Riverblindness is caused by a parasitic worm which is spread by a black fly's bite. It causes potential blindness and sever skin itching, which effects every day tasks. It is estimated that 18 million people are infected and the vast majority of them live in Africa. Riverblindness also plagues livestock, costing the African sheep industry over $200 million a year. Ivermectin is the only widely used drug to control this disease and lately there have been cases of built up resistance in the sheep worms. Today researchers are watching for any sign of this resistance in human strands.