Onchocerciasis in Chiapas, Mexico

By Vachon, Michael | The Geographical Review, April 1993 | Go to article overview

Onchocerciasis in Chiapas, Mexico


Vachon, Michael, The Geographical Review


ONCHOCERCIASIS, or river blindness, is commonly associated with Africa (Hunter 1966), but the disease also affects an estimated 100,000 persons in Latin America, with clusters of incidence in Mexico, Guatemala, Ecuador, Brazil, Colombia, and Venezuela (Shelley 1988). The disease was probably transmitted from Africa by the slave trade and established itself in Latin America in zones with appropriate ecological conditions (Ruiz Reyes 1952). This article examines the coevolution of onchocerciasis and a regional landscape in Mexico. The underlying components that help define the onchocerciasis system there are linked to population mobility and landscape change. Both these variables have placed people, parasite, and disease vector together in time and space.

TRANSMISSION MECHANISM

Onchocerciasis is a disease system involving the filarial worm Onchocerca volvulus, a Simulium or black fly, and a human host. The black fly breeds in streams: hence the common name of the disease, river blindness. Various stages in the life of the parasite are carried in both black fly and human. The human is the definitive host; the black fly is only an intermediate part of the disease system. Two stages of the parasite, adult worm and offspring, occur in the skin and muscle tissue of an infected human. The male and female adult worms live together in a mass known as a worm nodule. Some lie deep in muscle tissue, but others are superficial and palpable, especially in the scalp. The female worms are viviparous, continually bringing forth live offspring known as microfilaria. They may live as long as two years in a human host.

The number of microfilaria in the skin tissue varies according to the infection pressure in a community. Concentrations commonly range from sixty to seventy microfilaria per milligram of skin. The microfilaria develop further only when ingested by a black fly. The female fly inserts her proboscis into human skin tissue to feed. If microfilaria are ingested during the blood meal, they may enter the fly's stomach. From there they must penetrate the stomach lining and migrate to the thoracic muscles, which must occur rapidly or the microfilaria die in the stomach. The sudden change in environment, from human skin tissue to fly stomach, triggers the development of the microfilaria through the next two larval stages. It may take as long as ten days for the ingested microfilaria to mature into infective larvae. During that time the female fly will take more blood meals and lay eggs between them. After deposition of the larvae in a human host, they mature into adult worms, thereby completing the life cycle of the parasite.

The microfilaria, not the adult worms, are directly responsible for the clinical symptoms. The disease is insidious, gradually damaging the skin by causing an itch, loss of elasticity, and sagging. The microfilaria may invade the eyes and surrounding tissue, which sets off immune reactions that cause scarring of the cornea and optic nerves and eventually blindness. It may take ten years before any significant damage occurs to the eyes, but meanwhile the disease has caused skin lesions, itching, skin sagging in the groin and facial areas, and numerous nodules on the body ranging in size from a pea to a golf ball.

Control measures have traditionally focused on reducing black-fly populations and on the mass distribution of drugs (Cupp and others 1986, 1989; Philippon 1987). Control measures for the fly have never been given serious consideration in Latin America (Takaoka and Suzuki 1987). This situation results in part from the difficulty in applying pesticides to the numerous small streams and the lack of resources necessary to sustain a concerted effort. It has been estimated that more than fifteen thousand streams and watercourses in southern Chiapas alone would require chemical treatment to reduce effectively the fly populations (Davies 1974).

MEXICO

Data obtained from public-health officials (Secretaria de Salud) show three endemic zones of onchocerciasis in Mexico.

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