Black fly control. Officials adding insecticide to a backpack sprayer next to a river. The insecticide is used to eliminate the larvae of Simulium sp. black flies. The bites of these flies transmit the larvae of the parasitic roundworm Onchocerca volvulus, which is the cause of river blindness, or onchocerciasis. After transmission the parasite larvae develop into adults in the skin, causing nodules. The adults breed and produce many larvae that spread throughout the body, including to the eyes. When the larvae die they produce a strong immune reaction that causes inflammation. It is this inflammation that causes blindness. Photographed in Kigoyera, Uganda.