ONCHOCERCIASIS

Onchocerciasis, or "River Blindness," occurs in African south of the Sahara to Angola, Yemen, and smaller areas of Mexico, Guatemala, Colombia, Venezuela, Brazil and Ecuador. It is estimated that:

• 17.5 million people are infected
• 340,00 are completely blind
• 78.3 million are at risk

Transmission Cycle

Onchocerclasis is caused by a filaroid nematode, Onchocerca volvulus. which is transmitted by female black flies of the genus Simulium.

Onchocerca larvae mature to the infective stage in a fly. After maturing, they gain access to a human host through the fly's bite.

Within a year after entering a person, the adult worms pair and mate and are eventually encased in a fibrous nodule that results from the host's immune response. Female worms produce thousands of larval worms, or microfilariae, per day.

Symptoms

Millions of microfilariae migrate through the skin, causing intense rash and itching, and loss of skin pigmentation and elasticity. They frequently reach the eye, causing visual disturbances and blindness.

Economic Impact

Although onchocerciasis is not a life-threatening disease, it, has severe economic repercussions because family providers are often stricken with blindness and other chronic effects of the disease.

WHO estimates that 11 percent or more of some heavily Infected communities may be economically blind." In some villages in Burkina Faso and Ghana. the prevalence of blindness reaches 35 percent.

The intensity of onchocerciasis transmission is highest near rivers and streams, where the black flies breed. Some of the most fertile valleys in tropical Africa have been abandoned because of this disease, The presence of black flies and fear of river blindness also tends to discourage implementation of irrigation projects, dam building and other, development plans.

Onchocerciasis Control

The disease has been controlled through various mechanisms: drug treatment, surgical removal of nodules, and vector control with larvicides.

The larvae of black flies develop in running water. During development, which lasts approximately 10 days, the 'larval stage is vulnerable and can be killed by periodically applying insecticides to breeding, sites.

OCP

The most successful control effort to date, the Onchocerciasis Control Programme (OCP), uses chemical and biological larvicides with low environmental impact to kill black fly larvae. The program began in 1974. It covers virtually every river where the flies are found in 11 West African countries (over 1.2 million square kilometers), serving a population of 25 million.

Because of the efforts of the OCP, which receives $2.5 million/year from A.I.D., some areas are now completely free of onchocerciasis transmission. Fertile tracts of land that had to be abandoned because of the ravages of the disease and the flies have been resettled and are contributing to the financial well-being of host countries.

The adult worms of O. volvulus live for 10-15 years or longer and there is no drug that can kill them. Therefore, any control measures must be continued for 15 or more years to interrupt transmission. In addition to the obvious financial burden this approach creates, it may promote the development of insecticide resistance in the flies.

Treatment

Field trials have demonstrated that the drug ivermectin provides longer lasting control of onchocerciasis with fewer side effects than diethylcarbamazine (DEC), which had been the drug of choice. Ivermectin is being considered as a follow-up intervention after the OCP is disbanded through a process called "devolution."

Interrupting transmission with ivermectin is not considered feasible because the drug does not kill adult worms. Microfilariae can return to patients' skin within six months after treatment. Nevertheless, ivermectin is an important tool for blindness prevention.

The manufacturer of ivermectin, Merck, Sharp and Dohme, has made the drug available to qualified health providers at no cost.

Several donors, including the OCP and A.I.D., are developing systems or national programs for cost-effective distribution of ivermectin. A.I.D.'s Ivermectin Delivery Program, a three-year pilot project, is being carried out by private voluntary organizations and host-country counterparts.