Many years ago, while at a conference, I was having breakfast with a well-known US freshwater scientist, who told me a story. While the details of the story are a bit hazy, it went like this.
Last century, a group missionaries travelled to West Africa to convert the local people to Christianity. They were surprised to note that one village which they visited was situated several kilometres from the nearest water source, a large river. The people would treck each day to the river, carrying back water for their needs. When the missionaries asked the people why they did not live closer to the river, they were uncertain, but were convinced that they were not allowed to live closer than they did. The missionaries, wanting to help the village people, convinced them to move their huts to the edge of the river, thus cutting down the effort and time for water fetching considerably. Unfortunately, thereafter, many of the people contracted a disease that is known as ‘river blindness’ (Onchocerciasis) and is carried by blackflies, little ‘true flies’ in the family Simuliidae. It seemed that the adult blackflies could only fly a certain distance from the river, and the village was just beyond that distance, thus ensuring that the people were largely safe from the bites of the flies.
These small flies have an aquatic larval phase, and any student of freshwater ecology would know them as the little critters that live in fast-flowing water, hanging on to hard substrates, and filtering with the fans around their mouths, small particles that go whizzing past them.
Photograph of larval blackflies, attached to substrate and with fans extended catching particles of food as they are swept past (from https://notendur.hi.is/arnie).
Nematode worms are the source of river blindness and use blackflies as an intermediate host. Ocular lesions, caused by the nematodes, are the cause of the blindness. Up to half a million people have been recorded as being affected by the parasite at any one time. A paper by Maria-Gloria Basanez, from Imperial College London and colleagues gives a good overview of the issues, where they warn against complacency. This WHO site on the African Programme for the Control of Onchocerciasis Control gives details on all aspects of the disease and the program to control and eradicate it.
A schematic diagram of how river blindness works is given below (from Basanez et al. (2006) from the journal PloS Medecine 3(9): e371).
I am not able to verify the truth of the story told to me, and my reading tells me that the adult flies are in fact able to fly quite long distances, and so it is likely that the villagers would not have avoided the flies, but the story has stuck with me for many years. So I was interested to see a recent report by the BBC about catching blackflies in the Kou Valley, West Africa. Scientists and human health authorities are moving away from prevention, as they claim that the disease has largely been eliminated, and now more to surveillance. The BBC report largely discusses the use of people as sentinels for the presence of river blindness in the region. Read the BBC story here.
Blackflies, like many freshwater invertebrates, are a pest on many continents, and can cause diseases in mammals, birds, and like river blindness, in humans. Malarial mosquitoes, and aquatic snails, as intermediate hosts for the trematode parasite Bilharzia (Schistosomiasis), are just two more of the many aquatic invertebrates that cause huge problems for humans.