Agent – What is the causative agent? Scientific name? Image? Type of pathogen? Where is it found?
Onchocerca volvulus is the pathogen responsible for the disease onchocerciasis, or river blindness. Onchocerciasis is caused by nematodes (roundworms) that inhabit subcutaneous tissues. Onchocerciasis affects at least 17.7 million people worldwide. It is a non-fatal but severely debilitating disease. It has been found in 37 countries (30 in Africa, 6 in the Americas, and Yemen).
Onchocerciasis is transmitted through the bites of Simulium blackflies. Blackflies breed in fast-flowing rivers and streams, increasing the risk of infection to people living nearby.Adult Onchocerca volvulus worms can live for fifteen years in the human body. Common blackfly hosts include Similium damnosum and S. neavei in Africa and S. ochraceum, S. metallicum, S. callidum, and S. exiguum in the Americas.
Host – How are humans affected? Which systems are impacted? What treatments are available?
Onchocerciasis occurs when a blackfly bites an infected person during a blood meal. During the bite, microfilariae of the Onchocerca volvulus are transferred to the person from the fly. The microfilariae then develop into infected larvae over the next few weeks. Live microfilariae are barely noticed inside the body as they cause little inflammation, but dead and degenerating microfilariae in the skin cause severe dermatitis, intense itching, skin depigmentation, skin thickening, skin discoloration, cracking of skin, and loss of skin elasticity.
Quite often, microfilariae migrate to the eye, and their death causes intense inflammation. Over years this sclerosing (scarring) keratitis hardens the cornea and causes blindness.Adults can be found knotted together in pairs or groups in the subcutaneous tissues.
There are a few treatment options for onchocerciasis. One is a surgical operation to remove onchocercomas (especially those around the head) in order to reduce the rate of visual impairment and the number of new infections.
The most popular treatment for infected patients is chemotherapy with the drug ivermectin. A single oral dose given annually eliminates skin microfilariae, interrupts the transmission cycle by suppressing adult female microfilariae release, improves skin disease, and eventually kills adult worms. need to be given every 6 months for the life span of the adult worms or for as long as the infected person has evidence of skin or eye infection. Ivermectin kills the larvae and prevents them from causing damage but it does not kill the adults. There is a promising new treatment using doxycycline that kills the adult worms. They do this by killing the Wolbachia bacteria on which the adult worms depend in order to survive.
Thirdly, larvicides have been used to reduce the blackfly vector population. A process to spay chemicals over high concentrations of black have proved very successful. Between 1974 and 2002 using mainly the spray of insecticides against black-fly larvae (vector control) by helicopters and airplanes and upplemented by large-scale distribution of ivermectin since 1989, the OCP (Onchocerciasis Control Program) relieved 40 million people from infection. These measures prevented blindness in 600 000 people, and ensured that 18 million children were born free from the threat of the disease and blindness.
Environment – What environmental or behavioral factors result in disease transmission? What environmental or behavioral factors impact treating the disease.
Blackflies breed in fast-flowing rivers and streams, increasing the risk of infection to people living nearby. Transmission of the disease is through the bite of female blackflies (usually occurring during the daytime near rapidly flowing rivers and streams). Multiple bites are usually needed before being infected.
There are no vaccines or medications available to prevent becoming infected with O. volvulus. The best prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeves and long pants during the day when black flies bite, and wearing permethrin- treated clothing.