person's eye. When a female blackfly bites an infected person during a blood meal, the microfilaria are transferred from the person to the fly (figure a, below). Over the course of one to three weeks, the microfilaria develop inside the blackfly to form infective larvae (figure b, below). These are then passed on to other people when the blackfly takes another blood meal (figure c, below). The microfilaria migrate to the skin, lymph nodes and eyes of the infected person, causing inflammation and tissue damage.
In the human host, the larvae migrate into the skin, and nodules (swellings) form around them. They slowly mature into adult worms, which can live for 15 years in the human body. After mating, the female worm releases around 1,000 microfilaria a day into the surrounding tissue. Microfilaria live for one to two years, moving around the body. When they die, they cause an inflammatory response which leads to the clinical manifestations and complications such as blindness.