Onchocerciasis: changes in transmission in Mexico.

There are now three endemic foci of onchocerciasis in Mexico, all located in mountainous areas in the south-east: two in the state of Chiapas and one in the state of Oaxaca. Together, these three foci cover 16,900 km2 and contain about 286,000 people in 947 localities, most of the localities being small and scattered. The main economic activity in all the foci is coffee farming. Introduction of Mectizan (ivermectin, MSD) into Mexico in 1989 revolutionized control of onchocerciasis in the country and made elimination of the disease a reasonable goal. Concerted efforts between 1989 and 1997, supported by Merck & Co., the River Blindness Foundation and the Onchocerciasis Elimination Programme in the Americas, have led to steady decreases in the incidence of new cases, nodule prevalence and skin-snip positivities. The improvement has been most marked in the Oaxaca focus, where transmission may have been interrupted. As prior to attempts at control, the largest populations at risk are in the Chiapas foci, where logistical problems and migration have combined to limit the success of local control programmes.