Retina in onchocerciasis. Ophthalmoscope view of the damaged retina of a patient suffering from onchocerciasis (river blindness). The pigmented areas are abnormal, causing loss of vision. The retina is the light-sensitive layer at the back of each eye. River blindness is a type of filariasis, caused by the tiny worms (microfiliariae) that are the larval stage in the life cycle of the nematode worm Onchocerca volvulus. The larvae are spread by flies which breed in and around rivers, primarily in Africa. The adult worms (macrofiliariae) grow in the body and mate to produce the microfiliarae that cause an allergic reaction, causing blindness if near the eye. Drug treatments are effective.