Black Fly - Simulium (damnosum)

The last couple of days, when it wasn't snowing, I've been doing a bit of yardwork. I hit a large rock with my mower which put it out of commission until a shear pin could be replaced. My grass grew to about 8 inches high before I got my mower back. Tall grass resulted in me being outside during the worst season in the Adirondacks. But most Adirondackers know it can be difficult to work outside during the late Spring and early Summer.



So what does this have to do with Simulium? Simulium, better known as the black fly, is the terror of the Adirondacks from late May until July when the heat kills many of them off.

Black flies are a nuisance in the Adirondacks but carry a serious disease in Africa and South America called Onchocerciasis.

First the black fly, genus Simulium; it is a stout fly and a powerful flier. It breeds primarily in places that have fast running water, streams and rivers. The various stages of development of the black fly are shown in the below figure.



B, C, and D are the stages of the fly that are found in fast running water. The larval stage is the feeding stage that we attempt to control with the bacterium Bacillus thuringiensis(Bt). The larvae "eats" the Bacillus which then paralyzes the digestive tract of the black fly larvae.

The entire life cycle of the black fly is shown in the figure below.



The part of the life cycle that is necessary for reproduction is not shown. The female black fly must take a blood meal before it can lay eggs. In the Adirondacks, this causes nasty little, lesions that may take a long time to heal. In other areas of the world, the bite of a black fly which is infected with parasitic nematode Onchocerca volvulus. A person infected with Onchocerca volvulus gets a disease called onchocerciasis (river blindness in Africa). The figure below, found at the Carter Center depicts the disease progression.



The adult culprit of this disease are the nematodes shown below.



The adults can be found in nodules as depicted below. The nodules can be openned and the adult worms removed as one form of treatment.



But it is the offspring of the adults that are actually responsible for disease. The larval forms of the nematode are shown below.



The larvae live in the skin and cause a severe dermatitis, itching and infection of the eye which results in blindness. Blind individuals can often be seen being led around by younger individuals who have yet to become infected.



Treatment of the disease is with the drug Ivermectin which paralyzes the larvae and prevents the adult worms from producing more larvae. There is plenty of information on this disease at the Centers for Disease Control.

Maybe later we will discuss Wolbachia, an interesting bacterial endosymbiont of filarial worms.