To eliminate transmission of Onchocerca volvulus, semiannual mass treatment with ivermectin (Mectizan; donated by Merck & Co) has been underway in Guatemala since 2000. We applied the 2001 World Health Organization (WHO) elimination criteria in the Santa Rosa focus of onchocerciasis transmission in Guatemala (10,923 persons at risk). No evidence of parasite DNA was found in 2,221 Simulium ochraceum vectors (one-sided 95% confidence interval [CI], 0-0.086%), and no IgG4 antibody positives to recombinant antigen OV16 were found in a sample of 3,232 school children (95% CI, 0-0.009%). We also found no evidence of microfilariae in the anterior segment of the eye in 363 area residents (95% CI, 0-0.08%). Our interpretation of these data, together with historical information, suggest that transmission of O. volvulus is permanently interrupted in Santa Rosa and that ivermectin treatments there can be halted.

To identify possible immune mechanisms in human onchocerciasis, we compared a group of 12 individuals who had no clinical or parasitological evidence of infection, despite ongoing exposure to the parasite, with a group of 16 individuals from the same area who had active Onchocerca volvulus infection. Despite having less parasite-specific serum antibody, the infection-free ("putatively immune") individuals showed greater lymphocyte responsiveness, especially interleukin-2 (IL-2) production, to O. volvulus antigen (OVA) than did the infected subjects; lymphocyte responses (including IL-2 production) to mitogens and nonparasite antigen in both study groups were equivalent and normal. Our findings define differences in parasite-specific T cell subpopulations between infected and putatively immune subjects that could be a central element in developing or maintaining protective immunity to O. volvulus infection.

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