The worms responsible for causing parasitic disease in animals and humans are classified into three species – roundworms or nematodes (including filarial worms and soil-transmitted helminths), flatworms, and flukes. Filarial worms are spread by blood-feeding insect vectors, and invade different parts of the human body causing chronic disease. Wucheria bancrofti, Brugia malayi, and B. timori adult worms invade the lymphatic system, and Onchocerca volvulus and Loa loa form deep tissue and subcutaneous nodules.
Onchocerciasis is predominantly found in West and Central Africa where it causes river blindness, so-called because the black flies which spread disease breed in fast-flowing rivers and streams, and can produce blindness after many years of chronic infection. Before large control programmes started, blindness was highly prevalent in villages along rivers infested with blackflies, leading to the abandonment of fertile land, and increased poverty. An estimated 26 million people are infected with O. volvulus worms, which cause severe itching and may result in blindness or impaired vision.
Lymphatic filariasis (LF) is more widespread, found in tropical areas principally in Africa and Asia. Worms migrate to the lymph glands, resulting in swollen limbs and genitals, a disabling, painful, and highly stigmatizing affliction. 120 million people are infected and 40 million are estimated to be clinically affected by the symptoms.
Loiasis, also known as African eyeworm because of the migration of the adult worm through the conjunctiva, has less direct impact, with symptoms not considered to be as severe. But loiasis infection has important implications for LF and onchocerciasis control programmes using preventive mass drug administration (MDA) chemotherapy, as serious adverse events can occur in co-infected patients.
Filarial disease patients and doctors share their stories