River Blindness (Insects)

Onchocerciasis, or river blindness, is a nonfatal, human disease that affects the skin and vision and can ultimately lead to blindness in infected persons. The name river blindness reflects both the place where the disease is most common and severe, and its ultimate outcome.
The disease is caused by a filarial (threadlike) worm in the phylum Nematoda, Onchocerca volvulus. The disease is found in 37 countries, 30 of which are in Africa, 6 in the Americas, and 1 in the Arabian Peninsula; however, Africa, and particularly sub-Saharan West Africa, is by far the most affected area in terms of clinical manifestations of the disease, number of affected persons, and widespread occurrence.
The worm is most commonly transmitted to humans in Africa by black flies in the Simulium damnosum species complex, which contains about 40 different forms, several of which are distinct species. Other species of Simulium are vectors in other parts of the world. Larval and pupal stages of these species occur attached to near-surface substrates in fast-flowing rivers. Because the adults are capable of long flights (>200 km), the aquatic stages are considered the most vulnerable for control.
In humans, the disease takes three progressive forms: first, skin lesions and intense, often violent, itching occur; second, painless nodules containing adult worms form where tissues are thin over bones, such as pelvis, ribs, and scalp, and these nodules are the sites of reproduction for the worms; and third, eye lesions form that can lead to blindness. The third phase can be devastating to communities; 10% of the people in vast areas can be blind, with levels of adults being blind in individual villages sometimes exceeding 30%. This disease is most prevalent among the rural villages that occur in some of the poorest areas of the poorest countries of the world. In these areas, subsistence (whether through agriculture or freshwater fishing) is difficult to achieve; the large proportion of blind adults makes the burden of the disease devastating to these communities.
The life cycle involves both humans and black flies, and there are no known animal reservoirs of this disease. First, a biting, black fly female sucks blood from a human host; if that person is infected, she may also suck up some microfilaria (which are best thought of as worm embryos) of the parasite. Interestingly, evidence indicates that the worms migrate to the skin surface during the day, when black flies are active, which increases their chance of uptake by the biting females. Second, the microfilaria move from the fly’s stomach to its thoracic muscles, where they pass through three larval stages (L1, L2, and L3), and the infective L3 larva moves into the black fly vector’s head and mouthparts. Third, in the course of biting, L3 larvae are transferred from female black flies to humans. Fourth, the larval worm matures to an adult (which can be 40-45 cm in length) and

FIGURE 1 Blind adults being led through villages were a common sight in West Africa before the control of river blindness.
adults mate (in the nodules that form), producing millions of micro-filaria (each about 3 mm in length) that can be picked up from the skin of that human by a biting black fly. The death and disintegration of microfilaria result in an inflammatory reaction, which ultimately leads to itching, visual impairment, and eventually blindness.
The control of river blindness in West Africa has been one of the success stories of public health and economic development (Fig. 1 ). In 1974, the Onchocerciasis Control Programme in West Africa (OCP) began, with its objective being to eliminate onchocerciasis as a disease of public health importance and as an obstacle to socioeconomic development in this region. Eleven countries participated in this program, in a geographical region ranging from Senegal in the west to Benin in the east. Under the jurisdiction of the World Health Organization, and funded through the World Bank and 20 donor countries and organizations, the strategy was to interrupt transmission of the blinding strain of the worm O. volvulus for a period of about 12 years, which is the life span of the adult worm. This was done by aerial application of seven different selective insecticides, weekly, to rivers infested with black flies. Because the female black fly lays batches of eggs at or near the water surface in fast-flowing water, these were the sites sprayed with insecticides. Much of the current understanding of the environmental impact of pesticides on aquatic ecosystems comes from studies conducted as part of this control effort. Beginning in the late 1980s, an ivermectin drug (a microfilaricide originally used as a veterinary product for the treatment of dog heart-worm) was also distributed annually to people living in infected areas.
The success of the OCP is especially impressive when one considers the scope of the project: Onchocerciasis was eradicated in most parts of 11 different countries; at times, over 50,000 km of rivers had to be sprayed weekly for more than 10 (or sometimes 20) years; no permanent environmental damage resulted from the spraying activities because of the increased reliance on insecticides that selectively targeted the black fly vectors; the drugs were distributed to almost 7 million people by over 22,000 community volunteers; millions of children have been born that will be free of this disease; the drugs were donated to the program without charge by the producer of them, Merck; the drug distribution program has recently been expanded to cover 19 more countries (and 157 million people); and
plans include using this same system for dispensing drugs against lymphatic filariasis (elephantiasis) and pesticide-impregnated bed nets that are effective against malaria. The OCP ended in 2002; perhaps its most significant accomplishment is not only that the disease is under control, but also that the previously abandoned valleys, now free of river blindness, grow food for 17 million people in areas inhabited by the poorest of the poor, the people truly living at the ” end of the road.”